Linux basic commands

Document Id: RHEL002

In my previous post, I've shown how to install Red Hat Enterprise Linux, in this post I'll explain some basic commands every Linux administrator should know to operate Linux operating system.

Previous post: How to Install RedHat Enterprise Linux 8.2

hostname command, to check server hostname

[root@RHEL82 html]# hostname

RHEL82.localdomain

whoami command, to check login user

[root@RHEL82 html]# whoami

root

uname –a command, to get system identification, mainly kernel version

[root@RHEL82 html]# uname -a

Linux RHEL82.localdomain 4.18.0-193.el8.x86_64 #1 SMP Fri Mar 27 14:35:58 UTC 2020 x86_64 x86_64 x86_64 GNU/Linux

To get installed Red Hat Linux version details

[root@RHEL82 html]# cat /etc/redhat-release

Red Hat Enterprise Linux release 8.2 (Ootpa)

ifconfig command, to check server network interface details

[root@RHEL82 html]# ifconfig

ping command, to check connectivity to the server

[root@RHEL82 ~]# ping 192.168.10.10

PING 192.168.10.10 (192.168.10.10) 56(84) bytes of data.

64 bytes from 192.168.10.10: icmp_seq=1 ttl=128 time=20.8 ms

pwd command

Present working directory, when we first login to terminal we are in user home directory, pwd command shows absolute path of the current directory

[root@RHEL82 ~]# pwd

mkdir command

To create an empty directory

[root@RHEL82 ~]# mkdir newfolder

Output: It will create a directory with name newfolder in the current working directory

To create a directory in the specified location

[root@RHEL82 ~]# mkdir /tmp/newfolder

Output: It will create a directory with name newfolder in the /tmp directory

ls command

List of files and directory

[root@RHEL82 ~]# ls

Output: anaconda-ks.cfginitial-setup-ks.cfgnewfolder

To check list of files and directory in the specified location

[root@RHEL82 ~]# ls /tmp/

To check hidden files and folders

[root@RHEL82 ~]# ls –a

To check all files and folder with associated permission

[root@RHEL82 ~]# ls –la

cd command

Change directory- we use this command to change the working directory location.

Example:

[root@RHEL82 ~]# pwd

/root

[root@RHEL82 ~]# ls

anaconda-ks.cfg initial-setup-ks.cfg newfolder

[root@RHEL82 ~]# cd newfolder/

[root@RHEL82 newfolder]# pwd

/root/newfolder

cd.. (double dot) to go parent directory

Example:

[root@RHEL82 html]# pwd

/var/www/html

[root@RHEL82 html]# cd ..

[root@RHEL82 www]# pwd

/var/www

cd (only cd or cd~) to go home directory from anywhere

Example:

[root@RHEL82 ~]# cd /var/www/html/

[root@RHEL82 html]# pwd

/var/www/html

[root@RHEL82 html]# cd

[root@RHEL82 ~]# pwd

/root

cd – (hyphen) to come back previous working directory

Example:

[root@RHEL82 html]# pwd

/var/www/html

[root@RHEL82 html]# cd

[root@RHEL82 ~]# pwd

/root

[root@RHEL82 ~]# cd -

/var/www/html

[root@RHEL82 html]# pwd

/var/www/html

touch command

To create new blank file

[root@RHEL82 ~]# touch newfile

[root@RHEL82 ~]# ls

anaconda-ks.cfg initial-setup-ks.cfg newfile newfolder

echo command

There are various use of echo command, here we will use echo command to create file and to append string to already created file

[root@RHEL82 ~]# echo This is RedHat Enterprise Linux >> newfile

[root@RHEL82 ~]# echo Current version 8.2 >> newfile

[root@RHEL82 ~]# cat newfile

This is RedHat Enterprise Linux

Current version 8.2

In echo command “>>” use to append content into the file

In echo command “>” use to add content to the file by deleting existing content

[root@RHEL82 ~]# echo delete all content > newfile

[do not use this command to system configuration files]

