30 Useful Linux Commands for System
Administrators. In this article we are going to review some of it…
1. Uptime Command
In Linux uptime command
shows since how long your system is running and the number of users are
currently logged in and also displays load average for 1,5 and 15 minutes intervals.
# uptime
08:16:26 up 22 min, 1 user, load average: 0.00, 0.03, 0.22
Check Uptime Version
Uptime command don’t have other options
other than uptime and version. It gives information only in hours:mins if it less than 1 day.
[kkraj@sysadmin ~]$ uptime -V
procps version 3.2.8
2. W Command
It will displays users currently logged
in and their process along-with shows load
averages. also shows the login
name, tty name, remote host, login time, idle time,JCPU, PCPU, command and processes.
# w
08:27:44 up 34 min, 1 user, load average: 0.00, 0.00, 0.08
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
kkraj pts/0 192.168.50.1 07:59 0.00s 0.29s 0.09s w
Available options
1.
-h : displays no header entries.
2.
-s : without JCPU and PCPU.
3.
-f : Removes from field.
4.
-V : (upper letter) – Shows versions.
3. Users Command
Users command displays currently logged
in users. This command don’t have other parameters other than help and version.
# users
kkraj
4. Who Command
who command simply return user name, date, time and host information. who command is
similar to w command.
Unlike w command who doesn’t print what users are
doing. Lets illustrate and see the different between who and w commands.
# who
kkraj pts/0 2012-09-18 07:59 (192.168.50.1)
# w
08:43:58 up 50 min, 1 user, load average: 0.64, 0.18, 0.06
USER TTY FROM LOGIN@ IDLE JCPU PCPU WHAT
kkraj pts/0 192.168.50.1 07:59 0.00s 0.43s 0.10s w
Who command Options
1.
-b : Displays last system reboot date and time.
2.
-r : Shows current runlet.
3.
-a, –all : Displays all information in cumulatively.
5. Whoami Command
whoami command print the name of current
user. You can also use “who am i”
command to display the current user. If you are logged in as a root using sudo
command “whoami” command return root as current user. Use “who am i” command if you want to know
the exact user logged in.
# whoami
kkraj
6. ls Command
ls command display list of files in
human readable format.
# ls -l
total 114
dr-xr-xr-x. 2 root root 4096 Sep 18 08:46 bin
dr-xr-xr-x. 5 root root 1024 Sep 8 15:49 boot
Sort file as per last modified time.
# ls -ltr
total 40
-rw-r--r--. 1 root root 6546 Sep 17 18:42 install.log.syslog
-rw-r--r--. 1 root root 22435 Sep 17 18:45 install.log
-rw-------. 1 root root 1003 Sep 17 18:45 anaconda-ks.cfg
7. Crontab Command
List schedule jobs for current user with crontab command and -l option.
# crontab -l
00 10 * * * /bin/ls >/ls.txt
Edit your crontab with -e option. In the below example
will open schedule jobs in VI
editor. Make a necessary changes and quit pressing :wq keys which saves the setting
automatically.
# crontab -e
8. Less Command
less command allows quickly view file.
You can page up and down. Press ‘q‘
to quit from less window.
# less install.log
Installing setup-2.8.14-10.el6.noarch
warning: setup-2.8.14-10.el6.noarch: Header V3 RSA/SHA256 Signature, key ID c105b9de: NOKEY
Installing filesystem-2.4.30-2.1.el6.i686
Installing ca-certificates-2010.63-3.el6.noarch
Installing xml-common-0.6.3-32.el6.noarch
Installing tzdata-2010l-1.el6.noarch
Installing iso-codes-3.16-2.el6.noarch
9. More Command
more command allows quickly view file
and shows details in percentage. You can page up and down. Press ‘q‘ to quit out from more window.
# more install.log
Installing setup-2.8.14-10.el6.noarch
warning: setup-2.8.14-10.el6.noarch: Header V3 RSA/SHA256 Signature, key ID c105b9de: NOKEY
Installing filesystem-2.4.30-2.1.el6.i686
Installing ca-certificates-2010.63-3.el6.noarch
Installing xml-common-0.6.3-32.el6.noarch
Installing tzdata-2010l-1.el6.noarch
Installing iso-codes-3.16-2.el6.noarch
--More--(10%)
10. CP Command
Copy file from source to destination
preserving same mode.
