Most frequently used LINUX commands with examples for oracle database administrator

Tar command with examples

1)tar : The tar command manipulates archives by writing files to, or retrieving files from an archive storage medium. The files used by the tar command are represented by the File parameter.

1. Creating an archive using tar command

Creating an uncompressed tar archive using option cvf

This is the basic command to create a tar archive.

$ tar cvf archive_name.tar dirname/

In the above command:

• c – create a new archive
• v – verbosely list files which are processed.
• f – following is the archive file name

Creating a tar gzipped archive using option cvzf

The above tar cvf option, does not provide any compression. To use a gzip compression on the tar archive, use the z option as shown below.

$ tar cvzf archive_name.tar.gz dirname/

• z – filter the archive through gzip

Note: .tgz is same as .tar.gz

Note: I like to keep the ‘cvf’ (or tvf, or xvf) option unchanged for all archive creation (or view, or extract) and add additional option at the end, which is easier to remember. i.e cvf for archive creation, cvfz for compressed gzip archive creation, cvfj for compressed bzip2 archive creation etc., For this method to work properly, don’t give – in front of the options.

Creating a bzipped tar archive using option cvjf

Create a bzip2 tar archive as shown below:

$ tar cvfj archive_name.tar.bz2 dirname/

• j – filter the archive through bzip2

gzip vs bzip2: bzip2 takes more time to compress and decompress than gzip. bzip2 archival size is less than gzip.

Note: .tbz and .tb2 is same as .tar.bz2

2. Extracting (untar) an archive using tar command

Extract a *.tar file using option xvf

Extract a tar file using option x as shown below:

$ tar xvf archive_name.tar

• x – extract files from archive

Extract a gzipped tar archive ( *.tar.gz ) using option xvzf

Use the option z for uncompressing a gzip tar archive.

$ tar xvfz archive_name.tar.gz

Extracting a bzipped tar archive ( *.tar.bz2 ) using option xvjf

Use the option j for uncompressing a bzip2 tar archive.

$ tar xvfj archive_name.tar.bz2

Note: In all the above commands v is optional, which lists the file being processed.

3. Listing an archive using tar command

View the tar archive file content without extracting using option tvf

You can view the *.tar file content before extracting as shown below.

$ tar tvf archive_name.tar

View the *.tar.gz file content without extracting using option tvzf

You can view the *.tar.gz file content before extracting as shown below.

$ tar tvfz archive_name.tar.gz

View the *.tar.bz2 file content without extracting using option tvjf

You can view the *.tar.bz2 file content before extracting as shown below.

$ tar tvfj archive_name.tar.bz2

4. Listing out the tar file content with less command

When the number of files in an archive is more, you may pipe the output of tar to less. But, you can also use less command directly to view the tar archive output, as explained in one of our previous article 

5. Extract a single file from tar, tar.gz, tar.bz2 file

To extract a specific file from a tar archive, specify the file name at the end of the tar xvf command as shown below. The following command extracts only a specific file from a large tar file.

$ tar xvf archive_file.tar /path/to/file

Use the relevant option z or j according to the compression method gzip or bzip2 respectively as shown below.

$ tar xvfz archive_file.tar.gz /path/to/file

$ tar xvfj archive_file.tar.bz2 /path/to/file

6. Extract a single directory from tar, tar.gz, tar.bz2 file

To extract a single directory (along with it’s subdirectory and files) from a tar archive, specify the directory name at the end of the tar xvf command as shown below. The following extracts only a specific directory from a large tar file.

$ tar xvf archive_file.tar /path/to/dir/

To extract multiple directories from a tar archive, specify those individual directory names at the end of the tar xvf command as shown below.

$ tar xvf archive_file.tar /path/to/dir1/ /path/to/dir2/

Use the relevant option z or j according to the compression method gzip or bzip2 respectively as shown below.

$ tar xvfz archive_file.tar.gz /path/to/dir/

$ tar xvfj archive_file.tar.bz2 /path/to/dir/

7. Extract group of files from tar, tar.gz, tar.bz2 archives using regular expression

You can specify a regex, to extract files matching a specified pattern. For example, following tar command extracts all the files with pl extension.

$ tar xvf archive_file.tar --wildcards '*.pl'

Options explanation:

• –wildcards *.pl – files with pl extension

8. Adding a file or directory to an existing archive using option -r

You can add additional files to an existing tar archive as shown below. For example, to append a file to *.tar file do the following:

$ tar rvf archive_name.tar newfile

This newfile will be added to the existing archive_name.tar. Adding a directory to the tar is also similar,

$ tar rvf archive_name.tar newdir/

Note: You cannot add file or directory to a compressed archive. If you try to do so, you will get “tar: Cannot update compressed archives” error as shown below.

$ tar rvfz archive_name.tgz newfile

tar: Cannot update compressed archives

Try `tar --help' or `tar --usage' for more information.

9. Verify files available in tar using option -W

As part of creating a tar file, you can verify the archive file that got created using the option W as shown below.

$ tar cvfW file_name.tar dir/

If you are planning to remove a directory/file from an archive file or from the file system, you might want to verify the archive file before doing it as shown below.

$ tar tvfW file_name.tar

Verify 1/file1

1/file1: Mod time differs

1/file1: Size differs

Verify 1/file2

Verify 1/file3

If an output line starts with Verify, and there is no differs line then the file/directory is Ok. If not, you should investigate the issue.

Note: for a compressed archive file ( *.tar.gz, *.tar.bz2 ) you cannot do the verification.

Finding the difference between an archive and file system can be done even for a compressed archive. It also shows the same output as above excluding the lines with Verify.

Finding the difference between gzip archive file and file system

$ tar dfz file_name.tgz

Finding the difference between bzip2 archive file and file system

$ tar dfj file_name.tar.bz2

10. Estimate the tar archive size

The following command, estimates the tar file size ( in KB ) before you create the tar file.

$ tar -cf - /directory/to/archive/ | wc -c

20480

The following command, estimates the compressed tar file size ( in KB ) before you create the tar.gz, tar.bz2 files.

$ tar -czf - /directory/to/archive/ | wc -c

508

$ tar -cjf - /directory/to/archive/ | wc -c

Grep command with examples

First create the following demo_file that will be used in the examples below to demonstrate grep command.

$ cat demo_file

THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.

this line is the 1st lower case line in this file.

This Line Has All Its First Character Of The Word With Upper Case.

Two lines above this line is empty.

And this is the last line.

1. Search for the given string in a single file

The basic usage of grep command is to search for a specific string in the specified file as shown below.

Syntax:

grep "literal_string" filename

$ grep "this" demo_file

this line is the 1st lower case line in this file.

Two lines above this line is empty.

2. Checking for the given string in multiple files.

Syntax:

grep "string" FILE_PATTERN

This is also a basic usage of grep command. For this example, let us copy the demo_file to demo_file1. The grep output will also include the file name in front of the line that matched the specific pattern as shown below. When the Linux shell sees the meta character, it does the expansion and gives all the files as input to grep.

$ cp demo_file demo_file1

$ grep "this" demo_*

demo_file:this line is the 1st lower case line in this file.

demo_file:Two lines above this line is empty.

demo_file:And this is the last line.

demo_file1:this line is the 1st lower case line in this file.

demo_file1:Two lines above this line is empty.

demo_file1:And this is the last line.

3. Case insensitive search using grep -i

Syntax:

grep -i "string" FILE

This is also a basic usage of the grep. This searches for the given string/pattern case insensitively. So it matches all the words such as “the”, “THE” and “The” case insensitively as shown below.

$ grep -i "the" demo_file

THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.

this line is the 1st lower case line in this file.

This Line Has All Its First Character Of The Word With Upper Case.

And this is the last line.

4. Match regular expression in files

Syntax:

grep "REGEX" filename

This is a very powerful feature, if you can use use regular expression effectively. In the following example, it searches for all the pattern that starts with “lines” and ends with “empty” with anything in-between. i.e To search “lines[anything in-between]empty” in the demo_file.

$ grep "lines.*empty" demo_file

Two lines above this line is empty.

From documentation of grep: A regular expression may be followed by one of several repetition operators:

• ? The preceding item is optional and matched at most once.
• * The preceding item will be matched zero or more times.
• + The preceding item will be matched one or more times.
• {n} The preceding item is matched exactly n times.
• {n,} The preceding item is matched n or more times.
• {,m} The preceding item is matched at most m times.
• {n,m} The preceding item is matched at least n times, but not more than m times.

5. Checking for full words, not for sub-strings using grep -w

If you want to search for a word, and to avoid it to match the substrings use -w option. Just doing out a normal search will show out all the lines.

The following example is the regular grep where it is searching for “is”. When you search for “is”, without any option it will show out “is”, “his”, “this” and everything which has the substring “is”.

$ grep -i "is" demo_file

THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.

this line is the 1st lower case line in this file.

This Line Has All Its First Character Of The Word With Upper Case.

Two lines above this line is empty.

And this is the last line.

The following example is the WORD grep where it is searching only for the word “is”. Please note that this output does not contain the line “This Line Has All Its First Character Of The Word With Upper Case”, even though “is” is there in the “This”, as the following is looking only for the word “is” and not for “this”.

$ grep -iw "is" demo_file

THIS LINE IS THE 1ST UPPER CASE LINE IN THIS FILE.

this line is the 1st lower case line in this file.

Two lines above this line is empty.

And this is the last line.

6. Displaying lines before/after/around the match using grep -A, -B and -C

When doing a grep on a huge file, it may be useful to see some lines after the match. You might feel handy if grep can show you not only the matching lines but also the lines after/before/around the match.

Please create the following demo_text file for this example.

$ cat demo_text

4. Vim Word Navigation

You may want to do several navigation in relation to the words, such as:

 * e - go to the end of the current word.

 * E - go to the end of the current WORD.

 * b - go to the previous (before) word.

 * B - go to the previous (before) WORD.

 * w - go to the next word.

 * W - go to the next WORD.

WORD - WORD consists of a sequence of non-blank characters, separated with white space.

word - word consists of a sequence of letters, digits and underscores.

Example to show the difference between WORD and word

 * 192.168.1.1 - single WORD

 * 192.168.1.1 - seven words.

6.1 Display N lines after match

-A is the option which prints the specified N lines after the match as shown below.

Syntax:

grep -A <N> "string" FILENAME

The following example prints the matched line, along with the 3 lines after it.

$ grep -A 3 -i "example" demo_text

Example to show the difference between WORD and word

* 192.168.1.1 - single WORD

* 192.168.1.1 - seven words.

6.2 Display N lines before match

-B is the option which prints the specified N lines before the match.

Syntax:

grep -B <N> "string" FILENAME

When you had option to show the N lines after match, you have the -B option for the opposite.

$ grep -B 2 "single WORD" demo_text

Example to show the difference between WORD and word

* 192.168.1.1 - single WORD

6.3 Display N lines around match

-C is the option which prints the specified N lines before the match. In some occasion you might want the match to be appeared with the lines from both the side. This options shows N lines in both the side(before & after) of match.

$ grep -C 2 "Example" demo_text

word - word consists of a sequence of letters, digits and underscores.

Example to show the difference between WORD and word

* 192.168.1.1 - single WORD

7. Highlighting the search using GREP_OPTIONS

As grep prints out lines from the file by the pattern / string you had given, if you wanted it to highlight which part matches the line, then you need to follow the following way.

When you do the following export you will get the highlighting of the matched searches. In the following example, it will highlight all the this when you set the GREP_OPTIONS environment variable as shown below.

$ export GREP_OPTIONS='--color=auto' GREP_COLOR='100;8'

$ grep this demo_file

this line is the 1st lower case line in this file.

Two lines above this line is empty.

And this is the last line.

8. Searching in all files recursively using grep -r

When you want to search in all the files under the current directory and its sub directory. -r option is the one which you need to use. The following example will look for the string “ramesh” in all the files in the current directory and all it’s subdirectory.

$ grep -r "ramesh" *

9. Invert match using grep -v

You had different options to show the lines matched, to show the lines before match, and to show the lines after match, and to highlight match. So definitely You’d also want the option -v to do invert match.

When you want to display the lines which does not matches the given string/pattern, use the option -v as shown below. This example will display all the lines that did not match the word “go”.

$ grep -v "go" demo_text

4. Vim Word Navigation

You may want to do several navigation in relation to the words, such as:

WORD - WORD consists of a sequence of non-blank characters, separated with white space.

word - word consists of a sequence of letters, digits and underscores.

Example to show the difference between WORD and word

* 192.168.1.1 - single WORD

* 192.168.1.1 - seven words.

10. display the lines which does not matches all the given pattern.

Syntax:

grep -v -e "pattern" -e "pattern"

$ cat test-file.txt

a

b

c

d

$ grep -v -e "a" -e "b" -e "c" test-file.txt

d

11. Counting the number of matches using grep -c

When you want to count that how many lines matches the given pattern/string, then use the option -c.

Syntax:

grep -c "pattern" filename

$ grep -c "go" demo_text

6

When you want do find out how many lines matches the pattern

$ grep -c this demo_file

3

When you want do find out how many lines that does not match the pattern

$ grep -v -c this demo_file

4

12. Display only the file names which matches the given pattern using grep -l

If you want the grep to show out only the file names which matched the given pattern, use the -l (lower-case L) option.

When you give multiple files to the grep as input, it displays the names of file which contains the text that matches the pattern, will be very handy when you try to find some notes in your whole directory structure.

$ grep -l this demo_*

demo_file

demo_file1

13. Show only the matched string

By default grep will show the line which matches the given pattern/string, but if you want the grep to show out only the matched string of the pattern then use the -o option.

It might not be that much useful when you give the string straight forward. But it becomes very useful when you give a regex pattern and trying to see what it matches as

$ grep -o "is.*line" demo_file

is line is the 1st lower case line

is line

is is the last line

14. Show the position of match in the line

When you want grep to show the position where it matches the pattern in the file, use the following options as

Syntax:

grep -o -b "pattern" file

$ cat temp-file.txt

12345

12345

$ grep -o -b "3" temp-file.txt

2:3

8:3

Note: The output of the grep command above is not the position in the line, it is byte offset of the whole file.

15. Show line number while displaying the output using grep -n

To show the line number of file with the line matched. It does 1-based line numbering for each file. Use -n option to utilize this feature.

$ grep -n "go" demo_text

5: * e - go to the end of the current word.

6: * E - go to the end of the current WORD.

7: * b - go to the previous (before) word.

8: * B - go to the previous (before) WORD.

9: * w - go to the next word.

10: * W - go to the next WORD.

Find command with examples 

# find -mindepth 3 -maxdepth 5 -name passwd

./usr/bin/passwd

./etc/pam.d/passwd

4. Executing Commands on the Files Found by the Find Command.

In the example below, the find command calculates the md5sum of all the files with the name MyCProgram.c (ignoring case). {} is replaced by the current file name.

