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50 Most Frequently Used UNIX / Linux Commands (With Examples)

http://www.thegeekstuff.com/2010/11/50-linux-commands/

This article provides practical examples for 50 most frequently used commands in Linux / UNIX.

1.      tar command examples

     Create a new tar archive.

     > tar cvf archive_name.tar dirname/

     Extract from an existing tar archive.

     > tar xvf archive_name.tar

     View an existing tar archive.

     > tar tvf archive_name.tar

2. grep command examples

     Search for a given string in a file (case in-sensitive search).

     > grep -i "the" demo_file

     Print the matched line, along with the 3 lines after it.

     > grep -A 3 -i "example" demo_text

     Search for a given string in all files recursively

     > grep -r "ramesh" *

3. find command examples

     Find files using file-name ( case in-sensitve find)

     # find -iname "MyCProgram.c"

     Execute commands on files found by the find command

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

     Find all empty files in home directory

     # find ~ -empty

4. ssh command examples

     Login to remote host

     ssh -l jsmith remotehost.example.com

     Debug ssh client

     ssh -v -l jsmith remotehost.example.com

     Display ssh client version

     > ssh -V

     OpenSSH_3.9p1, OpenSSL 0.9.7a Feb 19 2003

5. sed command examples

     When you copy a DOS file to Unix, you could find \r\n in the end of each line. This example converts the DOS file format to Unix file format using sed command.

     > sed 's/.$//' filename

     Print file content in reverse order

     > sed -n '1!G;h;$p' thegeekstuff.txt

     Add line number for all non-empty-lines in a file

     > sed '/./=' thegeekstuff.txt | sed 'N; s/\n/ /'

6. awk command examples

     Remove duplicate lines using awk

     > awk '!($0 in array) { array[$0]; print }' temp

     Print all lines from /etc/passwd that has the same uid and gid

     > awk -F ':' '$3==$4' passwd.txt

     Print only specific field from a file.

     > awk '{print $2,$5;}' employee.txt

7. vim command examples

    Go to the 143rd line of file

    $ vim +143 filename.txt

    Go to the first match of the specified

    $ vim +/search-term filename.txt

    Open the file in read only mode.

    $ vim -R /etc/passwd

8. diff command examples

    Ignore white space while comparing.

    # diff -w name_list.txt name_list_new.txt

    2c2,3

    < John Doe --- > John M Doe

    > Jason Bourne

9. 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

10. 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

11. 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

12. ls command examples

    Display filesize in human readable format (e.g. KB, MB etc.,)

    $ ls -lh

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

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

    $ ls -ltr

    Visual Classification of Files With Special Characters Using ls -F

    $ ls -F

13. 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.

14. cd command examples

    Use "cd -" to toggle between the last two directories

15. 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

16. bzip2 command examples

    To create a *.bz2 compressed file:

    $ bzip2 test.txt

    To uncompress a *.bz2 file:

    bzip2 -d test.txt.bz2

17. 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

18. 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

19. ftp command examples

    Both ftp and secure ftp (sftp) has similar commands. To connect to a remote server and download multiple files, do the following.

    $ ftp IP/hostname

    ftp> mget *.html

    To view the file names located on the remote server before downloading, mls ftp command as shown below.

    ftp> mls *.html -

    /ftptest/features.html

    /ftptest/index.html

    /ftptest/othertools.html

    /ftptest/samplereport.html

    /ftptest/usage.html

20. crontab command examples

    View crontab entry for a specific user

    # crontab -u john -l

    Schedule a cron job every 10 minutes.

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

21. 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 status of all the services.

    service --status-all

    Restart a service.

    # service ssh restart

22. 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

23. 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

24. top command examples

    top command displays the top processes in the system ( by default sorted by cpu usage ). 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

      ........

    To displays only the processes that belong to a particular user use -u option. The following will show only the top processes that belongs to oracle user.

    $ top -u oracle

25. 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

26. kill command examples

    Use kill command to terminate a process. First get the process id using ps -ef command, then use kill -9 to kill the running Linux process as shown below. You can also use killall, pkill, xkill to terminate a unix process.

    $ ps -ef | grep vim

    ramesh    7243  7222  9 22:43 pts/2    00:00:00 vim

    $ kill -9 7243

27. 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

28. 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

29. 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

30. 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       }

31. 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

32. chmod command examples

    chmod command is used to change the permissions for a file or directory.

    Give full access to user and group (i.e read, write and execute ) on a specific file.

    $ chmod ug+rwx file.txt

    Revoke all access for the group (i.e read, write and execute ) on a specific file.

    $ chmod g-rwx file.txt

    Apply the file permissions recursively to all the files in the sub-directories.

    $ chmod -R ug+rwx file.txt

33. 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

34. 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

35. 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/

36. ifconfig command examples

   Use ifconfig command to view or configure a network interface on the Linux system.

   View all the interfaces along with status.

   $ ifconfig -a

   Start or stop a specific interface using up and down command as shown below.

   $ ifconfig eth0 up

   $ ifconfig eth0 down

37. 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

38. 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

39. 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

40. 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

41. 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.

   1.   General commands

   2.   System calls

   3.   C library functions

   4.   Special files (usually devices, those found in /dev) and drivers

   5.   File formats and conventions

   6.   Games and screensavers

   7.   Miscellaneous

   8.   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

42. 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

   More tail examples: 3 Methods To View tail -f output of Multiple Log Files in One Terminal

43. less command examples

   less is very efficient while viewing huge log files, as it doesn't need to load the full file while opening.

   $ less huge-log-file.log

   One you open a file using less command, following two keys are very helpful.

   CTRL+F - forward one window

   CTRL+B - backward one window

   More less examples: Unix Less Command: 10 Tips for Effective Navigation

44. 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

45. 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

47. rpm command examples

   To install apache using rpm.

   # rpm -ivh httpd-2.2.3-22.0.1.el5.i386.rpm

   To upgrade apache using rpm.

   # rpm -uvh httpd-2.2.3-22.0.1.el5.i386.rpm

   To uninstall/remove apache using rpm.

   # rpm -ev httpd

48. ping command examples

   Ping a remote host by sending only 5 packets.

   $ ping -c 5 gmail.com

49. 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

50. 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