[root@RHEL82 ~]# cat newfile

delete all content

cat command

cat is short of concatenate, there are various use of cat command

To see file content

[root@RHEL82 ~]# echo Create a new file > file01

[root@RHEL82 ~]# cat file01

Create a new file

[root@RHEL82 ~]# echo This is the second file content > file02

[root@RHEL82 ~]# cat file02

This is the second file content

To add two file contents to a single file “file03” and to see the content of “file03”

[root@RHEL82 ~]# cat file01 file02 >> file03

[root@RHEL82 ~]# cat file03

Create a new file

This is the second file content

adduser command

To add new user

[root@RHEL82 ~]# useradd user01

passwd command

To set user password

[root@RHEL82 ~]# passwd user01

Changing password for user user01.

New password:

BAD PASSWORD: The password is shorter than 8 characters

Retype new password:

passwd: all authentication tokens updated successfully.

su -

Switch to new user

[root@RHEL82 ~]# su - user01

[switch user from root user to another user password is not required, from any other user to any other user password is required]

[user01@RHEL82 ~]$ whoami

user01

exit command, to exit from login shell

[user01@RHEL82 ~]$ exit

Logout

cp command, to copy files or folder

Exampel:

To copy file01 to /var/www/html

[root@RHEL82 ~]# pwd

/root

[root@RHEL82 ~]# ls

anaconda-ks.cfg file01 file02 file03 initial-setup-ks.cfg newfile newfolder

[root@RHEL82 ~]# cp file01 /var/www/html/

[root@RHEL82 ~]# ls /var/www/html/

file01

To copy /var/ww/html/file01 to /mnt

[root@RHEL82 ~]# cp /var/www/html/file01 /mnt/

[root@RHEL82 ~]# ls /mnt/

file01

To copy directory/folder we have to use cp with (–R) option

To copy /root /newfolder to /mnt/

[root@RHEL82 ~]# cp -R /root/newfolder /mnt/

[root@RHEL82 ~]# cd /mnt/

[root@RHEL82 mnt]# ls

file01 newfolder

mv command, to move files

To move /root/file03 to /var/www/html/

[root@RHEL82 ~]# pwd

/root

[root@RHEL82 ~]# ls

anaconda-ks.cfg file01 file02 file03 initial-setup-ks.cfg newfile newfolder

[root@RHEL82 ~]# mv /root/file03 /var/www/html/

[root@RHEL82 ~]# ls

anaconda-ks.cfg file01 file02 initial-setup-ks.cfg newfile newfolder

[root@RHEL82 ~]# ls /var/www/html/

file01 file03

To move folder

[root@RHEL82 ~]# mv /mnt/newfolder /var/www/html/

[root@RHEL82 ~]# ls /var/www/html/

file01 file03 newfolder

rmdir command, to remove an empty directory

Example:

[root@RHEL82 ~]# mkdir emptydirectory

[root@RHEL82 ~]# ls

anaconda-ks.cfg emptydirectory file01 file02 initial-setup-ks.cfg newfile newfolder

[root@RHEL82 ~]# rmdir emptydirectory

[root@RHEL82 ~]# ls

anaconda-ks.cfg file01 file02 initial-setup-ks.cfg newfile newfolder

rm command, to delete files or folder

Example:

[root@RHEL82 ~]# ls

anaconda-ks.cfg file01 file02 initial-setup-ks.cfg newfile newfolder

[root@RHEL82 ~]# rm file01

rm: remove regular file 'file01'? y

[root@RHEL82 ~]# ls

anaconda-ks.cfg file02 initial-setup-ks.cfg newfile newfolder

rm –r command, to delete directory

Example:

[root@RHEL82 ~]# rm -r newfolder/

rm: descend into directory 'newfolder/'? y

rm: remove regular file 'newfolder/newfile'? y

rm: remove directory 'newfolder/'? y

[root@RHEL82 ~]# ls

anaconda-ks.cfg file02 initial-setup-ks.cfg newfile

rm –rf command, to delete entire directory without prompt

Exmaple:

[root@RHEL82 html]# pwd

/var/www/html

[root@RHEL82 html]# ls

file01 file03 newfolder

[root@RHEL82 html]# rm -rf newfolder

[use this command with caution , it will delete all data of the folder]

[root@RHEL82 html]# ls

file01 file03

grep command, to search full text of a file and display the full line of "matching word"

below command will display lines that contain word “NetowrkManager”

[root@RHEL82 ~]# grep NetworkManager /var/log/messages

Aug 31 22:12:56 RHEL82 NetworkManager[1430]: <info> [1598897576.2262] dhcp4 (eth0): option dhcp_lease_time => '86400'

Aug 31 22:12:56 RHEL82 NetworkManager[1430]: <info> [1598897576.2269] dhcp4 (eth0): option domain_name => 'mshome.net'

Aug 31 22:12:56 RHEL82 NetworkManager[1430]: <info> [1598897576.2270] dhcp4 (eth0): option domain_name_servers => '172.18.35.17'

Aug 31 22:12:56 RHEL82 NetworkManager[1430]: <info> [1598897576.2270] dhcp4 (eth0): option expiry => '1598983976'

Aug 31 22:12:56 RHEL82 NetworkManager[1430]: <info> [1598897576.2270] dhcp4 (eth0): option ip_address => '172.18.35.29'

ps command, to check system running process

To see every process on the system using standard syntax:

[root@RHEL82 ~]# ps –ef

Output: It will show all current services

To search for a particular service

[root@RHEL82 ~]# ps –ef |grep service_name

[root@RHEL82 ~]# ps -ef |grep nfs

root 2258 2156 0 20:57 ? 00:00:00 python3 /var/lib/pcp/pmdas/nfsclient/pmdanfsclient.python

root 23758 9955 0 23:36 pts/0 00:00:00 grep --color=auto nfs

free –h command, to see system memory usages

[root@RHEL82 html]# free -h

total used free shared buff/cache available

Mem: 2.5Gi 524Mi 1.5Gi 21Mi 498Mi 2.0Gi

Swap: 2.0Gi 0B 2.0Gi

df –h command , to see system disk usages

[root@RHEL82 html]# df -h

Filesystem Size Used Avail Use% Mounted on

devtmpfs 854M 0 854M 0% /dev

tmpfs 882M 0 882M 0% /dev/shm

tmpfs 882M 18M 865M 2% /run

tmpfs 882M 0 882M 0% /sys/fs/cgroup

/dev/mapper/rootvg-root 20G 5.3G 15G 27% /

/dev/sda1 1014M 231M 784M 23% /boot

/dev/mapper/rootvg-home 9.8G 112M 9.7G 2% /home

/dev/mapper/rootvg-var 9.8G 488M 9.3G 5% /var

tmpfs 177M 16K 177M 1% /run/user/42

tmpfs 259M 4.0K 259M 1% /run/user/0

head command, to see first lines of a file, normally it shows first 10 lines.

Example:

[root@RHEL82 ~]# head /etc/fstab

#

# /etc/fstab

# Created by anaconda on Thu Aug 27 22:50:21 2020

#

# Accessible filesystems, by reference, are maintained under '/dev/disk/'.

# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info.

#

# After editing this file, run 'systemctl daemon-reload' to update systemd

# units generated from this file.

To see number of lines from the top of the file we can use

#head –n number file_name

[root@RHEL82 ~]# head -n 15 /etc/fstab

#

# /etc/fstab

# Created by anaconda on Thu Aug 27 22:50:21 2020

#

# Accessible filesystems, by reference, are maintained under '/dev/disk/'.

# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info.

