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Linux_Commands.md

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Important files

System

Users and groups

Contents/Details Ubuntu Redhat/CentOS
User details /etc/passwd
Encrypted passwords /etc/shadow
Groups info /etc/group
Config for Sudo /etc/sudoers
log files /var/log/syslog
/var/log/auth.log		 var/log/messages
/var/log/secure
Switch to root without paasword sudo su

System commands

Usage Command
OS Version cat /etc/os-release#For Ubuntu
cat /etc/redhat-release#For Redhat
cat /etc/centos-release#For Centos
Kernel Version uname -a
hostnamectl status
Reboot sudo reboot #Server will be rebooted immediatly
sudo shutdown -r #Rebooted after a min
sudo shutdown -r +10 #Reboot after 10 min
sudo shutdown -r 16:15 #Scheduled reboot
sudo shutdown -c#Cancel the schedule reboot
Port status netstat -anp |grep <PortNum>
Memory usage free -m
Shell echo $SHELL

Install a Package/Software

CentOS / Redhat

Usage Command
Search for a package yum list <pkg>
Pachage info yum info <pkg>
Install a package yum install <pkg> -y
Install a package from rpm file yum localinstall <rpmfile> -y

Ref: https://access.redhat.com/sites/default/files/attachments/rh_yum_cheatsheet_1214_jcs_print-1.pdf

Ubuntu

Usage Command
List installed packages apt list --installed

Users

Usage Command
current user whoami
List of all logged in users who
Add an user sudo useradd <username>
Set password for an user sudo passwd <username>
Add a user without login access sudo useradd <uname> -s /sbin/nologin
Switch user
Swich to root user		 su <username>
su or su bash
Delete an user sudo userdel <username>
Modify an User sudo usermod
Change an user account password expiration sudo chage

It is also important to note that, as opposed to other Linux flavors, the user that is created when Ubuntu is first installed has superuser privileges out-of-the-box.

Groups

Usage Command
Add a group sudo groupadd <groupname>
Add an user to group sudo useradd -g <Groupname> <username>
list the UID,Groups for current user id
list the UID,Groups for a user id <user name>
Delete a group sudo groupdel <groupname>

grep (Search)

Usage Command
Search in a file grep <pattern> <filename>
Case insensitive grep -i <pattern> <filename>
Search results with line numbers grep -in <pattern> <filename>
Search that doesnt match grep -iv <pattern> <filename>
Search in a dir grep -ir <pattern> <dir>

Sorting

Usage Command
Case insensitive sorting sort -f
Reverse sort sort -r
Numeric sorting sort -n

Permissions

Usage Command
Change the owner chown <user> <file>
Change the permission chmod 777 <file>
Add read permission chmod +r <file>
Remove read permission chmod -r <file>

Files

Usage Command
List of opened files lsof
Files opened by a user lsof -u <username>

Zip / Unzip

Usage Command
Zip a file tar -cvf <tar-filename> <folder>
Unzip a tar file tar -xvf <tar-file>

Selected column o/p

Usage Command
only one column cut -c1 <file>
o/p first two columns cut -c1-2 <file>

SED (cli text editor)

Usage Command
Replace in a file sed 's/<string>/<new-string>' <file>

o/p only unique/Remove duplicates

Usage Command
o/p only unique/Remove duplicates unique <filename>

Monitor a command o/p

Usage Command
monitor a command
o/p refreshes every 2 secs		 watch <command>

Base64 encoding/decoding

base64 encoding: echo -n 'string' | base64 base64 decoding: echo "encodedstring" | base64 --decode

Networking

Usage Command
IP address ifconfig -a

Edit text files

Vi Editor