50 Most Frequently Used UNIX
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This article provides practical examples for 50 most frequently used commands in Linux / UNIX.
This is not a comprehensive list by any means, but this should give you a jumpstart on some of the common Linux commands. Bookmark this article for your future reference.
Create a new tar archive.
Extract from an existing tar archive.
View an existing tar archive.
More tar examples: The Ultimate Tar Command Tutorial with 10 Practical Examples
Search for a given string in a file (case in-sensitive search).
Print the matched line, along with the 3 lines after it.
Search for a given string in all files recursively
More grep examples: Get a Grip on the Grep! – 15 Practical Grep Command Examples
Find files using file-name ( case in-sensitve find)
Execute commands on files found by the find command
Find all empty files in home directory
More find examples: Mommy, I found it! — 15 Practical Linux Find Command Examples
Login to remote host
Debug ssh client
Display ssh client version
More ssh examples: 5 Basic Linux SSH Client Commands
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.
Print file content in reverse order
Add line number for all non-empty-lines in a file
More sed examples: Advanced Sed Substitution Examples
Remove duplicate lines using awk
Print all lines from /etc/passwd that has the same uid and gid
Print only specific field from a file.
More awk examples: 8 Powerful Awk Built-in Variables – FS, OFS, RS, ORS, NR, NF, FILENAME, FNR
Go to the 143rd line of file
Go to the first match of the specified
Open the file in read only mode.
More vim examples: How To Record and Play in Vim Editor
Ignore white space while comparing.
More diff examples: Top 4 File Difference Tools on UNIX / Linux – Diff, Colordiff, Wdiff, Vimdiff
Sort a file in ascending order
Sort a file in descending order
Sort passwd file by 3rd field.
To view oracle related environment variables.
To export an environment variable:
Copy all images to external hard-drive
Search all jpg images in the system and archive it.
Download all the URLs mentioned in the url-list.txt file
Display filesize in human readable format (e.g. KB, MB etc.,)
Order Files Based on Last Modified Time (In Reverse Order) Using ls -ltr
Visual Classification of Files With Special Characters Using ls -F
More ls examples: Unix LS Command: 15 Practical Examples
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 “cd -” to toggle between the last two directories
Use “shopt -s cdspell” to automatically correct mistyped directory names on cd
More cd examples: 6 Awesome Linux cd command Hacks
To create a *.gz compressed file:
To uncompress a *.gz file:
Display compression ratio of the compressed file using gzip -l
To create a *.bz2 compressed file:
To uncompress a *.bz2 file:
More bzip2 examples: BZ is Eazy! bzip2, bzgrep, bzcmp, bzdiff, bzcat, bzless, bzmore examples
To extract a *.zip compressed file:
View the contents of *.zip file (Without unzipping it):
Shutdown the system and turn the power off immediately.
Shutdown the system after 10 minutes.
Reboot the system using shutdown command.
Force the filesystem check during reboot.
Both ftp and secure ftp (sftp) has similar commands. To connect to a remote server and download multiple files, do the following.
To view the file names located on the remote server before downloading, mls ftp command as shown below.
More ftp examples: FTP and SFTP Beginners Guide with 10 Examples
View crontab entry for a specific user
Schedule a cron job every 10 minutes.
More crontab examples: Linux Crontab: 15 Awesome Cron Job 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:
Check the status of all the services.
Restart a service.
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.
To view current running processes in a tree structure. H option stands for process hierarchy.
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.
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.
If you want to see a total memory ( including the swap), use the -t switch, which will display a total line as shown below.
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.
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.
More top examples: Can You Top This? 15 Practical Linux Top Command Examples
Displays the file system disk space usage. By default df -k displays output in bytes.
df -h displays output in human readable form. i.e size will be displayed in GB’s.
Use -T option to display what type of file system.
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.
More kill examples: 4 Ways to Kill a Process – kill, killall, pkill, xkill
Get confirmation before removing the file.
It is very useful while giving shell metacharacters in the file name argument.
Print the filename and get confirmation before removing the file.
Following example recursively removes all files and directories under the example directory. This also removes the example directory itself.
Copy file1 to file2 preserving the mode, ownership and timestamp.
Copy file1 to file2. if file2 exists prompt for confirmation before overwritting it.
Rename file1 to file2. if file2 exists prompt for confirmation before overwritting it.
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.
You can view multiple files at the same time. Following example prints the content of file1 followed by file2 to stdout.
While displaying the file, following cat -n command will prepend the line number to each line of the output.
To mount a file system, you should first create a directory and mount it as shown below.
You can also add this to the fstab for automatic mounting. i.e Anytime system is restarted, the filesystem will be mounted.
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.
Revoke all access for the group (i.e read, write and execute ) on a specific file.
Apply the file permissions recursively to all the files in the sub-directories.
More chmod examples: 7 Chmod Command Examples for Beginners
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.
Use -R to change the ownership recursively.
Change your password from command line using passwd. This will prompt for the old password followed by the new password.
Super user can use passwd command to reset others password. This will not prompt for current password of the user.
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.
Following example creates a directory called temp under your home directory.
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.
Use ifconfig command to view or configure a network interface on the Linux system.
View all the interfaces along with status.
Start or stop a specific interface using up and down command as shown below.
More ifconfig examples: Ifconfig: 7 Examples To Configure Network Interface
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.
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.
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.
Whatis command displays a single line description about a command.
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.
Display the man page of a specific command.
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.
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.
Print the last 10 lines of a file by default.
Print N number of lines from the file named 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.
More tail examples: 3 Methods To View tail -f output of Multiple Log Files in One Terminal
less is very efficient while viewing huge log files, as it doesn’t need to load the full file while opening.
One you open a file using less command, following two keys are very helpful.
More less examples: Unix Less Command: 10 Tips for Effective Navigation
Switch to a different user account using su command. Super user can switch to any other user without entering their password.
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.
Login to a specified user account, and execute the specified shell instead of the default shell.
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.
To connect to a local mysql database.
If you want to specify the mysql root password in the command line itself, enter it immediately after -p (without any space).
To install apache using yum.
To upgrade apache using yum.
To uninstall/remove apache using yum.
To install apache using rpm.
To upgrade apache using rpm.
To uninstall/remove apache using rpm.
More rpm examples: RPM Command: 15 Examples to Install, Uninstall, Upgrade, Query RPM Packages
Ping a remote host by sending only 5 packets.
More ping examples: Ping Tutorial: 15 Effective Ping Command Examples
Set the system date:
Once you’ve changed the system date, you should syncronize the hardware clock with the system date as shown below.
The quick and effective method to download software, music, video from internet is using wget command.
Download and store it with a different name.
More wget examples: The Ultimate Wget Download Guide With 15 Awesome Examples
Did I miss any frequently used Linux commands? Leave a comment and let me know.