# cp -p fileA fileB
You will be prompted before overwrite to
file.
# cp -i fileA fileB
11. MV Command
Rename fileA to fileB. -i options
prompt before overwrite. Ask for confirmation if exist already.
# mv -i fileA fileB
12. Cat Command
cat command used to view multiple file
at the same time.
# cat fileA fileB
You combine more and less command with cat command to
view file contain if that doesn’t fit in single screen / page.
# cat install.log | less
# cat install.log | more
13. Cd command (change directory)
with cd command (change directory) it
will goes to fileA directory.
# cd /fileA
14. pwd command (print working directory)
pwd command return with present
working directory.
# pwd
/root
15. Sort command
Sorting lines of text files in ascending
order. with -r options
will sort in descending order.
#sort fileA.txt
#sort -r fileA.txt
16. VI Command
Vi is a most popular text editor
available most of the UNIX-like OS.
Below examples open file in read only with -R option.
Press ‘:q‘ to quit from vi
window.
# vi -R /etc/shadows
17. SSH Command (Secure Shell)
SSH command is used to login into remote
host. For example the below ssh command will connect to remote host (192.168.50.2) using user as narad.
# ssh kkraj@192.168.50.2
To check the version of ssh use option -V (uppercase) shows version of
ssh.
# ssh -V
OpenSSH_5.3p1, OpenSSL 1.0.0-fips 29 Mar 2010
18. Ftp or sftp Command
ftp or sftp command
is used to connect to remote ftp host. ftp is (file
transfer protocol) and sftp is (secure
file transfer protocol). For example the below commands will connect
to ftp host (192.168.50.2).
# ftp 192.168.50.2
# sftp 192.168.50.2
Putting multiple files in remote host
with mput similarly
we can do mget to
download multiple files from remote host.
# ftp > mput *.txt
# ftp > mget *.txt
19. Service Command
Service command call script located at /etc/init.d/ directory and execute
the script. There are two ways to start the any service. For example we start
the service called httpd with
service command.
# service httpd start
OR
# /etc/init.d/httpd start
20. Free command
Free command shows free, total and swap memory information in bytes.
# free
total used free shared buffers cached
Mem: 1030800 735944 294856 0 51648 547696
-/+ buffers/cache: 136600 894200
Swap: 2064376 0 2064376
Free with -t options
shows total memory used
and available to use in bytes.
# free -t
total used free shared buffers cached
Mem: 1030800 736096 294704 0 51720 547704
-/+ buffers/cache: 136672 894128
Swap: 2064376 0 2064376
Total: 3095176 736096 2359080
21. Top Command
top command displays processor
activity of your system and also displays tasks managed by kernel in real-time.
It’ll show processor and memory are being used. Use top
command with ‘u‘ option
this will display specific User process details as shown below. Press ‘O‘ (uppercase
letter) to sort as per desired by you. Press ‘q‘ to quit from top screen.
# top -u kkraj
top - 11:13:11 up 3:19, 2 users, load average: 0.00, 0.00, 0.00 Tasks: 116 total, 1 running, 115 sleeping, 0 stopped, 0 zombie Cpu(s): 0.0%us, 0.3%sy, 0.0%ni, 99.7%id, 0.0%wa, 0.0%hi, 0.0%si, 0.0%st Mem: 1030800k total, 736188k used, 294612k free, 51760k buffers Swap: 2064376k total, 0k used, 2064376k free, 547704k cached PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1889 kkraj 20 0 11468 1648 920 S 0.0 0.2 0:00.59 sshd 1890 kkraj 20 0 5124 1668 1416 S 0.0 0.2 0:00.44 bash 6698 kkraj 20 0 11600 1668 924 S 0.0 0.2 0:01.19 sshd 6699 kkraj 20 0 5124 1596 1352 S 0.0 0.2 0:00.11 bash
22. Tar Command
tar command is used to compress files
and folders in Linux. For example the below command will create a archive for /home directory with file name as archive-name.tar.
