Linux Cheatsheet

Users management
History
Network
Permissions
Redirection
Network sniffer
systemd
text manipulation - awk and sed

System info

What	How
name of host	hostname
FQDN of host	hostname -A
IP address of host	hostname -i
memory usage	free -h
Linux flavor	cat /etc/*-release cat /proc/version lsb_release -a
disk usage	df -h
folders size	du -ah
total disk usage off the current directory	du -sh

History tool

What	How
Run last command starting with abc	!abc
Print last command starting with abc	!abc:p
Last argument of previous command	!$
Search history	CONTROL-R
All arguments of last command	!*
First argument of last command	!^
Take second argument of last cp command	ls -l !cp:2

Network

netstat man page
netstat examples

View Only Established Connection	netstat -natu | grep 'ESTABLISHED' -a = Show both listening and non-listening (for TCP this means established connections) sockets -t = tcp -u = tudp -n = Show numerical addresses instead of trying to determine symbolic host
Listing all the LISTENING Ports of TCP and UDP connections	netstat -a
Listing all LISTENING Connections	netstat -l -l = Show only listening sockets
Listing TCP Ports connections	netstat -at
Listing all TCP Listening Ports	netstat -lt
Listing UDP Ports connections	netstat -au
View Port Number used by PID	netstat -anlp | grep $PID
which process using particular port	netstat -anlp | grep portnumber

Users management

Groups

What	How
/etc/group	[Group name]:[Group password]:[GID]:[Group members]
Group of current user	groups
Groups an user is a member of	groups tecmint id tecmint (first listed group is primary group)
List existing groups	getent group
Deleting a group	groupdel [group_name]
Create group	groupadd mynewgroup

Users

What	How
/etc/passwd	[username]:[x]:[UID]:[GID]:[Comment]:[Home directory]:[Default shell]
Change user shell	usermod --shell /bin/sh tecmint
Adding a user to a primary group and supplementary group	useradd -g “head” -G “faculty” anirban -g = primary group
Add user to group	usermod -a -G group user
Define primary group of user	usermod -g groupname username
View the numerical IDs associated with each group	id

Other:

What	How
Creating a new group for read and write access to files that need to be accessed by several users	groupadd common_group # Add a new group chown :common_group common.txt # Change the group owner of common.txt to common_group usermod -aG common_group user1 # Add user1 to common_group usermod -aG common_group user2 # Add user2 to common_group usermod -aG common_group user3 # Add user3 to common_group =======================================================
Look for files and replace strings in it	find . -type f -exec grep -q "opt/octane" {} \; -print -exec sed -i "s+/opt/octane+/temp/octane+g" {} \;

Permissions

Unix Permissions Calculator 1
Unix Permissions Calculator 2

0=---  1=--x  2=-w-  3=-wx  4=r-  5=r-x  6=rw-  7=rwx

644 u+rw-, g+r, o+r
750 u+rwx, g+rx
755 u+rwx, g+rx, o+rx
760 u+rwx, g+rw
770 u+rwx, g+rwx

What	How
owner	(u) The Owner permissions apply only the owner
group	(g) The Group permissions apply only to the group that has been assigned to the file or directory
all users	(o) The All Users permissions apply to all other users
add permissions	chmod a+rw file1
remove permissions	chmod a-rw file1
binary represntation of permissions	read = 4 write = 2 execute = 1
changing groups of files & directories	chgrp <group> <file>
changing ownership	chown -R tom <folder> chown -R tom:sales <folder>

IO Redirection

What	How
Error output (stderr) of cmd to file	cmd 2> file
stdout to same place as stderr	cmd 1>&2
stderr to same place as stdout	cmd 2>&1
Every output of cmd to file	cmd &> file

Network sniffer

What	How
Capture and display all packets on interface eth0	tcpdump -i eth0
Monitor all traffic on port 80	tcpdump -i eth0 'port 80'

systemd

redhat doc for unit file

What	How
Start service	systemctl start foobar
Stop service	systemctl stop foobar
low-level properties of a unit	systemctl show foobar
Loaded services	systemctl -t service
Installed services	systemctl list-unit-files -t service systemctl list-units --all
low-level properties of a unit	systemctl show foobar
Loaded services	systemctl -t service
Read all the service options	man systemd.service
Run if .service file has changed	systemctl daemon-reload
See all systemd logs	journalctl journalctl -f journalctl -u my_daemon.service
Display an overview of overridden or modified unit files	systemd-delta

Text manipulation

sed   https://likegeeks.com/sed-linux/

What	How
print only certain fields	awk '/a/ {print $3 "\t" $4}' marks.txt
count and print the number of lines	awk '/a/{++cnt} END {print "Count = ", cnt}' marks.txt
print lines that contain more than 18 characters	awk 'length($0) > 18' marks.txt
Split by different character	awk -F":" '{ print $1 " " $3 }' /etc/passwd
rint only lines that contain a floating point number	/[0-9]+\.[0-9]*/ { print }
sed delete	second line: sed '2d' myfile range: sed '2,3d' myfile third line to the end: sed '3,$d' myfile by pattern: sed '/test 1/d' myfile range: sed '/second/,/fourth/d' myfile
sed replace	specifying occurrence number: sed 's/test/another test/2' myfile prints line with pattern, -n = to print modified lines only: sed -n 's/test/another test/p' myfile use the exclamation mark (!) as string delimiter: sed 's!/bin/bash!/bin/csh!' /etc/passwd limit to range of lines: sed '2,3s/test/another test/' myfile