# find -iname "MyCProgram.c" -exec md5sum {} \;

d41d8cd98f00b204e9800998ecf8427e  ./mycprogram.c

d41d8cd98f00b204e9800998ecf8427e  ./backup/mycprogram.c

d41d8cd98f00b204e9800998ecf8427e  ./backup/MyCProgram.c

d41d8cd98f00b204e9800998ecf8427e  ./MyCProgram.c

5. Inverting the match.

Shows the files or directories whose name are not MyCProgram.c .Since the maxdepth is 1, this will look only under current directory.

# find -maxdepth 1 -not -iname "MyCProgram.c"

.

./MybashProgram.sh

./create_sample_files.sh

./backup

./Program.c

6. Finding Files by its inode Number.

Every file has an unique inode number, using that we can identify that file. Create two files with similar name. i.e one file with a space at the end.

# touch "test-file-name"

# touch "test-file-name "

[Note: There is a space at the end]

# ls -1 test*

test-file-name

test-file-name

From the ls output, you cannot identify which file has the space at the end. Using option -i, you can view the inode number of the file, which will be different for these two files.

# ls -i1 test*

16187429 test-file-name

16187430 test-file-name

You can specify inode number on a find command as shown below. In this example, find command renames a file using the inode number.

# find -inum 16187430 -exec mv {} new-test-file-name \;

# ls -i1 *test*

16187430 new-test-file-name

16187429 test-file-name

You can use this technique when you want to do some operation with the files which are named poorly as shown in the example below. For example, the file with name — file?.txt has a special character in it. If you try to execute “rm file?.txt”, all the following three files will get removed. So, follow the steps below to delete only the “file?.txt” file.

# ls

file1.txt  file2.txt  file?.txt

Find the inode numbers of each file.

# ls -i1

804178 file1.txt

804179 file2.txt

804180 file?.txt

Use the inode number to remove the file that had special character in it as shown below.

# find -inum 804180 -exec rm {} \;

# ls

file1.txt  file2.txt

[Note: The file with name "file?.txt" is now removed]

7. Find file based on the File-Permissions

Following operations are possible.

• Find files that match exact permission
• Check whether the given permission matches, irrespective of other permission bits
• Search by giving octal / symbolic representation

For this example, let us assume that the directory contains the following files. Please note that the file-permissions on these files are different.

# ls -l

total 0

-rwxrwxrwx 1 root root 0 2009-02-19 20:31 all_for_all

-rw-r--r-- 1 root root 0 2009-02-19 20:30 everybody_read

---------- 1 root root 0 2009-02-19 20:31 no_for_all

-rw------- 1 root root 0 2009-02-19 20:29 ordinary_file

-rw-r----- 1 root root 0 2009-02-19 20:27 others_can_also_read

----r----- 1 root root 0 2009-02-19 20:27 others_can_only_read

Find files which has read permission to group. Use the following command to find all files that are readable by the world in your home directory, irrespective of other permissions for that file.

# find . -perm -g=r -type f -exec ls -l {} \;

-rw-r--r-- 1 root root 0 2009-02-19 20:30 ./everybody_read

-rwxrwxrwx 1 root root 0 2009-02-19 20:31 ./all_for_all

----r----- 1 root root 0 2009-02-19 20:27 ./others_can_only_read

-rw-r----- 1 root root 0 2009-02-19 20:27 ./others_can_also_read

Find files which has read permission only to group.

# find . -perm g=r -type f -exec ls -l {} \;

----r----- 1 root root 0 2009-02-19 20:27 ./others_can_only_read

Find files which has read permission only to group [ search by octal ]

# find . -perm 040 -type f -exec ls -l {} \;

----r----- 1 root root 0 2009-02-19 20:27 ./others_can_only_read

8. Find all empty files (zero byte file) in your home directory and its subdirectory

Most files of the following command output will be lock-files and place holders created by other applications.

# find ~ -empty

List all the empty files only in your home directory.

# find . -maxdepth 1 -empty

List only the non-hidden empty files only in the current directory.

# find . -maxdepth 1 -empty -not -name ".*"

9. Finding the Top 5 Big Files

The following command will display the top 5 largest file in the current directory and its subdirectory. This may take a while to execute depending on the total number of files the command has to process.

# find . -type f -exec ls -s {} \; | sort -n -r | head -5

10. Finding the Top 5 Small Files

Technique is same as finding the bigger files, but the only difference the sort is ascending order.

# find . -type f -exec ls -s {} \; | sort -n  | head -5

In the above command, most probably you will get to see only the ZERO byte files ( empty files ). So, you can use the following command to list the smaller files other than the ZERO byte files.

# find . -not -empty -type f -exec ls -s {} \; | sort -n  | head -5

11. Find Files Based on file-type using option -type

Find only the socket files.

# find . -type s

Find all directories

# find . -type d

Find only the normal files

# find . -type f

Find all the hidden files

# find . -type f -name ".*"

Find all the hidden directories

# find -type d -name ".*"

12. Find files by comparing with the modification time of other file.

Show files which are modified after the specified file. The following find command displays all the files that are created/modified after ordinary_file.

# ls -lrt

total 0

-rw-r----- 1 root root 0 2009-02-19 20:27 others_can_also_read

----r----- 1 root root 0 2009-02-19 20:27 others_can_only_read

-rw------- 1 root root 0 2009-02-19 20:29 ordinary_file

-rw-r--r-- 1 root root 0 2009-02-19 20:30 everybody_read

-rwxrwxrwx 1 root root 0 2009-02-19 20:31 all_for_all

---------- 1 root root 0 2009-02-19 20:31 no_for_all

# find -newer ordinary_file

.

./everybody_read

./all_for_all

./no_for_all

13. Find Files by Size

Using the -size option you can find files by size.

Find files bigger than the given size

# find ~ -size +100M

Find files smaller than the given size

# find ~ -size -100M

Find files that matches the exact given size

# find ~ -size 100M

Note: – means less than the give size, + means more than the given size, and no symbol means exact given size.

14. Create Alias for Frequent Find Operations

If you find some thing as pretty useful, then you can make it as an alias. And execute it whenever you want.

Remove the files named a.out frequently.

# alias rmao="find . -iname a.out -exec rm {} \;"

# rmao

Remove the core files generated by c program.

# alias rmc="find . -iname core -exec rm {} \;"

# rmc

15. Remove big archive files using find command

The following command removes *.zip files that are over 100M.

# find / -type f -name *.zip -size +100M -exec rm -i {} \;"

Remove all *.tar file that are over 100M using the alias rm100m (Remove 100M). Use the similar concepts and create alias like rm1g, rm2g, rm5g to remove file size greater than 1G, 2G and 5G respectively.

# alias rm100m="find / -type f -name *.tar -size +100M -exec rm -i {} \;"

# alias rm1g="find / -type f -name *.tar -size +1G -exec rm -i {} \;"

# alias rm2g="find / -type f -name *.tar -size +2G -exec rm -i {} \;"

# alias rm5g="find / -type f -name *.tar -size +5G -exec rm -i {} \;"

# rm100m

# rm1g

# rm2g

# rm5g

SSH command with examples

Let us review the following 5 basic command line usage of the ssh client.

1. Identify SSH client version
2. Login to remote host
3. Transfer Files to/from remote host
4. Debug SSH client connection
5. SSH escape character usage: (Toggle SSH session, SSH session statistics etc.)

1. SSH Client Version:

Sometimes it may be necessary to identify the SSH client that you are currently running and it’s corresponding version number, which can be identified as shown below. Please note that Linux comes with OpenSSH.

$ ssh -V

OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003

$ ssh -V

ssh: SSH Secure Shell 3.2.9.1 (non-commercial version) on i686-pc-linux-gnu

2. Login to remote host:

• The First time when you login to the remotehost from a localhost, it will display the host key not found message and you can give “yes” to continue. The host key of the remote host will be added under .ssh2/hostkeys directory of your home directory, as shown below.

localhost$ ssh -l jsmith remotehost.example.com

Host key not found from database.

Key fingerprint:

xabie-dezbc-manud-bartd-satsy-limit-nexiu-jambl-title-jarde-tuxum

You can get a public key’s fingerprint by running

% ssh-keygen -F publickey.pub

on the keyfile.

Are you sure you want to continue connecting (yes/no)? yes

Host key saved to /home/jsmith/.ssh2/hostkeys/key_22_remotehost.example.com.pub

host key for remotehost.example.com, accepted by jsmith Mon May 26 2008 16:06:50 -0700

jsmith@remotehost.example.com password:

remotehost.example.com$

• The Second time when you login to the remote host from the localhost, it will prompt only for the password as the remote host key is already added to the known hosts list of the ssh client.

         localhost$ ssh -l jsmith remotehost.example.com
         jsmith@remotehost.example.com password: 
         remotehost.example.com$

• For some reason, if the host key of the remote host is changed after you logged in for the first time, you may get a warning message as shown below. This could be because of various reasons such as 1) Sysadmin upgraded/reinstalled the SSH server on the remote host 2) someone is doing malicious activity etc., The best possible action to take before saying “yes” to the message below, is to call your sysadmin and identify why you got the host key changed message and verify whether it is the correct host key or not.

localhost$ ssh -l jsmith remotehost.example.com

         @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

         @       WARNING: HOST IDENTIFICATION HAS CHANGED!         @

         @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@

         IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!

         Someone could be eavesdropping on you right now (man-in-the-middle attack)!

         It is also possible that the host key has just been changed.

         Please contact your system administrator.

         Add correct host key to "/home/jsmith/.ssh2/hostkeys/key_22_remotehost.example.com.pub"

         to get rid of this message.

        Received server key's fingerprint:

        xabie-dezbc-manud-bartd-satsy-limit-nexiu-jambl-title-jarde-tuxum

        You can get a public key's fingerprint by running

         % ssh-keygen -F publickey.pub

         on the keyfile.

         Agent forwarding is disabled to avoid attacks by corrupted servers.

         Are you sure you want to continue connecting (yes/no)? yes

         Do you want to change the host key on disk (yes/no)? yes

         Agent forwarding re-enabled.

         Host key saved to /home/jsmith/.ssh2/hostkeys/key_22_remotehost.example.com.pub

         host key for remotehost.example.com, accepted by jsmith Mon May 26 2008 16:17:31 -0700

         jsmith @remotehost.example.com's password: 

        remotehost$

3. File transfer to/from remote host:

Another common use of ssh client is to copy files from/to remote host using scp.

• Copy file from the remotehost to the localhost:

        localhost$scp jsmith@remotehost.example.com:/home/jsmith/remotehostfile.txt remotehostfile.txt

• Copy file from the localhost to the remotehost:

        localhost$scp localhostfile.txt jsmith@remotehost.example.com:/home/jsmith/localhostfile.txt

4. Debug SSH Client: 

Sometimes it is necessary to view debug messages to troubleshoot any SSH connection issues. For this purpose, pass -v (lowercase v) option to the ssh as shown below.

• Example without debug message:

        localhost$ ssh -l jsmith remotehost.example.com
        warning: Connecting to remotehost.example.com failed: No address associated to the name
        localhost$

• Example with debug message:

        locaclhost$ ssh -v -l jsmith remotehost.example.com
        debug: SshConfig/sshconfig.c:2838/ssh2_parse_config_ext: Metaconfig parsing stopped at line 3.
        debug: SshConfig/sshconfig.c:637/ssh_config_set_param_verbose: Setting variable 'VerboseMode' to 'FALSE'.
        debug: SshConfig/sshconfig.c:3130/ssh_config_read_file_ext: Read 17 params from config file.
        debug: Ssh2/ssh2.c:1707/main: User config file not found, using defaults. (Looked for '/home/jsmith/.ssh2/ssh2_config')
        debug: Connecting to remotehost.example.com, port 22... (SOCKS not used)
        warning: Connecting to remotehost.example.com failed: No address associated to the name

5. Escape Character: (Toggle SSH session, SSH session statistics etc.)

Escape character ~ get’s SSH clients attention and the character following the ~ determines the escape command. 
Toggle SSH Session: When you’ve logged on to the remotehost using ssh from the localhost, you may want to come back to the localhost to perform some activity and go back to remote host again. In this case, you don’t need to disconnect the ssh session to the remote host. Instead follow the steps below.

• Login to remotehost from localhost: localhost$ssh -l jsmith remotehost
• Now you are connected to the remotehost: remotehost$
• To come back to the localhost temporarily, type the escape character ~ and Control-Z. When you type ~ you will not see that immediately on the screen until you press <Control-Z> and press enter. So, on the remotehost in a new line enter the following key strokes for the below to work: ~<Control-Z>

    remotehost$ ~^Z
    [1]+  Stopped                 ssh -l jsmith remotehost
    localhost$ 

• Now you are back to the localhost and the ssh remotehost client session runs as a typical unix background job, which you can check as shown below:

    localhost$ jobs
    [1]+  Stopped                 ssh -l jsmith remotehost

• You can go back to the remote host ssh without entering the password again by bringing the background ssh remotehost session job to foreground on the localhost

    localhost$ fg %1
    ssh -l jsmith remotehost
    remotehost$

SSH Session statistics: To get some useful statistics about the current ssh session, do the following. This works only on SSH2 client.

• Login to remotehost from localhost: localhost$ssh -l jsmith remotehost
• On the remotehost, type ssh escape character ~ followed by s as shown below. This will display lot of useful statistics about the current SSH connection.

        remotehost$  [Note: The ~s is not visible on the command line when you type.] 
        remote host: remotehost
        local host: localhost
        remote version: SSH-1.99-OpenSSH_3.9p1
        local version:  SSH-2.0-3.2.9.1 SSH Secure Shell (non-commercial)
        compressed bytes in: 1506
        uncompressed bytes in: 1622
        compressed bytes out: 4997
        uncompressed bytes out: 5118
        packets in: 15
        packets out: 24
        rekeys: 0
        Algorithms:
        Chosen key exchange algorithm: diffie-hellman-group1-sha1
        Chosen host key algorithm: ssh-dss
        Common host key algorithms: ssh-dss,ssh-rsa
        Algorithms client to server:
        Cipher: aes128-cbc
        MAC: hmac-sha1
        Compression: zlib
        Algorithms server to client:
        Cipher: aes128-cbc
        MAC: hmac-sha1
        Compression: zlib
        localhost$

SED command with  examples

I. Sed Substitution Delimiter

As we discussed in our previous post, we can use the different delimiters such as @ % | ; : in sed substitute command.

Let us first create path.txt file that will be used in all the examples mentioned below.

$ cat path.txt

/usr/kbos/bin:/usr/local/bin:/usr/jbin:/usr/bin:/usr/sas/bin

/usr/local/sbin:/sbin:/bin/:/usr/sbin:/usr/bin:/opt/omni/bin:

/opt/omni/lbin:/opt/omni/sbin:/root/bin

Example 1 – sed @ delimiter: Substitute /opt/omni/lbin to /opt/tools/bin

When you substitute a path name which has ‘/’, you can use @ as a delimiter instead of ‘/’. In the sed example below, in the last line of the input file, /opt/omni/lbin was changed to /opt/tools/bin.

