50
Most Frequently Used UNIX / Linux Commands With Examples
1.tar command
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
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
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
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.
$ 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 belong 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 j-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 optionand give path as argument to it. This searches for the
executable lsmk in the /tmp directory, and displays it, if itis 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 keep 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 lscommand 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 (withoutany 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 synchronize 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
