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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