#

# After editing this file, run 'systemctl daemon-reload' to update systemd

# units generated from this file.

#

/dev/mapper/rootvg-root / xfs defaults 0 0

UUID=aa8807bd-6213-4159-a6ec-ff606a4cf3f0 /boot xfs defaults 0 0

/dev/mapper/rootvg-home /home xfs defaults 0 0

/dev/mapper/rootvg-var /var xfs defaults 0 0

tail command, to see last lines of a file, normally it shows last 10 lines

[root@RHEL82 ~]# tail /etc/fstab

# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info.

#

# After editing this file, run 'systemctl daemon-reload' to update systemd

# units generated from this file.

#

/dev/mapper/rootvg-root / xfs defaults 0 0

UUID=aa8807bd-6213-4159-a6ec-ff606a4cf3f0 /boot xfs defaults 0 0

/dev/mapper/rootvg-home /home xfs defaults 0 0

/dev/mapper/rootvg-var /var xfs defaults 0 0

UUID=be0efc87-f61b-4b50-abef-ebbe26ae19f3 swap swap defaults 0 0

To see number of lines form the last of the file we can use

#tail –n number files_name

[root@RHEL82 ~]# tail -n 15 /etc/fstab

#

# /etc/fstab

# Created by anaconda on Thu Aug 27 22:50:21 2020

#

# Accessible filesystems, by reference, are maintained under '/dev/disk/'.

# See man pages fstab(5), findfs(8), mount(8) and/or blkid(8) for more info.

#

# After editing this file, run 'systemctl daemon-reload' to update systemd

# units generated from this file.

#

/dev/mapper/rootvg-root / xfs defaults 0 0

UUID=aa8807bd-6213-4159-a6ec-ff606a4cf3f0 /boot xfs defaults 0 0

/dev/mapper/rootvg-home /home xfs defaults 0 0

/dev/mapper/rootvg-var /var xfs defaults 0 0

UUID=be0efc87-f61b-4b50-abef-ebbe26ae19f3 swap swap defaults 0 0

tail –f command , f=follow , output appended data as the file grows, very important command to check logs in progress

[root@RHEL82 ~]# tail -f /var/log/messages

Aug 31 22:07:56 RHEL82 NetworkManager[1430]: <info> [1598897276.2174] dhcp4 (eth0): option requested_subnet_mask => '1'

Aug 31 22:07:56 RHEL82 NetworkManager[1430]: <info> [1598897276.2174] dhcp4 (eth0): option requested_time_offset => '1'

Aug 31 22:07:56 RHEL82 NetworkManager[1430]: <info> [1598897276.2175] dhcp4 (eth0): option requested_wpad => '1'

Aug 31 22:07:56 RHEL82 NetworkManager[1430]: <info> [1598897276.2175] dhcp4 (eth0): option routers => '172.18.35.17'

Aug 31 22:07:56 RHEL82 NetworkManager[1430]: <info> [1598897276.2175] dhcp4 (eth0): option subnet_mask => '255.255.255.240'

Aug 31 22:07:56 RHEL82 NetworkManager[1430]: <info> [1598897276.2175] dhcp4 (eth0): state changed extended -> extended

Aug 31 22:07:56 RHEL82 dbus-daemon[891]: [system] Activating via systemd: service name='org.freedesktop.nm_dispatcher' unit='dbus-org.freedesktop.nm-dispatcher.service' requested by ':1.23' (uid=0 pid=1430 comm="/usr/sbin/NetworkManager --no-daemon " label="system_u:system_r:NetworkManager_t:s0")

Aug 31 22:07:56 RHEL82 systemd[1]: Starting Network Manager Script Dispatcher Service...

Aug 31 22:07:56 RHEL82 dbus-daemon[891]: [system] Successfully activated service 'org.freedesktop.nm_dispatcher'

Aug 31 22:07:56 RHEL82 systemd[1]: Started Network Manager Script Dispatcher Service.

man command, an interface to the online reference manual, to see command manual

[root@RHEL82 ~]# man ls

Output will show the details of ls command and syntax

[root@RHEL82 ~]# man adduser

Output will show details of adduser command and syntax

That's the end of this post, thank you!

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