$ sed 's@/opt/omni/lbin@/opt/tools/bin@g' path.txt

/usr/kbos/bin:/usr/local/bin:/usr/jbin/:/usr/bin:/usr/sas/bin

/usr/local/sbin:/sbin:/bin/:/usr/sbin:/usr/bin:/opt/omni/bin:

/opt/tools/bin:/opt/omni/sbin:/root/bin

II. Sed ‘&’ Get Matched String

The precise part of an input line on which the Regular Expression matches is represented by &, which can then be used in the replacement part.

Example 1 – sed & Usage: Substitute /usr/bin/ to /usr/bin/local

$ sed 's@/usr/bin@&/local@g' path.txt

/usr/kbos/bin:/usr/local/bin:/usr/jbin/:/usr/bin/local:/usr/sas/bin

/usr/local/sbin:/sbin:/bin/:/usr/sbin:/usr/bin/local:/opt/omni/bin:

/opt/omni/lbin:/opt/omni/sbin:/root/bin

In the above example ‘&’ in the replacement part will replace with /usr/bin which is matched pattern and add it with /local. So in the output all the occurrance of /usr/bin will be replaced with /usr/bin/local

Example 2 – sed & Usage: Match the whole line

& replaces whatever matches with the given REGEXP.

$ sed 's@^.*$@<<<&>>>@g' path.txt

<<</usr/kbos/bin:/usr/local/bin:/usr/jbin/:/usr/bin:/usr/sas/bin>>>

<<</usr/local/sbin:/sbin:/bin/:/usr/sbin:/usr/bin:/opt/omni/bin:>>>

<<</opt/omni/lbin:/opt/omni/sbin:/root/bin>>>

In the above example regexp has “^.*$” which matches the whole line. Replacement part <<<&>>> writes the whole line with <<< and >>> in the beginning and end of the line respectively.

III. Grouping and Back-references in Sed

Grouping can be used in sed like normal regular expression. A group is opened with “\(” and closed with “\)”.Grouping can be used in combination with back-referencing.

Back-reference is the re-use of a part of a Regular Expression selected by grouping. Back-references in sed can be used in both a Regular Expression and in the replacement part of the substitute command.

Example 1: Get only the first path in each line

$ sed 's/\(\/[^:]*\).*/\1/g' path.txt

/usr/kbos/bin

/usr/local/sbin

/opt/omni/lbin

In the above example, \(\/[^:]*\) matches the path available before first : comes. \1 replaces the first matched group.

Example 2: Multigrouping

In the file path.txt change the order of field in the last line of the file.

$ sed '$s@\([^:]*\):\([^:]*\):\([^:]*\)@\3:\2:\1@g' path.txt

/usr/kbos/bin:/usr/local/bin:/usr/jbin:/usr/bin:/usr/sas/bin

/usr/local/sbin:/sbin:/bin:/usr/sbin:/usr/bin:/opt/omni/bin:

/root/bin:/opt/omni/sbin:/opt/omni/lbin

In the above command $ specifies substitution to happen only for the last line.Output shows that the order of the path values in the last line has been reversed.

Example 3: Get the list of usernames in /etc/passwd file

This sed example displays only the first field from the /etc/passwd file.

$sed 's/\([^:]*\).*/\1/' /etc/passwd

root

bin

daemon

adm

lp

sync

shutdown

Example 4: Parenthesize first character of each word

This sed example prints the first character of every word in paranthesis.

$ echo "Welcome To The Geek Stuff" | sed 's/\(\b[A-Z]\)/\(\1\)/g'

(W)elcome (T)o (T)he (G)eek (S)tuff

Example 5: Commify the simple number.

Let us create file called numbers which has list of numbers. The below sed command example is used to commify the numbers till thousands.

$ cat  numbers

1234

12121

3434

123

$sed 's/\(^\|[^0-9.]\)\([0-9]\+\)\([0-9]\{3\}\)/\1\2,\3/g' numbers

1,234

12,121

3,434

123

 Awk command with examples

Awk reads and parses each line from input based on whitespace character by default and set the variables $1,$2 and etc. Awk FS variable is used to set the field separator for each record. Awk FS can be set to any single character or regular expression. You can use input field separator using one of the following two options:

1. Using -F command line option.
2. Awk FS can be set like normal variable.

Syntax:

$ awk -F 'FS' 'commands' inputfilename

(or)

$ awk 'BEGIN{FS="FS";}'

• Awk FS is any single character or regular expression which you want to use as a input field separator.
• Awk FS can be changed any number of times, it retains its values until it is explicitly changed. If you want to change the field separator, its better to change before you read the line. So that change affects the line what you read.

Here is an awk FS example to read the /etc/passwd file which has “:” as field delimiter.

$ cat etc_passwd.awk

BEGIN{

FS=":";

print "Name\tUserID\tGroupID\tHomeDirectory";

}

{

        print $1"\t"$3"\t"$4"\t"$6;

}

END {

        print NR,"Records Processed";

}

$awk -f etc_passwd.awk /etc/passwd

Name    UserID  GroupID        HomeDirectory

gnats   41      41      /var/lib/gnats

libuuid 100     101     /var/lib/libuuid

syslog  101     102     /home/syslog

hplip   103     7       /var/run/hplip

avahi   105     111     /var/run/avahi-daemon

saned   110     116     /home/saned

pulse   111     117     /var/run/pulse

gdm     112     119     /var/lib/gdm

8 Records Processed

2. Awk OFS Example: Output Field Separator Variable

Awk OFS is an output equivalent of awk FS variable. By default awk OFS is a single space character. Following is an awk OFS example.

$ awk -F':' '{print $3,$4;}' /etc/passwd

41 41

100 101

101 102

103 7

105 111

110 116

111 117

112 119

Concatenator in the print statement “,” concatenates two parameters with a space which is the value of awk OFS by default. So, Awk OFS value will be inserted between fields in the output as shown below.

$ awk -F':' 'BEGIN{OFS="=";} {print $3,$4;}' /etc/passwd

41=41

100=101

101=102

103=7

105=111

110=116

111=117

112=119

3. Awk RS Example: Record Separator variable

Awk RS defines a line. Awk reads line by line by default

Let us take students marks are stored in a file, each records are separated by double new line, and each fields are separated by a new line character.

$cat student.txt

Jones

2143

78

84

77

Gondrol

2321

56

58

45

RinRao

2122

38

37

65

Edwin

2537

78

67

45

Dayan

2415

30

47

20

Now the below Awk script prints the Student name and Rollno from the above input file.

$cat student.awk

BEGIN {

        RS="\n\n";

        FS="\n";

}

{

        print $1,$2;

}

$ awk -f student.awk  student.txt

Jones 2143

Gondrol 2321

RinRao 2122

Edwin 2537

Dayan 2415

In the script student.awk, it reads each student detail as a single record,because awk RS has been assigned to double new line character and each line in a record is a field, since FS is newline character.

4. Awk ORS Example: Output Record Separator Variable

Awk ORS is an Output equivalent of RS. Each record in the output will be printed with this delimiter. Following is an awk ORS example:

$  awk 'BEGIN{ORS="=";} {print;}' student-marks

Jones 2143 78 84 77=Gondrol 2321 56 58 45=RinRao 2122 38 37 65=Edwin 2537 78 67 45=Dayan 2415 30 47 20=

In the above script,each records in the file student-marks file is delimited by the character “=”.

5. Awk NR Example: Number of Records Variable

Awk NR gives you the total number of records being processed or line number. In the following awk NR example, NR variable has line number, in the END section awk NR tells you the total number of records in a file.

$ awk '{print "Processing Record - ",NR;}END {print NR, "Students Records are processed";}' student-marks

Processing Record -  1

Processing Record -  2

Processing Record -  3

Processing Record -  4

Processing Record -  5

5 Students Records are processed

6. Awk NF Example: Number of Fields in a record

Awk NF gives you the total number of fields in a record. Awk NF will be very useful for validating whether all the fields are exist in a record.

Let us take in the student-marks file, Test3 score is missing for to students as shown below.

$cat student-marks

Jones 2143 78 84 77

Gondrol 2321 56 58 45

RinRao 2122 38 37

Edwin 2537 78 67 45

Dayan 2415 30 47

The following Awk script, prints Record(line) number, and number of fields in that record. So It will be very simple to find out that Test3 score is missing.

$ awk '{print NR,"->",NF}' student-marks

1 -> 5

2 -> 5

3 -> 4

4 -> 5

5 -> 4

7. Awk FILENAME Example: Name of the current input file

FILENAME variable gives the name of the file being read. Awk can accept number of input files to process.

$ awk '{print FILENAME}' student-marks

student-marks

student-marks

student-marks

student-marks

student-marks

In the above example, it prints the FILENAME i.e student-marks for each record of the input file.

8. Awk FNR Example: Number of Records relative to the current input file

When awk reads from the multiple input file, awk NR variable will give the total number of records relative to all the input file. Awk FNR will give you number of records for each input file.

$ awk '{print FILENAME, FNR;}' student-marks bookdetails

student-marks 1

student-marks 2

student-marks 3

student-marks 4

student-marks 5

bookdetails 1

bookdetails 2

bookdetails 3

bookdetails 4

bookdetails 5

In the above example, instead of awk FNR, if you use awk NR, for the file bookdetails the you will get from 6 to 10 for each record.

Vi and Vim Macro: How To Record and Play

Using Vim Macro feature you can record and play a sequence of actions inside the editor.

This article explains how to perform record and play inside Vi and Vim editor using two detailed examples.

High Level Steps to Record and Play inside Vim

1. Start recording by pressing q, followed by a lower case character to name the macro
2. Perform any typical editing, actions inside Vim editor, which will be recorded
3. Stop recording by pressing q
4. Play the recorded macro by pressing @ followed by the macro name
5. To repeat macros multiple times, press : NN @ macro name. NN is a number

Example 1: Sequence number generation inside a file using Vim Macro

1. Start a sequence-test.txt file to generate the sequence.

$ vim sequence-test.txt

2. Go to insert mode and type “1 .” as shown below

Type: Esc i followed by 1.

$ vim sequence-test.txt

1.

3. Start the Recording and store it in register a.

Type: Esc q followed by a

• q indicates to start the recording
• a indicates to store the recordings in register a
• When you do q a, it will display “recording” at the bottom of the vi.

4. Copy the 1st line to 2nd line to have two lines with 1 . as shown below

Type: Esc yy followed by p

• yy will copy the current line
• p will paste the line that was just copied

$ vim sequence-test.txt

1.

1.

Note: Vim will still show recording at the bottom as shown below.

Fig: Vim showing recording message at the bottom

5. Increment the number.

Type: Control a

By placing the cursor at the 2nd line, press Ctrl+a which increment the number to 2. as shown below.

$ vim sequence-test.txt

1.

2.

Note: vim will still show recording at the bottom

6. Stop the recording

Type: q

Press q to stop the recording. You’ll notice that recording message at the bottom of the vim is now gone.

7. Repeat the recording 98 times.

Type: 98@a

• Now repeat this job, by typing 98 @ a
• @a repeats the macro “a” one time.
• 98@a repeats the macros “a” 98 times generating the sequence number 1 – 100 as shown below using macros.

Fig: Generate Sequence Number in Vim using Macro

Example 2: Repeat Vim Macro with different arguments

This example explains how you can executing the same command, with different input for it. i.e Framing the same command, with different arguments.

Before Executing the Macro: change-password.sql

$ vim change-password.sql

Annette

Warren

Anthony

Preston

Kelly

Taylor

Stiller

Dennis

Schwartz

After Recording and executing the Macro: change-password.sql

$ vim change-password.sql

ALTER USER Annette IDENTIFIED BY 'Annette';

ALTER USER Warren IDENTIFIED BY 'Warren';

ALTER USER Anthony IDENTIFIED BY 'Anthony ';

ALTER USER Preston IDENTIFIED BY 'Preston';

ALTER USER Kelly IDENTIFIED BY 'Kelly ';

ALTER USER Taylor IDENTIFIED BY 'Taylor';

ALTER USER Stiller IDENTIFIED BY 'Stiller';

ALTER USER Dennis IDENTIFIED BY 'Dennis';

ALTER USER Schwart IDENTIFIED BY 'Schwart';

1. Open the change-password.sql that has only the names.

$ vim change-password.sql

Annette

Warren

Anthony

Preston

Kelly

Taylor

Stiller

Dennis

Schwartz

2. Start the Recording and store it in register a

Type: q a

• q indicates to start the recording
• a indicates to store the recordings in register a
• When you do q a, it will display the message recording at the bottom of the vi.

3. Go to Insert Mode and Type ALTER USER

Type: I (Upper case i) followed by “ALTER USER ”

Place the cursor anywhere in the first line, and then press I. Which will take you to the first character of the line. Type ALTER USER

4. Copy the Next Word (i.e the name)

Type: Esc w yw

• Press Esc, and then press w to go to the next word ( name ).
• yw, copies the current word ( name ).

5.Go to the end and type IDENTIFIED BY ‘

Type: Esc A followed by ” IDENTIFIED BY ‘”

• Press Esc, and A to move the cursor to the end of the line, and then type space.
• Type IDENTIFIED BY ‘

6. Paste the copied Name

Type: Esc p

Press Esc, and then type p to paste the name that was copied in the step #4.

7. Complete the quote at the end.

Type: Esc A followed by ‘;

Press Esc, and A to go to the end of the line, and ‘;

8. Jump to the next line and stop the record.

Type: Esc j followed by q

• j to move to the next line.
• q to stop the recording

Note: The recording message shown in the bottom of the vi will now disappear. At this stage, the change-password.sql will look like the following.

Fig: Vim Macro completed the recording

9. Repete the Macro with the arguments in the corresponding line

Type: 8 @ a

• Now repeat this job 8 times by typing 8@a
• @a repeats the macro “a” one time.
• 8@a repeats the macros “a” 8 times completing the rest of the line automatically as shown below

Fig: Vim Macro Play completed

Diff command with examples

For finding difference between two versions of a file on Linux, you can use any one of the 4 tools explained in this article — diff, colordiff, wdiff, and vimdiff.

The screenshots provided for these tools shows the difference between the following two empfile1.txt and empfile2.txt.

$ cat empfile1.txt

John Smith     1001    Sr. Engineer

Peter          1002    Engineer

Fernandous     1003    Sr. Engineer

Kraml          1004    Jr. Engineer

$ cat empfile2.txt

John Smith     1001    Sr. Engineer

Peter          1002    Engineer

Fernandous     1003    Resigned

Kraml          1004    Jr. Engineer

Raj            1005    Engineer

1. Diff Command

This is the traditional way to find out the difference two files is using diff command.

$ diff empfile1.txt empfile2.txt

3c3

< Fernandous   1003    Sr. Engineer

---

> Fernandous   1003    Resigned

5c5

< 

---

> Raj          1005    Engineer

sort command examples
Sort a file in ascending order



$ sort names.txt
Sort a file in descending order



$ sort -r names.txt
Sort passwd file by 3rd field.



$ sort -t: -k 3n /etc/passwd | more

 

 

 export
command examples
To view oracle related environment variables.



