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Displays the file system disk space usage. By default df -k displays output in bytes.
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$ 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
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
More kill examples:4 Ways to Kill a Process – kill, killall, pkill, xkill
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
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
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
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 }
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
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
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.
$ chown oracle:dba dbora.sh
Use -R to change the ownership recursively.
$ chown -R oracle:dba /home/oracle
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
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/
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
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.
$ uname -a Linux john-laptop 2.6.32-24-generic #41-Ubuntu SMP Thu Aug 19 01:12:52 UTC 2010 i686 GNU/Linux
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 displays a single line description about a command.
$ whatis ls ls (1) - list directory contents $ whatis ifconfig ifconfig (8) - configure a network interface
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
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
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
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
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 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).
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
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
More rpm examples:RPM Command: 15 Examples to Install, Uninstall, Upgrade, Query RPM Packages
Ping a remote host by sending only 5 packets.
$ ping -c 5 gmail.com
More ping examples:Ping Tutorial: 15 Effective Ping 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
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
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.