# tar -cvf archive-name.tar /home
To extract tar archive file use the
option as follows.
# tar -xvf archive-name.tar
23. Grep Command
grep search for a given string in a
file. Only kkraj user displays from /etc/passwd file.
we can use -i option
for ignoring case sensitive.
# grep
kkraj/etc/passwd kkraj:x:500:500::/home/kkraj:/bin/bash
24. Find Command
Find command used to search files, strings and directories. The below example of find
command search kkraj word in ‘/‘ partition and return the output.
# find / -name kkraj
/var/spool/mail/kkraj /home/kkraj /root/home/kkraj
25. lsof Command
lsof mean List of all open files. Below
lsof command list of all opened files by user kkraj.
# lsof -u kkraj
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
sshd 1889 kkraj cwd DIR 253,0 4096 2 /
sshd 1889 kkraj txt REG 253,0 532336 298069 /usr/sbin/sshd
sshd 1889 kkraj DEL REG 253,0 412940 /lib/libcom_err.so.2.1
sshd 1889 kkraj DEL REG 253,0 393156 /lib/ld-2.12.so
sshd 1889 kkraj DEL REG 253,0 298643 /usr/lib/libcrypto.so.1.0.0
sshd 1889 kkraj DEL REG 253,0 393173 /lib/libnsl-2.12.so
sshd 1889 kkraj DEL REG 253,0 412937 /lib/libkrb5support.so.0.1
sshd 1889 kkraj DEL REG 253,0 412961 /lib/libplc4.so
26. last command
With last command we can watch user’s
activity in the system. This command can execute normal user also. It will
display complete user’s info like terminal, time, date, system reboot orboot and kernel version. Useful command to
troubleshoot.
# last
kkraj pts/1 192.168.50.1 Tue Sep 18 08:50 still logged in
kkraj pts/0 192.168.50.1 Tue Sep 18 07:59 still logged in
reboot system boot 2.6.32-279.el6.i Tue Sep 18 07:54 - 11:38 (03:43)
root pts/1 192.168.50.1 Sun Sep 16 10:40 - down (03:53)
root pts/0 :0.0 Sun Sep 16 10:36 - 13:09 (02:32)
root tty1 :0 Sun Sep 16 10:07 - down (04:26)
reboot system boot 2.6.32-279.el6.i Sun Sep 16 09:57 - 14:33 (04:35)
narad pts/2 192.168.50.1 Thu Sep 13 08:07 - down (01:15)
You can use last with username to know for specific
user’s activity as shown below.
# last kkraj
kkraj pts/1 192.168.50.1 Tue Sep 18 08:50 still logged in
kkraj pts/0 192.168.50.1 Tue Sep 18 07:59 still logged in
kkraj pts/1 192.168.50.1 Thu Sep 13 08:07 - down (01:15)
kkraj pts/4 192.168.50.1 Wed Sep 12 10:12 - 12:29 (02:17)
27. ps command
ps command displays about processes
running in the system. Below example show initprocess
only.
# ps -ef | grep init
root 1 0 0 07:53 ? 00:00:04 /sbin/init
root 7508 6825 0 11:48 pts/1 00:00:00 grep init
28. kill command
Use kill command
to terminate process. First find process id with ps command as shown below and kill
process with kill -9 command.
# ps -ef | grep init
root 1 0 0 07:53 ? 00:00:04 /sbin/init
root 7508 6825 0 11:48 pts/1 00:00:00 grep init
# kill- 9 7508
29. rm command
rm command used to remove or delete a
file without prompting for confirmation.
# rm filename
Using -i option
to get confirmation before removing it. Using options ‘-r‘ and ‘-f‘
will remove the file forcefully without confirmation.
# rm -i test.txt
rm: remove regular file `test.txt'?
30. mkdir command example.
mkdir command is used to create
directories under Linux.
# mkdir directoryname
This is a handy day to day useable basic
commands in Linux / Unix-like operating system. Kindly share through our
comment box if we missed out.
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