$ export | grep ORACLE
declare -x ORACLE_BASE="/u01/app/oracle"
declare -x ORACLE_HOME="/u01/app/oracle/product/10.2.0"
declare -x ORACLE_SID="med"
declare -x ORACLE_TERM="xterm"
To export an environment variable:




$ export ORACLE_HOME=/u01/app/oracle/product/10.2.0


 

 xargs
command examples
Copy all images to external hard-drive



# ls *.jpg | xargs -n1 -i cp {} /external-hard-drive/directory
Search all jpg images in the system and archive it.



# find / -name *.jpg -type f -print | xargs tar -cvzf images.tar.gz
Download all the URLs mentioned in the url-list.txt file



# cat url-list.txt | xargs wget –c


Ls  command with examples

ls – Unix users and sysadmins cannot live without this two letter command.
Whether you use it 10 times a day or 100 times a day, knowing the power of ls
command can make your command line journey enjoyable.



In this article, let us review 15 practical examples of the mighty ls
command.




1. Open Last Edited File Using ls -t
To open the last edited file in the current directory use the combination of
ls, head and vi commands as shown below.



ls -t sorts the file by modification time, showing the last
edited file first. head -1 picks up this first file.



$ vi first-long-file.txt
$ vi second-long-file.txt
 
$ vi `ls -t | head -1`
[Note: This will open the last file you edited (i.e second-long-file.txt)]

2. Display One File Per Line Using ls -1
To show single entry per line, use -1 option as shown below.



$ ls -1
bin
boot
cdrom
dev
etc
home
initrd
initrd.img
lib

3. Display All Information About Files/Directories Using ls -l
To show long listing information about the file/directory.





$
ls -l



-rw-r-----
1 ramesh team-dev 9275204 Jun 13 15:27 mthesaur.txt.gz


1st Character – File Type: First character specifies the type of the file.

     In the example above the hyphen (-) in the 1st character indicates that
     this is a normal file. Following are the possible file type options in the
     1st character of the ls -l output.

Field Explanation
- normal file
d directory
s socket file
l link file

Field 1 – File Permissions: Next 9 character specifies the files permission. Each
     3 characters refers to the read, write, execute permissions for user,
     group and world In this example, -rw-r—– indicates read-write permission
     for user, read permission for group, and no permission for others.
Field 2 – Number of links: Second field specifies the number of links for that
     file. In this example, 1 indicates only one link to this file.
Field 3 – Owner:
     Third field specifies owner of the file. In this example, this file is
     owned by username ‘ramesh’.
Field 4 – Group:
     Fourth field specifies the group of the file. In this example, this file
     belongs to ”team-dev’ group.
Field 5 – Size:
     Fifth field specifies the size of file. In this example, ’9275204′
     indicates the file size.
Field 6 – Last modified date & time: Sixth field specifies the date and time of the last
     modification of the file. In this example, ‘Jun 13 15:27′ specifies the
     last modification time of the file.
Field 7 – File name: The
     last field is the name of the file. In this example, the file name is
     mthesaur.txt.gz.



4.
Display File Size in Human Readable Format Using ls -lh



Use ls -lh (h stands for
human readable form), to display file size in easy to read format. i.e M for
MB, K for KB, G for GB.



$
ls -l



-rw-r-----
1 ramesh team-dev 9275204 Jun 12 15:27 arch-linux.txt.gz*



$
ls -lh



-rw-r-----
1 ramesh team-dev 8.9M Jun 12 15:27
arch-linux.txt.gz



5.
Display Directory Information Using ls -ld



When you use “ls -l” you will get
the details of directories content. But if you want the details of directory
then you can use -d option as., For example, if you use ls -l /etc will display
all the files under etc directory. But, if you want to display the information
about the /etc/ directory, use -ld option as shown below.



$
ls -l /etc



total
3344



-rw-r--r--   1 root root  
15276 Oct  5  2004 a2ps.cfg



-rw-r--r--   1 root root    2562 Oct 
5  2004 a2ps-site.cfg



drwxr-xr-x   4 root root    4096 Feb 
2  2007 acpi



-rw-r--r--   1 root root      48 Feb 
8  2008 adjtime



drwxr-xr-x   4 root root    4096 Feb 
2  2007 alchemist



$
ls -ld /etc



drwxr-xr-x
21 root root 4096 Jun 15 07:02 /etc



6.
Order Files Based on Last Modified Time Using ls -lt



To sort the file names displayed in
the order of last modification time use the -t option. You will be finding it
handy to use it in combination with -l option.



$
ls -lt



total
76



drwxrwxrwt  14 root root 
4096 Jun 22 07:36 tmp



drwxr-xr-x
121 root root  4096 Jun 22 07:05 etc



drwxr-xr-x  13 root root 13780 Jun 22 07:04 dev



drwxr-xr-x  13 root root 
4096 Jun 20 23:12 root



drwxr-xr-x  12 root root 
4096 Jun 18 08:31 home



drwxr-xr-x   2 root root 
4096 May 17 21:21 sbin



lrwxrwxrwx   1 root root    11 May 17 20:29 cdrom -> media/cdrom



drwx------   2 root root 16384 May 17 20:29 lost+found



drwxr-xr-x  15 root root 
4096 Jul  2  2008 var



7.
Order Files Based on Last Modified Time (In Reverse Order) Using ls -ltr



To sort the file names in the last
modification time in reverse order. This will be showing the last edited file
in the last line which will be handy when the listing goes beyond a page. This
is my default ls usage. Anytime I do ls, I always use ls -ltr as I find this
very convenient.



$
ls -ltr



total
76



drwxr-xr-x  15 root root 
4096 Jul  2  2008 var



drwx------   2 root root 16384 May 17 20:29 lost+found



lrwxrwxrwx   1 root root    11 May 17 20:29 cdrom -> media/cdrom



drwxr-xr-x   2 root root 
4096 May 17 21:21 sbin



drwxr-xr-x  12 root root 
4096 Jun 18 08:31 home



drwxr-xr-x  13 root root 
4096 Jun 20 23:12 root



drwxr-xr-x  13 root root 13780 Jun 22 07:04 dev



drwxr-xr-x
121 root root  4096 Jun 22 07:05 etc



drwxrwxrwt  14 root root 
4096 Jun 22 07:36 tmp



8.
Display Hidden Files Using ls -a (or) ls -A



To show all the hidden files in the
directory, use ‘-a option’. Hidden files in Unix starts with ‘.’ in its file
name.



$
ls -a



[rnatarajan@asp-dev
~]$ ls -a



.                             Debian-Info.txt



..                            CentOS-Info.txt



.bash_history                 Fedora-Info.txt



.bash_logout                  .lftp



.bash_profile                 libiconv-1.11.tar.tar



.bashrc                      
libssh2-0.12-1.2.el4.rf.i386.rpm



It will show all the files including
the ‘.’ (current directory) and ‘..’ (parent directory). To show the hidden
files, but not the ‘.’ (current directory) and ‘..’ (parent directory), use
option -A.



$
ls -A



Debian-Info.txt               Fedora-Info.txt



CentOS-Info.txt               Red-Hat-Info.txt



.bash_history                 SUSE-Info.txt



.bash_logout                  .lftp



.bash_profile                 libiconv-1.11.tar.tar



.bashrc                      
libssh2-0.12-1.2.el4.rf.i386.rpm



[Note:
. and .. are not displayed here]



9.
Display Files Recursively Using ls -R



$
ls  /etc/sysconfig/networking



devices  profiles



$
ls  -R /etc/sysconfig/networking



/etc/sysconfig/networking:



devices  profiles



/etc/sysconfig/networking/devices:



/etc/sysconfig/networking/profiles:



default



/etc/sysconfig/networking/profiles/default:



To show all the files recursively,
use -R option. When you do this from /, it shows all the unhidden files in the
whole file system recursively.



10.
Display File Inode Number Using ls -i



Sometimes you may want to know the
inone number of a file for internal maintenance. Use -i option as shown below
to display inone number. Using inode number you can remove files that has
special characters in it’s name as explained in the example#6 of the find command article.



$
ls -i /etc/xinetd.d/



279694
chargen      279724 cups-lpd  279697 daytime-udp



279695
chargen-udp  279696 daytime   279698 echo



11.
Hide Control Characters Using ls -q



To print question mark instead of
the non graphics control characters use the -q option.



ls
-q



12.
Display File UID and GID Using ls -n



Lists the output like -l, but shows
the uid and gid in numeric format instead of names.



$
ls -l ~/.bash_profile



-rw-r--r--  1 ramesh ramesh 909 Feb  8 11:48 /home/ramesh/.bash_profile



$
ls -n ~/.bash_profile



-rw-r--r--  1 511 511 909 Feb  8 11:48 /home/ramesh/.bash_profile



[Note:
This display 511 for uid and 511 for gid]



13.
Visual Classification of Files With Special Characters Using ls -F



Instead of doing the ‘ls -l’ and
then the checking for the first character to determine the type of file. You
can use -F which classifies the file with different special character for
different kind of files.



$
ls -F



Desktop/  Documents/ 
Ubuntu-App@  firstfile  Music/ 
Public/  Templates/



Thus in the above output,


/ – directory.
nothing – normal file.
@ – link file.
* – Executable file



14.
Visual Classification of Files With Colors Using
ls -F



Recognizing the file type by the
color in which it gets displayed is an another kind in classification of file.
In the above output directories get displayed in blue, soft links get displayed
in green, and ordinary files gets displayed in default color.



$
ls --color=auto



Desktop  Documents
Examples firstfile Music  Pictures 
Public  Templates  Videos



15.
Useful ls Command Aliases



You can take some required ls
options in the above, and make it as aliases. We suggest the following.


Long list the file with size in human understandable
     form. 



alias
ll="ls -lh"


Classify the file type by appending special characters.
     



alias
lv="ls -F"


Classify the file type by both color and special
     character. 



alias
ls="ls -F --color=auto"


pwd command



pwd is Print working directory. What
else can be said about the good old pwd who has been printing the current
directory name for ages.




Use
CDPATH to define the base directory for cd command



If you are frequently doing cd to
subdirectories of a specific parent directory, you can set the CDPATH to the
parent directory and perform cd to the subdirectories without giving the parent
directory path as explained below.



[ramesh@dev-db
~]# pwd



/home/ramesh



[ramesh@dev-db
~]# cd mail



-bash:
cd: mail: No such file or directory



[Note: This is looking for mail directory under current directory]



[ramesh@dev-db
~]# export CDPATH=/etc



[ramesh@dev-db
~]# cd mail



[Note: This is looking for mail under /etc and not under current
directory]



[ramesh@dev-db
/etc/mail]# pwd



/etc/mail



To make this change permanent, add export
CDPATH=/etc to your ~/.bash_profile



This hack can be very helpful under
the following situations:


Oracle DBAs frequently working under $ORACLE_HOME, can
     set the CDPATH variable to the oracle home
Unix sysadmins frequently working under /etc, can set
     the CDPATH variable to /etc
Developers frequently working under project directory
     /home/projects, can set the CDPATH variable to /home/projects
End-users frequently accessing the subdirectories under
     their home directory, can set the CDPATH variable to ~ (home directory)



Use
cd alias to navigate up the directory effectively



When you are navigating up a very
long directory structure, you may be using cd ..\..\ with multiple ..\’s
depending on how many directories you want to go up as shown below.



#
mkdir -p /tmp/very/long/directory/structure/that/is/too/deep



#
cd /tmp/very/long/directory/structure/that/is/too/deep



#
pwd



/tmp/very/long/directory/structure/that/is/too/deep



#
cd ../../../../



#
pwd



/tmp/very/long/directory/structure



Instead of executing cd ../../../..
to navigate four levels up, use one of the following alias methods:



Navigate up the directory using ..n : In the example below, ..4 is used
to go up 4 directory level, ..3 to go up 3 directory level, ..2 to go up 2
directory level. Add the following alias to the .bash_profile and re-login.



alias
..="cd .."



alias
..2="cd ../.."



alias
..3="cd ../../.."



alias
..4="cd ../../../.."



alias
..5="cd ../../../../.."



#
cd /tmp/very/long/directory/structure/that/is/too/deep



#..4



[Note: use ..4 to go up 4 directory level]



#
pwd



/tmp/very/long/directory/structure/





Navigate up the directory using only dots: In the example below, …..
(five dots) is used to go up 4 directory level.  Typing 5 dots to go up 4
directory structure is really easy to remember, as when you type the first two
dots, you are thinking “going up one directory”, after that every additional
dot, is to go one level up. So, use …. (four dots) to go up 3 directory level
and .. (two dots) to go up 1 directory level. Add the following alias to the
.bash_profile and re-login for the ….. (five dots) to work properly.



alias
..="cd .."



alias
...="cd ../.."



alias
....="cd ../../.."



alias
.....="cd ../../../.."



alias
......="cd ../../../../.."



#
cd /tmp/very/long/directory/structure/that/is/too/deep



#
.....



[Note: use ..... (five dots) to go up 4 directory level]



#
pwd



/tmp/very/long/directory/structure/





Navigate up the directory using cd followed by consecutive dots: In the
example below, cd….. (cd followed by five dots) is used to go up 4 directory
level. Making it 5 dots to go up 4 directory structure is really easy to
remember, as when you type the first two dots, you are thinking “going up one
directory”, after that every additional dot, is to go one level up. So, use
cd…. (cd followed by four dots) to go up 3 directory level and cd… (cd followed
by three dots) to go up 2 directory level. Add the following alias to the
.bash_profile and re-login for the above cd….. (five dots) to work properly.



alias
cd..="cd .."



alias
cd...="cd ../.."



alias
cd....="cd ../../.."



alias
cd.....="cd ../../../.."



alias
cd......="cd ../../../../.."



#
cd /tmp/very/long/directory/structure/that/is/too/deep



#
cd.....



[Note: use cd..... to go up 4 directory level]



#
pwd



/tmp/very/long/directory/structure



Perform
mkdir and cd using a single command



Sometimes when you create a new
directory, you may cd to the new directory immediately to perform some work as
shown below.



#
mkdir -p /tmp/subdir1/subdir2/subdir3



#
cd /tmp/subdir1/subdir2/subdir3



#
pwd



/tmp/subdir1/subdir2/subdir3



Wouldn’t it be nice to combine both
mkdir and cd in a single command? Add the following to the .bash_profile and
re-login.



function
mkdircd () { mkdir -p "$@" && eval cd
"\"\$$#\""; }



Now, perform both mkdir and cd at
the same time using a single command as shown below:



#
mkdircd /tmp/subdir1/subdir2/subdir3



[Note: This creates the directory and cd to it automatically]



#
pwd



/tmp/subdir1/subdir2/subdir3



Use
“cd -” to toggle between the last two directories



You can toggle between the last two
current directories using cd – as shown below.



#
cd /tmp/very/long/directory/structure/that/is/too/deep



#
cd /tmp/subdir1/subdir2/subdir3



#
cd -



#
pwd



/tmp/very/long/directory/structure/that/is/too/deep



#
cd -



#
pwd



/tmp/subdir1/subdir2/subdir3



#
cd -



#
pwd



/tmp/very/long/directory/structure/that/is/too/deep



Note: You can also substitute an argument from other commands in
the history to the cd command using example#12 and #13 mentioned in the command line history
examples article.



