50 most frequently used commands in Linux / UNIX
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 jump start on some of the common Linux commands. Bookmark this article for your
future reference.
Did I miss any frequently used Linux commands? Leave
a comment
and let me know.
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
Use “shopt -s cdspell” to automatically correct mistyped directory names on
cd
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 steatus 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.
- 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
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
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
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