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.
[arun@arun ~]$ 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
arun pts/0
192.168.50.1 07:59 0.00s
0.29s 0.09s w
Available options
- -h : displays no header entries.
- -s : without JCPU and PCPU.
- -f : Removes from field.
- -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
root
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
arun 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
arun pts/0
192.168.50.1 07:59 0.00s
0.43s 0.10s w
Who command Options
- -b : Displays last system reboot date and time.
- -r : Shows current runlet.
- -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
arun
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
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
For
more examples of Linux Cron Command, please read our earlier article on 11 Cron Scheduling Task Examples in
Linux.
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 user.
# ssh user@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 arun
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 arun 20 0
11468 1648 920 S 0.0
0.2 0:00.59 sshd
1890 arun 20
0 5124 1668 1416 S 0.0
0.2 0:00.44 bash
6698 arun 20 0
11600 1668 924 S 0.0
0.2 0:01.19 sshd
6699 arun 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 arun user displays from /etc/passwd
file. we can use -i option for ignoring case sensitive.
# grep arun /etc/passwd
arun:x:500:500::/home/arun:/bin/bash
24. Find Command
Find
command used to search files, strings and directories. The
below example of find command search arun word in ‘/‘ partition
and return the output.
# find / -name arun
/var/spool/mail/arun
/home/arun
/root/home/arun
25. lsof Command
lsof mean List of all
open files. Below lsof command list of all opened files by user arun.
# lsof -u arun
COMMAND PID
USER FD TYPE
DEVICE SIZE/OFF NODE NAME
sshd 1889 arun
cwd DIR 253,0
4096 2 /
sshd 1889 arun
txt REG 253,0
532336 298069 /usr/sbin/sshd
sshd 1889 arun
DEL REG 253,0 412940 /lib/libcom_err.so.2.1
sshd 1889 arun
DEL REG 253,0 393156 /lib/ld-2.12.so
sshd 1889 arun
DEL REG 253,0 298643 /usr/lib/libcrypto.so.1.0.0
sshd 1889 arun
DEL REG 253,0 393173 /lib/libnsl-2.12.so
sshd 1889 arun
DEL REG 253,0 412937 /lib/libkrb5support.so.0.1
sshd 1889 arun
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 or boot and kernel
version. Useful command to troubleshoot.
# last
arun pts/1
192.168.50.1 Tue Sep 18
08:50 still logged in
arun 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 arun
arun pts/1
192.168.50.1 Tue Sep 18
08:50 still logged in
arun pts/0
192.168.50.1 Tue Sep 18
07:59 still logged in
arun pts/1
192.168.50.1 Thu Sep 13 08:07
- down (01:15)
arun 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 init process
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
Display of Top Command
In
this example, it will show information like tasks, memory, cpu
and swap. Press ‘q‘ to quit window.
# top
Linux
Top Command
2. Sorting with -O (Uppercase Letter
‘O’).
Press
(Shift+O) to Sort field via field letter, for example press ‘a‘
letter to sort process with PID (Process ID).
Sorting
Process ID’s with Top
Type
any key to return to main top window with sorted PID order as shown in
below screen. Press ‘q‘ to quit exit the window.
Sorting
Process ID’s
3. Display Specific User Process
Use
top command with ‘u‘ option will display specific User process
details.
# top -u arun
Top
with Specific User Processes
4. Highlight Running Process in Top
Press
‘z‘ option in running top command will display running process in color
which may help you to identified running process easily.
Top
Process with Colorful
5. Shows Absolute Path of Processes
Press
‘c‘ option in running top command, it will display absolute path of
running process.
Top
with Specific Process Path
6. Change Delay or Set ‘Screen Refresh
Interval’ in Top
By
default screen refresh interval is 3.0 seconds, same can be change
pressing ‘d‘ option in running top command and change it as desired as
shown below.
Top
– Set Refresh Time
7. Kill running process with argument
‘k’
You
can kill a process after finding PID of process by pressing ‘k‘
option in running top command without exiting from top window as shown below.
Top
– Kill Process ID
8. Sort by CPU Utilisation
Press
(Shift+P) to sort processes as per CPU utilization. See
screenshot below.
Top
– High CPU Utilization
9. Renice a Process
You
can use ‘r‘ option to change the priority of the process also called
Renice.
Top
– Renice Process
10. Save Top Command Results
Press
(Shift+W) to save the running top command results under /root/.toprc.
Top
Command Save Results
11. Getting Top Command Help
Press
‘h‘ option to obtain the top command help.
Top
Command Help
12. Exit Top Command After Specific
repetition
Top
output keep refreshing until you press ‘q‘. With below command top
command will automatically exit after 10 number of repetition.
# top -n 10
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