Use
dirs, pushd and popd to manipulate directory stack



You can use directory stack to push
directories into it and later pop directory from the stack. Following three
commands are used in this example.


dirs: Display the directory stack
pushd: Push directory into the stack
popd: Pop directory from the stack and cd to it



Dirs will always print the current
directory followed by the content of the stack. Even when the directory stack
is empty, dirs command will still print only the current directory as shown
below.



#
popd



-bash:
popd: directory stack empty



#
dirs



~



#
pwd



/home/ramesh



How to use pushd and popd? Let us
first create some temporary directories and push them to the directory stack as
shown below.



#
mkdir /tmp/dir1



#
mkdir /tmp/dir2



#
mkdir /tmp/dir3



#
mkdir /tmp/dir4



#
cd /tmp/dir1



#
pushd .



#
cd /tmp/dir2



#
pushd .



#
cd /tmp/dir3



#
pushd .



#
cd /tmp/dir4



#
pushd .



#
dirs



/tmp/dir4
/tmp/dir4 /tmp/dir3 /tmp/dir2 /tmp/dir1



[Note:
The first directory (/tmp/dir4) of the dir command output is always



          the current directory and not the
content from the stack.]



At this stage, the directory stack
contains the following directories:



/tmp/dir4



/tmp/dir3



/tmp/dir2



/tmp/dir1



The last directory that was pushed
to the stack will be at the top. When you perform popd, it will cd to the top
directory entry in the stack and remove it from the stack. As shown above, the
last directory that was pushed into the stack is /tmp/dir4. So, when we do a
popd, it will cd to the /tmp/dir4 and remove it from the directory stack as
shown below.



#
popd



#
pwd



/tmp/dir4



[Note:
After the above popd, directory Stack Contains:



/tmp/dir3



/tmp/dir2



/tmp/dir1]



#
popd



#
pwd



/tmp/dir3



[Note:
After the above popd, directory Stack Contains:



/tmp/dir2



/tmp/dir1]



#
popd



#
pwd



/tmp/dir2



[Note:
After the above popd, directory Stack Contains: /tmp/dir1]



#
popd



#
pwd



/tmp/dir1



[Note:
After the above popd, directory Stack is empty!]



#
popd



-bash:
popd: directory stack empty



Use
“shopt -s cdspell” to automatically correct mistyped directory names on cd



Use shopt -s cdspell to
correct the typos in the cd command automatically as shown below. If you are
not good at typing and make lot of mistakes, this will be very helpful.



#
cd /etc/mall



-bash:
cd: /etc/mall: No such file or directory



#
shopt -s cdspell



#
cd /etc/mall



#
pwd



/etc/mail



[Note: By mistake, when I typed mall instead of mail,



          cd corrected it
automatically]


gzip command examples
To create a *.gz compressed file:



$ gzip test.txt
To uncompress a *.gz file:



$ gzip -d test.txt.gz
Display compression ratio of the compressed file using gzip -l



$ gzip -l *.gz
         compressed        uncompressed  ratio uncompressed_name
              23709               97975  75.8% asp-patch-rpms.txt

 


bzip2 command examples
 
bzip2 command is used for compression and decompression of files. The main
advantage of bzip2 is the best compression size. bzip2 vs gzip: bzip2 will
compress better than gzip. Speed of bzip2 is somewhat slower than gzip and zip.
bzip2 provides high rate of compression with reasonable speed.



There are several Linux bz commands available to manipulate the bzip2 files.
This article explains various bz commands with 6 practical examples.








Example 1: Compressing a file using bzip2
When you compress a file using bzip2 command, it creates a compressed file
with *.bz2 extension as shown below.



$ bzip2 trace
 
$ ls -l trace.bz2
-rw-r--r-- 1 root root       54167 Jan 23  2009 trace.bz2

Example 2: Search operation in bzip2 file using bzgrep
bzgrep command is used to search for a string or a pattern (regular
expression) on bzip2 compressed files.



bzgrep will apply grep to data from files in the bzip2 format without
requiring on-disk decompression. So all the options of a grep command will be
applied for bzgrep also.



Syntax:
bzgrep grep-options -e pattern filename
In the below example, trace.bz2 is a compressed trace file which is of size
58M.











$ bzgrep -i "CONSOLE=.*"  trace.bz2
2010-10-11T08:40:28.100 gs(16985): CONSOLE=/dev/pts/0
2010-10-11T08:40:29.772 gs(17031): CONSOLE=/dev/pts/0
2010-10-11T08:40:58.140 gs(17099): CONSOLE=/dev/pts/0
2010-10-11T08:41:27.547 gs(17164): CONSOLE=/dev/pts/0
2010-10-11T08:41:57.962 gs(17233): CONSOLE=/dev/pts/0
2010-10-11T08:42:28.392 gs(17294): CONSOLE=/dev/pts/0
2010-10-11T08:42:57.721 gs(17439): CONSOLE=/dev/pts/0
If bzgrep is not there, you have to decompress the file manually and do a
grep on that, where bzgrep does this process internally and gives you the
required output. bzegrep and bzfgrep commands will apply egrep and freg
operation on bzip2 files respectively.




Example 3: View the bzip2 file using bzcat
If you want only to read the compressed file without decompressing it, use
the bzcat command as shown below.



$ bzcat trace.bz2
0: ERR: Wed Sep 22 09:59:42 2010: gs(11153/47752677794640): [chk_sqlcode.scp:92]: Database: ORA-01653: unable to extend table OPC_OP.OP
C_HIST_MESSAGES (OpC50-15)
0: ERR: Wed Sep 22 09:59:47 2010: gs(11153/47752677794640): [chk_sqlcode.scp:92]: Database: ORA-01653: unable to extend table OPC_OP.OP
C_HIST_MESSAGES (OpC50-15)
Retry. (OpC51-22)
Database: ORA-01653: unable to extend table OPC_OP.OPC_HIST_MESSAGES by 64 in tablespace OPC_6
(OpC50-15)
.
.
bzcat command displays the uncompressed content into standard output file
for the users to view the content.




Example 4. Paging bzip2 compressed file with bzless & bzmore
bzless and bzmore command allows you to view the content of bzip2 compressed
files page by page. bzmore works on files compressed with bzip2 and also on
uncompressed files.



$ bzless trace.bz2
 
$ bzmore trace.bz2
 
0: ERR: Wed Sep 22 09:59:42 2010: gs(11153/47752677794640): [chk_sqlcode.scp:92]: Database: ORA-01653: unable to extend table OPC_OP.OP
C_HIST_MESSAGES (OpC50-15)
0: ERR: Wed Sep 22 09:59:47 2010: gs(11153/47752677794640): [chk_sqlcode.scp:92]: Database: ORA-01653: unable to extend table OPC_OP.OP
C_HIST_MESSAGES (OpC50-15)
Retry. (OpC51-22)
Database: ORA-01653: unable to extend table OPC_OP.OPC_HIST_MESSAGES by 64 in tablespace OPC_6
(OpC50-15)
.
.
--More--

Example 5. Compare bzip2 files using bzcmp
You can compare two bzip2 compressed file using bzcmp command. It internally
uses cmp command to compare the content of the compressed contents. Here you
can see the output of comparison of the two normal files and compressed files.



$ cmp System.txt.001 System.txt.002
System.txt.001 System.txt.002 differ: byte 20, line 2
 
$ bzcmp System.txt.001.bz2 System.txt.002.bz2
- /tmp/bzdiff.csgqG32029 differ: byte 20, line 2

Example 6. Find the difference of two bzip2 files using bzdiff
In Linux, diff command will compare two files and give you the lowdown on
just how different they are. If you give bz2 files to diff command, it will not
be in a position to explain the difference.



For bzip2 compressed files, bzdiff command gives the differences of two
bzip2 compressed files as shown below.



$ bzdiff System.txt.001.bz2 System.txt.002.bz2
2c2
< 0: ERR: Mon Sep 27 12:19:34 2010: gs(11153/1105824064): [chk_sqlcode.scp:92]: Database: ORA-01654: unable to extend index OPC_OP.OPCX
_ANNO_NUM by 64 in tablespace OPC_INDEX1
---
> 0: ERR: Wed Sep 22 09:59:42 2010: gs(11153/47752677794640): [chk_sqlcode.scp:92]: Database: ORA-01653: unable to extend table OPC_OP.
OPC_HIST_MESSAGES by 64 in tablespace OPC_6
4,5c4
< Retry. (OpC51-22)
< Database: ORA-01654: unable to extend index OPC_OP.OPCX_ANNO_NUM by 64 in tablespace OPC_INDEX1
---
> 0: ERR: Wed Sep 22 09:59:47 2010: gs(11153/47752677794640): [chk_sqlcode.scp:92]: Database: ORA-01653: unable to extend table OPC_OP.
OPC_HIST_MESSAGES by 64 in tablespace OPC_6

unzip command examples
To extract a *.zip compressed file:



$ unzip test.zip
View the contents of *.zip file (Without unzipping it):



$ unzip -l jasper.zip
Archive:  jasper.zip
  Length     Date   Time    Name
 --------    ----   ----    ----
    40995  11-30-98 23:50   META-INF/MANIFEST.MF
    32169  08-25-98 21:07   classes_
    15964  08-25-98 21:07   classes_names
    10542  08-25-98 21:07   classes_ncomp
 
-----------------------------------------------------------------------------

shutdown command examples
Shutdown the system and turn the power off immediately.



# shutdown -h now
Shutdown the system after 10 minutes.



# shutdown -h +10
Reboot the system using shutdown command.



# shutdown -r now
Force the filesystem check during reboot.




# shutdown -Fr now



ftp command with examples

FTP is File Transfer Protocol. SFTP is secure FTP. In this article let us
review how to connect and login to a remote ftp server for downloading and
uploading files using ftp or sftp command. Most of the ftp commands are
applicable to sftp. So, wherever ftp is mentioned, you can use sftp also.








1. Connect to a FTP site
Connect to a particular FTP server using ftp command as shown below.

Syntax:



$ ftp IP/hostname
 
or 
 
$ ftp
ftp> open IP/hostname
You can directly open connection with a remote host using it’s IP or host
name from the command line. You can also go to ftp prompt and use open command
to connect with remote host.



It will ask you for the user name and password to login. On some public
domain FTP server, you can use “anonymous” username with any email address as
the password to connect. 




2. Download a file using ftp
Use the get command to download file from a remote ftp server as shown
below.



ftp> get FILENAME
You have to be in the right mode to download files. i.e binary or ascii
mode. Use ascii mode for transferring text files, and binary mode for all other
type of files.



Download the file and save it with another name. In the following example,
index.html file will be downloaded and saved as my.html on the local server.






ftp>
get index.html my.html



Fetching
/home/groups/index.html to my.html



/home/groups/index.html                          100% 2886     1.4KB/s  
00:02



3.
Changing FTP Mode to binary or ascii



Go to ftp Ascii mode



ftp>
ascii



200
Type set to A.



Go to ftp Binary mode



ftp>
binary



200
Type set to I.



4.
Uploading a file to FTP server



Use put command to upload a file to
a remote ftp server as shown below.



ftp>
put filename



5.
Changing the remote and local directory



Apart from downloading or uploading
a file, you may want to change either the remote or local directory, which you
can do using cd and lcd respectively. 



Change the remote server current
directory using cd command



ftp>
pwd



257
"/myftpserver" is current directory.



ftp>
cd dir1



250
CWD command successful. "/myftpserver/dir1" is current directory.



ftp>
pwd



257
"/myftpserver/dir1" is current directory.



Change the local machine current
directory using lcd command



ftp>
!



$
pwd



/home/sathiya/FTP



$
exit



exit



ftp>
lcd /tmp



Local
directory now /tmp



ftp>
!



$
pwd



/tmp



Note: 


executing ! takes you to the shell. 
prompt starts with ftp> is ftp prompt. 
prompt starts with $ is shell command line. 



6.
Listing the contents of remote directory from FTP



You can view the content of a remote
directory using the ls / dir command.



ftp>
ls



7.
FTP Help



Type help or ? to view
list of all available ftp commands.



For a detailed help on a particular
ftp command use:



ftp>
help COMMAND



8.
Downloading multiple files with mget command



mget is for fetching multiple files
from ftp server. You can use globs to download multiple files. For example,
*.html will download all html files. The glob expansion are done on the remote
server. So, it depends on the operating system of the remote server. 



ftp>
mget *.html



Fetching
/ftptest/features.html to features.html



/ftptest/features.html                       100% 2256     2.2KB/s  
00:01



Fetching
/ftptest/index.html to index.html



/ftptest/index.html                          100% 2886     2.8KB/s  
00:01



Fetching
/ftptest/othertools.html to othertools.html



/ftptest/othertools.html                     100% 2282     2.2KB/s  
00:01



Fetching
/ftptest/samplereport.html to samplereport.html



/ftptest/samplereport.html                   100%   15KB  
7.3KB/s   00:02



Fetching
/ftptest/usage.html to usage.html



/ftptest/usage.html                          100% 2340     2.3KB/s  
00:01



To view the file names before
downloading, you can also use mls command as shown below.



ftp>
mls *.html -



/ftptest/features.html



/ftptest/index.html



/ftptest/othertools.html



/ftptest/samplereport.html



/ftptest/usage.html



9.
Uploading multiple files with mput command



Use mput to upload multiple files
together. This works similar to the mget command. The following example uploads
all the *.html file from local server to remote server.



ftp>
mput *.html



10.
Close a FTP connection



Without exiting the ftp prompt you
may want to open a connection to another server. In that case, execute close
command.



ftp>
open ftp.your_server.com



Already
connected to NNN.com, use close first.



ftp>
close



221
Goodbye.




ftp> open ftp.your_server.com




crontab commands with
examples



An experienced Linux sysadmin knows the importance of
running the routine maintenance jobs in the background automatically.



Linux Cron utility is an effective way to schedule a routine background job at
a specific time and/or day on an on-going basis.



Linux
Crontab Format



MIN
HOUR DOM MON DOW CMD





Table: Crontab Fields and Allowed Ranges (Linux Crontab Syntax)




Field



Description



Allowed Value




MIN



Minute field



0 to 59




HOUR



Hour field



0 to 23




DOM



Day of Month



1-31




MON



Month field



1-12




DOW



Day Of Week



0-6




CMD



Command



Any command to be executed.






1.
Scheduling a Job For a Specific Time



The basic usage of cron is to
execute a job in a specific time as shown below. This will execute the Full
backup shell script (full-backup) on 10th June 08:30 AM.



Please note that the time field uses 24 hours format. So, for 8 AM use 8, and
for 8 PM use 20.



30
08 10 06 * /home/ramesh/full-backup


30 –
     30th Minute
08 – 08
     AM
10 –
     10th Day
06 –
     6th Month (June)
* –
     Every day of the week



2.
Schedule a Job For More Than One Instance (e.g. Twice a Day)



The following script take a
incremental backup twice a day every day.



This example executes the specified incremental backup shell script
(incremental-backup) at 11:00 and 16:00 on every day. The comma separated value
in a field specifies that the command needs to be executed in all the mentioned
time.



00
11,16 * * * /home/ramesh/bin/incremental-backup


00 –
     0th Minute (Top of the hour)
11,16
     – 11 AM and 4 PM
* –
     Every day
* –
     Every month
* –
     Every day of the week



3.
Schedule a Job for Specific Range of Time (e.g. Only on Weekdays)



If you wanted a job to be scheduled
for every hour with in a specific range of time then use the following.



Cron
Job everyday during working hours



This example checks the status of
the database everyday (including weekends) during the working hours 9 a.m – 6
p.m



00
09-18 * * * /home/ramesh/bin/check-db-status


00 –
     0th Minute (Top of the hour)
09-18
     – 9 am, 10 am,11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm
* –
     Every day
* –
     Every month
* –
     Every day of the week



Cron
Job every weekday during working hours



This example checks the status of
the database every weekday (i.e excluding Sat and Sun) during the working hours
9 a.m – 6 p.m.



00
09-18 * * 1-5 /home/ramesh/bin/check-db-status


00 –
     0th Minute (Top of the hour)
09-18
     – 9 am, 10 am,11 am, 12 am, 1 pm, 2 pm, 3 pm, 4 pm, 5 pm, 6 pm
* –
     Every day
* –
     Every month
1-5
     -Mon, Tue, Wed, Thu and Fri (Every Weekday)



4.
How to View Crontab Entries?



View
Current Logged-In User’s Crontab entries



To view your crontab entries type
crontab -l from your unix account as shown below.



ramesh@dev-db$
crontab -l



@yearly
/home/ramesh/annual-maintenance



*/10
* * * * /home/ramesh/check-disk-space



[Note:
This displays crontab of the current logged in user]



View
Root Crontab entries



Login as root user (su – root) and
do crontab -l as shown below.



root@dev-db#
crontab -l



no
crontab for root



Crontab
HowTo: View Other Linux User’s Crontabs entries



To view crontab entries of other
Linux users, login to root and use -u {username} -l as shown below.



root@dev-db#
crontab -u sathiya -l



@monthly
/home/sathiya/monthly-backup



00
09-18 * * * /home/sathiya/check-db-status



5.
How to Edit Crontab Entries?



Edit
Current Logged-In User’s Crontab entries



To edit a crontab entries, use
crontab -e as shown below. By default this will edit the current logged-in
users crontab.



ramesh@dev-db$
crontab -e



@yearly
/home/ramesh/centos/bin/annual-maintenance



*/10
* * * * /home/ramesh/debian/bin/check-disk-space



~



"/tmp/crontab.XXXXyjWkHw"
2L, 83C



[Note:
This will open the crontab file in Vim editor for editing.



Please
note cron created a temporary /tmp/crontab.XX... ]



When you save the above temporary
file with :wq, it will save the crontab and display the following message
indicating the crontab is successfully modified.



~



"crontab.XXXXyjWkHw"
2L, 83C written



crontab:
installing new crontab



Edit
Root Crontab entries



Login as root user (su – root) and
do crontab -e as shown below.



root@dev-db#
crontab -e



Edit
Other Linux User’s Crontab File entries



To edit crontab entries of other
Linux users, login to root and use -u {username} -e as shown below.



root@dev-db#
crontab -u sathiya -e



@monthly
/home/sathiya/fedora/bin/monthly-backup



00
09-18 * * * /home/sathiya/ubuntu/bin/check-db-status



~



~



~



"/tmp/crontab.XXXXyjWkHw"
2L, 83C



6.
Schedule a Job for Every Minute Using Cron.



Ideally you may not have a
requirement to schedule a job every minute. But understanding this example will
will help you understand the other examples mentioned below in this article.



*
* * * * CMD



The * means all the possible unit —
i.e every minute of every hour through out the year. More than using this *
directly, you will find it very useful in the following cases.


When you specify */5 in minute field means every 5
     minutes.
When you specify 0-10/2 in minute field mean every 2
     minutes in the first 10 minute.
Thus the above convention can be used for all the other
     4 fields.



7.
Schedule a Background Cron Job For Every 10 Minutes.



Use the following, if you want to
check the disk space every 10 minutes.



*/10
* * * * /home/ramesh/check-disk-space



It executes the specified command
check-disk-space every 10 minutes through out the year. But you may have a
requirement of executing the command only during office hours or vice versa.
The above examples shows how to do those things.



Instead of specifying values in the 5 fields, we can specify it using a single
keyword as mentioned below.



There are special cases in which instead of the above 5 fields you can use @
followed by a keyword — such as reboot, midnight, yearly, hourly.





Table: Cron special keywords and its meaning




Keyword



Equivalent




@yearly



0 0 1 1 *




@daily



0 0 * * *




@hourly



0 * * * *




@reboot



Run at startup.






8.
Schedule a Job For First Minute of Every Year using @yearly



If you want a job to be executed on
the first minute of every year, then you can use the @yearly cron
keyword as shown below.



This will execute the system annual maintenance using annual-maintenance shell
script at 00:00 on Jan 1st for every year.



@yearly
/home/ramesh/red-hat/bin/annual-maintenance



9.
Schedule a Cron Job Beginning of Every Month using @monthly



It is as similar as the @yearly as
above. But executes the command monthly once using @monthly cron
keyword.



This will execute the shell script tape-backup at 00:00 on 1st of every month.



@monthly
/home/ramesh/suse/bin/tape-backup



10.
Schedule a Background Job Every Day using @daily



Using the @daily cron
keyword, this will do a daily log file cleanup using cleanup-logs shell
scriptat 00:00 on every day.



@daily
/home/ramesh/arch-linux/bin/cleanup-logs "day started"



11.
How to Execute a Linux Command After Every Reboot using @reboot?



Using the @reboot cron
keyword, this will execute the specified command once after the machine got
booted every time.



@reboot
CMD



12.
How to Disable/Redirect the Crontab Mail Output using MAIL keyword?



By default crontab sends the job
output to the user who scheduled the job. If you want to redirect the output to
a specific user, add or update the MAIL variable in the crontab as shown below.



ramesh@dev-db$
crontab -l



MAIL="ramesh"



@yearly
/home/ramesh/annual-maintenance



*/10
* * * * /home/ramesh/check-disk-space



[Note:
Crontab of the current logged in user with MAIL variable]





If you wanted the mail not to be sent to anywhere, i.e to stop the crontab
output to be emailed, add or update the MAIL variable in the crontab as shown
below.



MAIL=""



13.
How to Execute a Linux Cron Jobs Every Second Using Crontab.



You cannot schedule a every-second
cronjob. Because in cron the minimum unit you can specify is minute. In a
typical scenario, there is no reason for most of us to run any job every second
in the system.



14.
Specify PATH Variable in the Crontab



All the above examples we specified
absolute path of the Linux command or the shell-script that needs to be
executed.



For example, instead of specifying /home/ramesh/tape-backup, if you want to
just specify tape-backup, then add the path /home/ramesh to the PATH variable
in the crontab as shown below.



ramesh@dev-db$
crontab -l



PATH=/bin:/sbin:/usr/bin:/usr/sbin:/home/ramesh



@yearly
annual-maintenance



*/10
* * * * check-disk-space



[Note:
Crontab of the current logged in user with PATH variable]



15.
Installing Crontab From a Cron File



Instead of directly editing the
crontab file, you can also add all the entries to a cron-file first. Once you
have all thoese entries in the file, you can upload or install them to the cron
as shown below.



ramesh@dev-db$
crontab -l



no
crontab for ramesh



$
cat cron-file.txt



@yearly
/home/ramesh/annual-maintenance



*/10
* * * * /home/ramesh/check-disk-space



ramesh@dev-db$
crontab cron-file.txt



ramesh@dev-db$
crontab -l



@yearly
/home/ramesh/annual-maintenance



*/10
* * * * /home/ramesh/check-disk-space



Note: This will install the cron-file.txt to your crontab, which
will also remove your old cron entries. So, please be careful while uploading
cron entries from a cron-file.txt.


service command examples
Service command is used to run the system V init scripts. i.e Instead of
calling the scripts located in the /etc/init.d/ directory with their full path,
you can use the service command.



Check the status of a service:



# service ssh status
Check the steatus of all the services.



service --status-all
Restart a service.




# service ssh restart


 

ps command examples
ps command is used to display information about the processes that are
running in the system.



While there are lot of arguments that could be passed to a ps command,
following are some of the common ones.



To view current running processes.



$ ps -ef | more
To view current running processes in a tree structure. H option stands for
process hierarchy.




$ ps -efH | more


 

 free command
examples
This command is used to display the free, used, swap memory available in the
system.



Typical free command output. The output is displayed in bytes.



$ free
             total       used       free     shared    buffers     cached
Mem:       3566408    1580220    1986188          0     203988     902960
-/+ buffers/cache:     473272    3093136
Swap:      4000176          0    4000176
If you want to quickly check how many GB of RAM your system has use the -g
option. -b option displays in bytes, -k in kilo bytes, -m in mega bytes.



$ free -g
             total       used       free     shared    buffers     cached
Mem:             3          1          1          0          0          0
-/+ buffers/cache:          0          2
Swap:            3          0          3
If you want to see a total memory ( including the swap), use the -t switch,
which will display a total line as shown below.



ramesh@ramesh-laptop:~$ free -t
             total       used       free     shared    buffers     cached
Mem:       3566408    1592148    1974260          0     204260     912556
-/+ buffers/cache:     475332    3091076
Swap:      4000176          0    4000176
Total:     7566584    1592148    5974436


TOP command with examples


1. Show Processes Sorted by any Top Output Column – Press O
By default top command displays the processes in the order of CPU
usage.  When the top command is running, press M (upper-case) to display
processes sorted by memory usage as shown below.







Fig: Press M to sort
by memory usage – Unix top command

To sort top output by any column, Press O (upper-case O) ,
which will display all the possible columns that you can sort by as shown
below.



Current Sort Field:  P  for window 1:Def
Select sort field via field letter, type any other key to return 
 
  a: PID        = Process Id              v: nDRT       = Dirty Pages count
  d: UID        = User Id                 y: WCHAN      = Sleeping in Function
  e: USER       = User Name               z: Flags      = Task Flags
  ........
When the linux top command is running, Press R, which does
the sort in reverse order.




2. Kill a Task Without Exiting From Top – Press k
Once you’ve located a process that needs to be killed, press ‘k’ which will ask for the process id, and signal to send.  If you have the privilege to kill that particular PID, it will get killed successfPID to kill: 1309


Kill
PID 1309 with signal [15]: 



  PID USER     
PR  NI  VIRT 
RES  SHR S %CPU %MEM    TIME+ 
COMMAND



 1309 geek  
23   0 2483m 1.7g  27m S   
0 21.8  45:31.32 gagent



 1882 geek  
25   0 2485m 1.7g  26m S   
0 21.7  22:38.97 gagent



 5136 root   
16   0 38040  14m 9836 S   
0  0.2   0:00.39 nautilus



3.
Renice a Unix Process Without Exiting From Top – Press r



Press r, if you want to just change the priority of the process (and
not kill the process). This will ask PID for renice, enter the PID and
priority.



PID
to renice: 1309



Renice
PID 1309 to value: 



  PID USER     
PR  NI  VIRT 
RES  SHR S %CPU %MEM    TIME+ 
COMMAND



 1309 geek  
23   0 2483m 1.7g  27m S   
0 21.8  45:31.32 gagent



 1882 geek  
25   0 2485m 1.7g  26m S   
0 21.7  22:38.97 gagent



4.
Display Selected User in Top Output Using top -u



Use top -u to display a specific
user processes only in the top command output.



$
top -u geek



While unix top command is running,
press u which will ask for username as shown below.



Which
user (blank for all): geek



  PID USER     
PR  NI  VIRT 
RES  SHR S %CPU %MEM    TIME+ 
COMMAND



 1309 geek  
23   0 2483m 1.7g  27m S   
0 21.8  45:31.32 gagent



 1882 geek  
25   0 2485m 1.7g  26m S   
0 21.7  22:38.97 gagent



Display
Only Specific Process with Given PIDs Using top -p



Use top -p as shown below to display
specific PIDs.



$
top -p 1309, 1882



  PID USER     
PR  NI  VIRT 
RES  SHR S %CPU %MEM    TIME+ 
COMMAND



 1309 geek  
23   0 2483m 1.7g  27m S   
0 21.8  45:31.32 gagent



 1882 geek  
25   0 2485m 1.7g  26m S   
0 21.7  22:38.97 gagent



5.
Display All CPUs / Cores in the Top Output – Press 1 (one)



Top output by default shows CPU line
for all the CPUs combined together as shown below.



top
- 20:10:39 up 40 days, 23:02,  1
user,  load average: 4.97, 2.01, 1.25



Tasks:
310 total,   1 running, 309
sleeping,   0 stopped,   0 zombie



Cpu(s):  0.5%us, 
0.7%sy,  0.0%ni, 92.3%id,  6.4%wa, 
0.0%hi,  0.0%si,  0.0%st



Press 1 (one), when the top command
is running, which will break the CPU down and show details for all the
individual CPUs running on the system as shown below.



top
- 20:10:07 up 40 days, 23:03,  1
user,  load average: 5.32, 2.38, 1.39



Tasks:
341 total,   3 running, 337
sleeping,   0 stopped,   1 zombie



Cpu0  : 
7.7%us,  1.7%sy,  0.0%ni, 79.5%id, 11.1%wa,  0.0%hi, 
0.0%si,  0.0%st



Cpu1  : 
0.3%us,  0.0%sy,  0.0%ni, 94.9%id,  4.7%wa, 
0.0%hi,  0.0%si,  0.0%st



Cpu2
:  3.3%us,  0.7%sy, 
0.0%ni, 55.7%id, 40.3%wa, 
0.0%hi,  0.0%si,  0.0%st



Cpu3
:  5.0%us,  1.0%sy, 
0.0%ni, 86.2%id,  7.4%wa,  0.0%hi, 
0.3%si,  0.0%st



Cpu4  : 38.5%us, 
5.4%sy,  0.3%ni,  0.0%id, 54.8%wa,  0.0%hi, 
1.0%si,  0.0%st



Cpu5  :  0.0%us,  0.0%sy, 
0.0%ni,100.0%id,  0.0%wa,  0.0%hi, 
0.0%si,  0.0%st



Cpu6  : 
0.3%us,  0.7%sy,  0.0%ni, 97.3%id,  1.7%wa, 
0.0%hi,  0.0%si,  0.0%st



Cpu7  : 
5.4%us,  4.4%sy,  0.0%ni, 82.6%id,  7.7%wa, 
0.0%hi,  0.0%si,  0.0%st



Cpu8
:  1.7%us,  1.7%sy, 
0.0%ni, 72.8%id, 23.8%wa, 
0.0%hi,  0.0%si,  0.0%st



6.
Refresh Unix Top Command Output On demand (or) Change Refresh Interval



By default, linux top command
updates the output every 3.0 seconds. When you want to update the output
on-demand, press space bar.



To change the output update
frequency, press d in interactive mode, and enter the time in seconds as shown
below.



Change
delay from 3.0 to: 10



  PID USER     
PR  NI  VIRT 
RES  SHR S %CPU %MEM    TIME+ 
COMMAND



 1309 geek  
23   0 2483m 1.7g  27m S   
0 21.8  45:31.32 gagent



 1882 geek  
25   0 2485m 1.7g  26m S   
0 21.7  22:38.97 gagent



7.
Highlight Running Processes in the Linux Top Command Output – Press z or b



Press z or b, which will highlight
all running process as shown below.





Fig: Ubuntu Linux – top command highlights running process



8.
Display Absolute Path of the Command and its Arguments – Press c



Press c which will show / hide
command absolute path, and arguments as shown below.



  PID USER     
PR  NI  VIRT 
RES  SHR S %CPU %MEM    TIME+ 
COMMAND



 1309 geek  
23   0 2483m 1.7g  27m S   
0 21.8  45:31.32 /usr/sbin/gagent



 1882 geek  
25   0 2485m 1.7g  26m S   
0 21.7  22:38.97 /usr/sbin/gagent
-l 0 -u pre



9.
Quit Top Command After a Specified Number of Iterations Using top -n



Until you press q, top continuously
displays the output. If you would like to view only a certain iteration and
want the top to exit automatically use -n option as shown below.



The following example will show 2
iterations of unix top command output and exit automatically



$
top -n 2



10.
Executing Unix Top Command in Batch Mode



If you want to execute top command
in the batch mode use option -b as shown below.



$
top -b -n 1



.



11.
Split Top Output into Multiple Panels – Press A



To display multiple views of top
command output on the terminal, press A. You can cycle through these windows
using ‘a’. This is very helpful, when you can sort the output on multiple
windows using different top output columns.



12.
Get Top Command Help from Command Line and Interactively



Get a quick command line option help
using top -h as shown below.



$
top -h



        top: procps version 3.2.0



usage:  top -hv | -bcisS -d delay -n iterations [-u
user | -U user] -p pid [,pid ...]



Press h while top command is
running, which will display help for interactive top commands.



Help
for Interactive Commands - procps version 3.2.0



Window
1:Def: Cumulative mode Off.  System:
Delay 3.0 secs; Secure mode Off.



  Z,B      
Global: 'Z' change color mappings; 'B' disable/enable bold



  l,t,m    
Toggle Summaries: 'l' load avg; 't' task/cpu stats; 'm' mem info



  1,I      
Toggle SMP view: '1' single/separate states; 'I' Irix/Solaris mode



  ..........



13.
Decrease Number of Processes Displayed in Top Output – Press n



Press n in the Interactive mode,
which prompts for a number and shows only that. Following example will display
only 2 process as a time.



Maximum
tasks = 0, change to (0 is unlimited): 2



  PID USER     
PR  NI  VIRT 
RES  SHR S %CPU %MEM    TIME+ 
COMMAND



 1309 geek  
23   0 2483m 1.7g  27m S   
0 21.8  45:31.32 gagent



 1882 geek  
25   0 2485m 1.7g  26m S   
0 21.7  22:38.97 gagent



14.
Toggle Top Header to Increase Number of Processes Displayed



By default top displays total number
process based on the window height. If you like to see additional process you
might want to eliminate some of the top header information.



Following is the default header
information provided by top.



top
- 23:47:32 up 179 days,  3:36,  1 user, 
load average: 0.01, 0.03, 0.00



Tasks:  67 total,  
1 running,  66 sleeping,   0 stopped,  
0 zombie



Cpu(s):   0.7% user,  
1.2% system,   0.0% nice,  98.0% idle



Mem:   1017136k total,   954652k used,    62484k free,   138280k buffers



Swap:  3068404k total,    22352k used,  3046052k free,   586576k cached


Press l – to hide / show the load average. 1st header
     line.
Press t – to hide / show the CPU states. 2nd and 3rd
     header line.
Press m – to hide / show the memory information. 4th
     and 5th line.



15.
Save Top Configuration Settings – Press W



If you’ve made any interactive top
command configurations suggested in the above examples, you might want to save
those for all future top command output. Once you’ve saved the top
configuration, next time when you invoke the top command all your saved top configuration
options will be used automatically.



To save the top configuration, press
W, which will write the configuration files to ~/.toprc. This will display the
write confirmation message as shown below.



top
- 23:47:32 up 179 days,  3:36,  1 user, 
load average: 0.01, 0.03, 0.00



Tasks:  67 total,  
1 running,  66 sleeping,   0 stopped,  
0 zombie



Cpu(s):   0.7% user,  
1.2% system,   0.0% nice,  98.0% idle



Mem:   1017136k total,   954652k used,    62484k free,   138280k buffers



Swap:  3068404k total,    22352k used,  3046052k free,   586576k cached



Wrote
configuration to '/home/ramesh/.toprc'


 


 


 

 df command
examples
Displays the file system disk space usage. By default df -k displays output
in bytes.



$ df -k
Filesystem           1K-blocks      Used Available Use% Mounted on
/dev/sda1             29530400   3233104  24797232  12% /
/dev/sda2            120367992  50171596  64082060  44% /home
df -h displays output in human readable form. i.e size will be displayed in
GB’s.



ramesh@ramesh-laptop:~$ df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              29G  3.1G   24G  12% /
/dev/sda2             115G   48G   62G  44% /home
Use -T option to display what type of file system.



ramesh@ramesh-laptop:~$ df -T
Filesystem    Type   1K-blocks      Used Available Use% Mounted on
/dev/sda1     ext4    29530400   3233120  24797216  12% /
/dev/sda2     ext4   120367992  50171596  64082060  44% /home


Kill command with examples

Kill command is use to send signal to a process or to kill a process. We
typically use kill -SIGNAL PID, where you know the PID of the process. 



There are other ways to effectively kill a process —
killing a process by name, killing a process by specifying part of the name,
killing a process by pointing out the process with cursor etc.,



In this article, let us review 4 ways to kill a process.




1. Kill Command – Kill the process by specifying its PID
All the below kill conventions will send the TERM signal to the specified
process. For the signals, either the signal name or signal number can be used.
You need to lookup the pid for the process and give it as an argument to kill.



$ kill -TERM pid
 
$ kill -SIGTERM pid
 
$ kill -15 pid


Example: Kill the firefox process.

$ ps -ef | grep firefox
1986 ?        Sl     7:22 /usr/lib/firefox-3.5.3/firefox
 
$ kill -9 1986

2. Killall Command – Kill processes by name
Instead of specifying a process by its PID, you can specify the name of the
process. If more than one process runs with that name, all of them will be
killed.

Example: Kill all the firefox processes



$ killall -9 firefox

3. Pkill Command – Send signal to the process based on its name
You can send signal to any process by specifying the full name or partial
name. So there is no need for you to find out the PID of the process to send
the signal.



Example: Send SIGTERM to all the process which has sample in its name.



$ pkill sample

Pkill Example:
Before sending signal, you can verify which are all the process is matching
the criteria using “pgrep -l”, which displays the process ID and process name
of the matching processes.



In this example, all the processes are designed to log the signal to
signal-log, along with its PID.



$ pgrep -l sample
12406 sample-server.p
12425 sample-server.p
12430 sample-garbagec
 
$ pkill -USR1 sample
 
$ cat signal-log
Name: ./sample-server.pl Pid: 12406 Signal Received: USR1
Name: ./sample-server.pl Pid: 12425 Signal Received: USR1
Name: ./sample-garbagecollector.pl Pid: 12430 Signal Received: USR1
Note: The part of name which you specify should be in the
character within the first 15 character of the process name.




4. Xkill Command – kill a client by X resource
xkill is the simplest way to kill a malfunctioning program. When you want to
kill a process, initiate xkill which will offer an cross-hair cursor. Click on
the window with left cursor which will kill that process.



$ xkill
Select the window whose client you wish to kill with button 1....
xkill:  killing creator of resource 0x1200003
Note: Actually, xkill instructs XServer to terminate the
client.





rm command examples
Get confirmation before removing the file.



$ rm -i filename.txt
It is very useful while giving shell metacharacters in the file name
argument.



Print the filename and get confirmation before removing the file.



$ rm -i file*
Following example recursively removes all files and directories under the
example directory. This also removes the example directory itself.




$ rm -r example


 

cp command examples
Copy file1 to file2 preserving the mode, ownership and timestamp.



$ cp -p file1 file2
Copy file1 to file2. if file2 exists prompt for confirmation before
overwritting it.




$ cp -i file1 file2


 

 mv command
examples
Rename file1 to file2. if file2 exists prompt for confirmation before
overwritting it.



$ mv -i file1 file2
Note: mv -f is just the opposite, which will overwrite file2 without
prompting.



mv -v will print what is happening during file rename, which is useful while
specifying shell metacharacters in the file name argument.




$ mv -v file1 file2


 

cat command examples
You can view multiple files at the same time. Following example prints the
content of file1 followed by file2 to stdout.



$ cat file1 file2
While displaying the file, following cat -n command will prepend the line number
to each line of the output.



$ cat -n /etc/logrotate.conf
    1   /var/log/btmp {
    2       missingok
    3       monthly
    4       create 0660 root utmp
    5       rotate 1

    6   }

 

mount command examples
To mount a file system, you should first create a directory and mount it as
shown below.



# mkdir /u01
 
# mount /dev/sdb1 /u01
You can also add this to the fstab for automatic mounting. i.e Anytime
system is restarted, the filesystem will be mounted.



/dev/sdb1 /u01 ext2 defaults 0 2


Following are the symbolic
representation of three different roles:


u is for user,
g is for group,
and o is for others.



Following are the symbolic
representation of three different permissions:


r is for read permission,
w is for write permission,
x is for execute permission.



Following are few examples on how to
use the symbolic representation on chmod.



1.
Add single permission to a file/directory



Changing permission to a single set.
+ symbol means adding permission. For example, do the following to give execute
permission for the user irrespective of anything else:



$
chmod u+x filename



2.
Add multiple permission to a file/directory



Use comma to separate the multiple
permission sets as shown below.



$
chmod u+r,g+x filename



3.
Remove permission from a file/directory



Following example removes read and
write permission for the user.



$
chmod u-rx filename



4.
Change permission for all roles on a file/directory



Following example assigns execute
privilege to user, group and others (basically anybody can execute this file).

$ chmod a+x filename

5. Make permission for a file same as another file (using reference)
If you want to change a file permission same as another file, use the
reference option as shown below. In this example, file2′s permission will be
set exactly same as file1′s permission.



$ chmod --reference=file1 file2

6. Apply the permission to all the files under a directory recursively
Use option -R to change the permission recursively as shown below.



$ chmod -R 755 directory-name/

7. Change execute permission only on the directories (files are not
affected)
On a particular directory if you have multiple sub-directories and files,
the following command will assign execute permission only to all the
sub-directories in the current directory (not the files in the current
directory).



$ chmod u+X *



Note: If the files has
execute permission already for either the group or others, the above command
will assign the execute permission to the user



chown command examples
chown command is used to change the owner and group of a file. \



To change owner to oracle and group to db on a file. i.e Change both owner
and group at the same time.



$ chown oracle:dba dbora.sh
Use -R to change the ownership recursively.



$ chown -R oracle:dba /home/oracle

 

 

passwd command examples
Change your password from command line using passwd. This will prompt for
the old password followed by the new password.



$ passwd
Super user can use passwd command to reset others password. This will not
prompt for current password of the user.



# passwd USERNAME
Remove password for a specific user. Root user can disable password for a
specific user. Once the password is disabled, the user can login without
entering the password.



# passwd -d USERNAME

 

 

mkdir command examples
Following example creates a directory called temp under your home directory.



$ mkdir ~/temp
Create nested directories using one mkdir command. If any of these
directories exist already, it will not display any error. If any of these
directories doesn’t exist, it will create them.



$ mkdir -p dir1/dir2/dir3/dir4/
 

 

 
 
Ifconfig command with examples
Ifconfig command is used to configure network interfaces. ifconfig stands
for interface configurator. Ifconfig is widely used to initialize the network
interface and to enable or disable the interfaces.



In this article, let us review 7 common usages of ifconfig command.










1. View Network Settings of an Ethernet Adapter
Ifconfig, when invoked with no arguments will display all the details of
currently active interfaces. If you give the interface name as an argument, the
details of that specific interface will be displayed.



# ifconfig eth0
 
eth0   Link encap:Ethernet  HWaddr 00:2D:32:3E:39:3B
inet addr:192.168.2.2  Bcast:192.168.2.255  Mask:255.255.255.0
inet6 addr: fe80::21d:92ff:fede:499b/64 Scope:Link
UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
RX packets:977839669 errors:0 dropped:1990 overruns:0 frame:0
TX packets:1116825094 errors:8 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:2694625909 (2.5 GiB)  TX bytes:4106931617 (3.8 GiB)
Interrupt:185 Base address:0xdc00

2. Display Details of All interfaces Including Disabled Interfaces
# ifconfig -a

3. Disable an Interface
# ifconfig eth0 down

4. Enable an Interface
# ifconfig eth0 up

5. Assign ip-address to an Interface
Assign 192.168.2.2 as the IP address for the interface eth0.



# ifconfig eth0 192.168.2.2
Change Subnet mask of the interface eth0.



# ifconfig eth0 netmask 255.255.255.0
Change Broadcast address of the interface eth0.



# ifconfig eth0 broadcast 192.168.2.255
Assign ip-address, netmask and broadcast at the same time to interface eht0.



# ifconfig eth0 192.168.2.2 netmask 255.255.255.0 broadcast 192.168.2.255

6. Change MTU
This will change the Maximum transmission unit (MTU) to XX. MTU is the
maximum number of octets the interface is able to handle in one transaction.
For Ethernet the Maximum transmission unit by default is 1500.



# ifconfig eth0 mtu XX

7. Promiscuous mode
By default when a network card receives a packet, it checks whether the
packet belongs to itself. If not, the interface card normally drops the packet.
But in promiscuous mode, the card doesn’t drop the packet. Instead, it will
accept all the packets which flows through the network card.





Superuser privilege is required to set an interface in promiscuous mode. Most
network monitor tools use the promiscuous mode to capture the packets and to
analyze the network traffic.





Following will put the interface in promiscuous mode.



# ifconfig eth0 promisc
Following will put the interface in normal mode.



# ifconfig eth0 -promisc


uname command examples



Uname command displays important
information about the system such as — Kernel name, Host name, Kernel release
number,

Processor type, etc.,



Sample uname output from a Ubuntu
laptop is shown below.



$
uname -a




Linux john-laptop
2.6.32-24-generic #41-Ubuntu SMP Thu Aug 19 01:12:52 UTC 2010 i686 GNU/Linux




whereis command examples



When you want to find out where a
specific Unix command exists (for example, where does ls command exists?), you
can execute the following command.



$
whereis ls



ls:
/bin/ls /usr/share/man/man1/ls.1.gz /usr/share/man/man1p/ls.1p.gz



When you want to search an
executable from a path other than the whereis default path, you can use -B
option and give path as argument to it. This searches for the executable lsmk
in the /tmp directory, and displays it, if it is available.



$
whereis -u -B /tmp -f lsmk




lsmk: /tmp/lsmk




whatis command examples



Whatis command displays a single
line description about a command.



$
whatis ls



ls             (1) 
- list directory contents



$
whatis ifconfig




ifconfig (8)         - configure a network interface




locate command examples



Using locate command you can quickly
search for the location of a specific file (or group of files). Locate command
uses the database created by updatedb.



The example below shows all files in
the system that contains the word crontab in it.



$
locate crontab



/etc/anacrontab



/etc/crontab



/usr/bin/crontab



/usr/share/doc/cron/examples/crontab2english.pl.gz



/usr/share/man/man1/crontab.1.gz



/usr/share/man/man5/anacrontab.5.gz



/usr/share/man/man5/crontab.5.gz




/usr/share/vim/vim72/syntax/crontab.vim




man command examples



Display the man page of a specific
command.



$
man crontab



When a man page for a command is
located under more than one section, you can view the man page for that command
from a specific section as shown below.



$
man SECTION-NUMBER commandname



Following 8 sections are available
in the man page.


General commands
System calls
C library functions
Special files (usually devices, those found in /dev)
     and drivers
File formats and conventions
Games and screensavers
Miscellaneous
System administration commands and daemons



For example, when you do whatis
crontab, you’ll notice that crontab has two man pages (section 1 and section
5). To view section 5 of crontab man page, do the following.



$
whatis crontab



crontab
(1)          - maintain crontab files for
individual users (V3)



crontab
(5)          - tables for driving cron




$ man 5 crontab




tail command examples



Print the last 10 lines of a file by
default.



$
tail filename.txt



Print N number of lines from the
file named filename.txt



$
tail -n N filename.txt



View the content of the file in real
time using tail -f. This is useful to view the log files, that keeps growing.
The command can be terminated using CTRL-C.


$ tail -f log-file

 


1.
Less Command – Search Navigation



Once you’ve opened a log file (or
any file) using less file-name, use the following keys to search.
Please note that the match will be highlighted automatically by default.



Forward
Search


/ – search for a pattern which will take you to the
     next occurrence.
n – for next match in forward
N – for previous match in backward



Backward
Search


? – search for a pattern which will take you to the
     previous occurrence.
n – for next match in backward direction
N – for previous match in forward direction



Tip: If you dont bother about which direction the search is
happening, and you want to search file path, or URL, such as “/home/ramesh/”,
you can use backward search (?pattern) which will be handy as you don’t want to
escape slashes each time.



Search
Path



In
forward: /\/home\/ramesh\/



In
backward: ?/home/ramesh/



2.
Less Command – Screen Navigation



Use the following screen navigation
commands while viewing large log files.


CTRL+F – forward one window
CTRL+B – backward one window
CTRL+D – forward half window
CTRL+U – backward half window



3.
Less Command – Line navigation



In a smaller chunk of data, where
you want to locate particular error, you may want to navigate line by line
using these keys:


j – navigate forward by one line
k – navigate backward by one line



4.
Less Command – Other Navigations



The following are other navigation
operations that you can use inside the less pager

 

G – go to the end of file
g – go to the start of file
q or ZZ – exit the less pager



5.
Simulate tail -f inside less pager – Press F



Once you’ve opened a file using less
command, any content that is appended to the file after that will not be
displayed automatically. However, you can press F less command will show
the status ‘waiting for data‘. This is as similar to ‘tail -f



6.
Less Command – Count magic



Similar to Vim editor navigation
command, you can give 10j to scroll 10 lines down, or 10k to go up by 10 lines.


10j – 10 lines forward.
10k – 10 lines backward.
CTRL+G – show the current file name along with line,
     byte and percentage statistics.



7.
Other useful Less Command Operations


v – using the configured editor edit the current file.
h – summary of less commands
&pattern – display only the matching lines, not
     all.



9.
Less Command – Marked navigation



When you are viewing a large log
file using less command, you can mark a particular position and return back to
that place again by using that mark.


ma – mark the current position with the letter ‘a’,
‘a – go to the marked position ‘a’.



10.
Less Command – Multiple file paging



Method 1: You can open multiple files by passing the file names as
arguments.



$
less file1 file2



Method 2: While you are viewing file1, use :e to open the file2 as
shown below.



$
less file1



:e
file2



Navigation across files: When you opened more than two files ( for e.g – less * ),
use the following keys to navigate between files.


:n – go to the next file.
:p – go to the previous file.


su command examples
Switch to a different user account using su command. Super user can switch
to any other user without entering their password.



$ su - USERNAME
Execute a single command from a different account name. In the following
example, john can execute the ls command as raj username. Once the command is
executed, it will come back to john’s account.



[john@dev-server]$ su - raj -c 'ls'
 
[john@dev-server]$
Login to a specified user account, and execute the specified shell instead
of the default shell.



$ su -s 'SHELLNAME' USERNAME

 

 

mysql command examples
mysql is probably the most widely used open source database on Linux. Even
if you don’t run a mysql database on your server, you might end-up using the
mysql command ( client ) to connect to a mysql database running on the remote
server.



To connect to a remote mysql database. This will prompt for a password.



$ mysql -u root -p -h 192.168.1.2
To connect to a local mysql database.



$ mysql -u root -p



If you want to specify the mysql root
password in the command line itself, enter it immediately after -p (without any
space).



46. yum command examples
To install apache using yum.



$ yum install httpd
To upgrade apache using yum.



$ yum update httpd
To uninstall/remove apache using yum.



$ yum remove httpd
 

 

 
 
Rpm command with examples


RPM command is used for installing, uninstalling,
upgrading, querying, listing, and checking RPM packages on your Linux system.



RPM stands for Red Hat Package
Manager.



With root privilege, you can use the
rpm command with appropriate options to manage the RPM software packages.



In this article, let us review 15 practical examples of rpm command.



Let us take an rpm of Mysql Client
and run through all our examples.



1.
Installing a RPM package Using rpm -ivh



RPM filename has packagename,
version, release and architecture name.



For example, In the
MySQL-client-3.23.57-1.i386.rpm file:


MySQL-client – Package Name
3.23.57 – Version
1 – Release
i386 – Architecture



When you install a RPM, it checks
whether your system is suitable for the software the RPM package contains,
figures out where to install the files located inside the rpm package, installs
them on your system, and adds that piece of software into its database of
installed RPM packages.



The following rpm command installs
Mysql client package.



#
rpm -ivh  MySQL-client-3.23.57-1.i386.rpm



Preparing...               
########################################### [100%]



   1:MySQL-client          
########################################### [100%]



rpm command and options


-i : install a package
-v : verbose
-h : print hash marks as the package archive is
     unpacked.



2.
Query all the RPM Packages using rpm -qa



You can use rpm command to query all
the packages installed in your system.



#
rpm -qa



cdrecord-2.01-10.7.el5



bluez-libs-3.7-1.1



setarch-2.0-1.1



.



.


-q query operation
-a queries all installed packages



To identify whether a particular rpm
package is installed on your system, combine rpm and grep command as shown
below. Following command checks whether cdrecord package is installed on your
system.



#
rpm -qa | grep 'cdrecord'



3.
Query a Particular RPM Package using rpm -q



The above example lists all
currently installed package. After installation of a package to check the
installation, you can query a particular package and verify as shown below.



#
rpm -q MySQL-client



MySQL-client-3.23.57-1



#
rpm -q MySQL



package
MySQL is not installed



Note: To query a package, you should
specify the exact package name. If the package name is incorrect, then rpm
command will report that the package is not installed.



4.
Query RPM Packages in a various format using rpm –queryformat



Rpm command provides an option
–queryformat, which allows you to give the header tag names, to list the
packages. Enclose the header tag with in {}.



#
rpm -qa --queryformat '%{name-%{version}-%{release} %{size}\n'



cdrecord-2.01-10.7
12324



bluez-libs-3.7-1.1
5634



setarch-2.0-1.1
235563



.



.



#



5.
Which RPM package does a file belong to? – Use rpm -qf



Let us say, you have list of files
and you would want to know which package owns all these files. rpm command has
options to achieve this.



The following example shows that
/usr/bin/mysqlaccess file is part of the MySQL-client-3.23.57-1 rpm.



#
rpm -qf /usr/bin/mysqlaccess



MySQL-client-3.23.57-1


-f : file name



6.
Locate documentation of a package that owns file using rpm -qdf



Use the following to know the list
of documentations, for a package that owns a file. The following command, gives
the location of all the manual pages related to mysql package.



#
rpm -qdf /usr/bin/mysqlaccess



/usr/share/man/man1/mysql.1.gz



/usr/share/man/man1/mysqlaccess.1.gz



/usr/share/man/man1/mysqladmin.1.gz



/usr/share/man/man1/mysqldump.1.gz



/usr/share/man/man1/mysqlshow.1.gz


-d : refers documentation.



7.
Information about Installed RPM Package using rpm -qi



rpm command provides a lot of
information about an installed pacakge using rpm -qi as shown below:



#
rpm -qi MySQL-client



Name        : MySQL-client                 Relocations: (not relocatable)



Version     : 3.23.57                           Vendor: MySQL AB



Release     : 1                             Build Date: Mon 09
Jun 2003 11:08:28 PM CEST



Install
Date: Mon 06 Feb 2010 03:19:16 AM PST               Build Host: build.mysql.com



Group       : Applications/Databases        Source RPM: MySQL-3.23.57-1.src.rpm



Size        : 5305109                          License: GPL / LGPL



Signature   : (none)



Packager    : Lenz Grimmer



URL         : http://www.mysql.com/



Summary     : MySQL - Client



Description
: This package contains the standard MySQL clients.



If you have an RPM file that you
would like to install, but want to know more information about it before
installing, you can do the following:



#
rpm -qip MySQL-client-3.23.57-1.i386.rpm



Name        : MySQL-client                 Relocations: (not relocatable)



Version     : 3.23.57                           Vendor: MySQL AB



Release     : 1                             Build Date: Mon 09
Jun 2003 11:08:28 PM CEST



Install
Date: (not installed)               Build
Host: build.mysql.com



Group       : Applications/Databases        Source RPM: MySQL-3.23.57-1.src.rpm



Size        : 5305109                          License: GPL / LGPL



Signature   : (none)



Packager    : Lenz Grimmer



URL         : http://www.mysql.com/



Summary     : MySQL - Client



Description
: This package contains the standard MySQL clients.


-i : view information about an rpm
-p : specify a package name



8.
List all the Files in a Package using rpm -qlp



To list the content of a RPM
package, use the following command, which will list out the files without
extracting into the local directory folder.



$
rpm -qlp ovpc-2.1.10.rpm



/usr/bin/mysqlaccess



/usr/bin/mysqldata



/usr/bin/mysqlperm



.



.



/usr/bin/mysqladmin


q : query the rpm file
l : list the files in the package
p : specify the package name



9.
List the Dependency Packages using rpm -qRP



To view the list of packages on which
this package depends,



#
rpm -qRp MySQL-client-3.23.57-1.i386.rpm



/bin/sh



/usr/bin/perl



10.
Find out the state of files in a package using rpm -qsp



The following command is to find
state (installed, replaced or normal) for all the files in a RPM package.



#
rpm -qsp MySQL-client-3.23.57-1.i386.rpm



normal        /usr/bin/msql2mysql



normal        /usr/bin/mysql



normal        /usr/bin/mysql_find_rows



normal        /usr/bin/mysqlaccess



normal        /usr/bin/mysqladmin



normal        /usr/bin/mysqlbinlog



normal        /usr/bin/mysqlcheck



normal        /usr/bin/mysqldump



normal        /usr/bin/mysqlimport



normal        /usr/bin/mysqlshow



normal        /usr/share/man/man1/mysql.1.gz



normal        /usr/share/man/man1/mysqlaccess.1.gz



normal        /usr/share/man/man1/mysqladmin.1.gz



normal        /usr/share/man/man1/mysqldump.1.gz



normal        /usr/share/man/man1/mysqlshow.1.gz



11.
Verify a Particular RPM Package using rpm -Vp



Verifying a package compares
information about the installed files in the package with information about the
files taken from the package metadata stored in the rpm database. In the
following command, -V is for verification and -p option is used to specify a
package name to verify.



#
rpm -Vp MySQL-client-3.23.57-1.i386.rpm



S.5....T
c        /usr/bin/msql2mysql



S.5....T
c        /usr/bin/mysql



S.5....T
c        /usr/bin/mysql_find_rows



S.5....T
c        /usr/bin/mysqlaccess



The character in the above output
denotes the following:


S file Size differs
M Mode differs (includes permissions and file type)
5 MD5 sum differs
D Device major/minor number mismatch
L readlink(2) path mismatch
U User ownership differs
G Group ownership differs
T mTime differs



12.
Verify a Package Owning file using rpm -Vf



The following command verify the package
which owns the given filename.



#
rpm -Vf /usr/bin/mysqlaccess



S.5....T
c /usr/bin/mysql



#



13.
Upgrading a RPM Package using rpm -Uvh



Upgrading a package is similar to
installing one, but RPM automatically un-installs existing versions of the
package before installing the new one. If an old version of the package is not
found, the upgrade option will still install it.



#
rpm -Uvh MySQL-client-3.23.57-1.i386.rpm



Preparing...
                  ###########################################
[100%]



1:MySQL-client          ###########################################



14.
Uninstalling a RPM Package using rpm -e



To remove an installed rpm package
using -e as shown below. After uninstallation, you can query using rpm -qa and
verify the uninstallation.



#
rpm -ev MySQL-client



15.
Verifying all the RPM Packages using rpm -Va



The following command verifies all
the installed packages.



#
rpm -Va



S.5....T
c /etc/issue



S.5....T
c /etc/issue.net



S.5....T
c /var/service/imap/ssl/seed



S.5....T
c /home/httpd/html/horde/ingo/config/backends.php



.



.



S.5....T
c /home/httpd/html/horde/ingo/config/prefs.php




S.5....T c
/etc/printcap




Ping command with examples

As you already know, ping command is used to find out whether the peer
host/gateway is reachable. 



If you are thinking ping is such a simple command and why do I need 15
examples, you should read the rest of the article. 



Ping command provides lot more options than what you might already know.




Ping Example 1. Increase or Decrease the Time Interval Between Packets
By default ping waits for 1 second before sending the next packet. You can
increase or decrease this using option -i as shown below.




Increase Ping Time Interval
Example: Wait for 5 seconds before sending the next packet.



$ ping -i 5 IP

Decrease Ping Time Interval
Example: Wait 0.1 seconds before sending the next packet.





date command examples
Set the system date:



# date -s "01/31/2010 23:59:53"
Once you’ve changed the system date, you should syncronize the hardware
clock with the system date as shown below.



# hwclock –systohc
 
# hwclock --systohc –utc
 

 

 

wget command examples
The quick and effective method to download software, music, video from internet
is using wget command.



$ wget http://prdownloads.sourceforge.net/sourceforge/nagios/nagios-3.2.1.tar.gz
Download and store it with a different name.



$ wget -O taglist.zip http://www.vim.org/scripts/download_script.php?src_id=7701