Linux Cheat Sheet

Table of Contents

Command Line Essentials

The bare essential you need to navigate the terminal.

cmd	description
pwd	print working directory
cd	change directory
ls	list contents of current directory
touch <name>	make a new file
mkdir <name>	make a directory (i.e. folder)
cp <what> <where>	copy a file or directory
mv <what> <where>	move a file or directory
mv <name> <newName>	rename a file or directory
rm <path>	delete a file or directory (⚠️)
Ctrl+C	kill a process (SIGINT)
clear (Ctrl+L)	clear terminal
man <command>	read the manual of a program

• Note: Both cp & mv allow you to rename a file while performing their respective (copy or move) action and can accept full filepaths.

Extensions of Commands Line Essentials

More complex than the bare essentials, but still essential.

cmd	description
cat <filename>	concatenates (useful for viewing files)
echo <string>	outputs string(s) passed (useful for bash scripting)
cd ../	move 1 directory up
ls <path>	list contents of a given directory
ls -l	list contents of directory with permissions (⚠️)
rm -r[i] *	delete the contents of a directory (⚠️)
rm -r[i] <path>	delete a folder and its contents (⚠️)
Ctrl+Z	suspend a process (SIGSTOP)
script <output>	log terminal to a file

Notes:

• Whenever this guide uses [] the contents inside of the bracket represent an optional argument.
• The optional argument [i] triggers rm's interactive mode which will prompt you before performing each and every delete. This is a safer approach to deletion.
• The author of this guide will like to make note of the optional [f] argument (used as -rf) which forcefully deletes a directory or file. While useful this can also be dangerous, please exert caution and ensure you most definitely can and should delete the file or directory before executing this action.

Absolute vs. Relative Paths

type	symbol	description
absolute	/	paths beginning from the root directory
relative	omit / (or ./)	paths beginning from the current directory
home	~	shortcut to user’s home directory

Admin Privileges

cmd	description
sudo <cmd>	super user do
!!	“bang bang” shorthand for last command
su <username>	switch user
sudo su	switch to super user

• Note: !! is useful to run a previous command with elevated privileges (e.g. sudo !!)

Package Manager (apt)

Note: This part of the guide is only true for Debian based distros that use apt (e.g Ubuntu)

cmd	description
apt install <name>	install a program
apt remove <name>	uninstall a program
apt search <name>	search for a program
apt policy <name>	checks if a program is installed
apt update	updates the package listings
apt upgrade	upgrades packages with newer versions

Note: It’s not uncommon to chain update and upgrade by running apt update && apt upgrade. && is used to chain commands together if and only the previous command exited without errors.

Installing packages not in the repository by manually downloading files:

1. In a web browser, navigate to the download page of the program
2. Download & save the correct package based on your distro and architecture (.deb for Debian)
3. Run sudo dpkg -i ./<path> where <path> is the filepath of the debian package (e.g .deb)

File Permissions & Ownership

Understanding File Permissions

After running ls -l on a directory you should get something like this:

The 3 columns on the left represent 3 associations every file/directory has:

• user (i.e. the owner of the file/directory) [red]
• group [blue]
• other (i.e. the public, the “world) [yellow]

The letters represent file permissions:

• r = read permission
• w = write permission
• x = execute permission
• and the prefix d = directory

In the case of garden_vintage.jpg, danielmenjivar owns this file and is part of the staff group.
danielmenjivar can write and read to this file, both the staff group and the public may read only.

Changing Ownership (chown)

cmd	description
chown [-R] <user>:<group> <path>	change ownership

Note: The optional argument [-R] is used to recursively change ownership of all files inside a directory.

Why/When do we use chown? A common scenario to use chown is when certain files or directories are owned by the root user and need to be owned by another non-root user. In this scenario a file’s permission would look something like this (after executing ls -l):
-rw-r--r-- 1 root staff 0 Jun 22 09:17 landscape.jpg.
By executing: sudo chown danielmenjivar:staff landscape.jpg,
danielmenjivar gains ownership the file and as a result can read and write to it.

Modifying Permissions (chmod)

cmd	description
chmod [-R] <shorthand> <path>	modify the file’s permissions

Note: The optional argument [-R] is used to recursively modify permissions of all the files inside a directory.

chmod using shorthands (recommended)

groups	permissions	operator
u = user	x = execute	+ Add
g = group	w = write	- Remove
o = other	r = read	= Equals
a = all of the above	(blank) = no permissions	, to chain assignments

Example To assign an user read & write, add write to group, and assign no permissions to other.

chmod u=rw,g+w,o=

chmod using octals (harder)

Octal permissions are set using numbers from 0 to 7.

All you need to know are the basics:

octal base	permissions
0	none
1	execute
2	write
4	read

From there, the rest are just combinations (sums)

combo permission	sum
execute + write	1 + 2 = 3
execute + read	1 + 4 = 5
write + read	2 + 4 = 6
execute + write + read	1 + 2 + 4 = 7

Tip: if the number is odd, it includes execute

cmd	description
chmod [-R] <ugo> <path>	modify the file’s permissions

Where ugo is the corresponding user mapped to an octal (user, group, other).

Example To give read/write permissions to user, read to group, and none to others you’d run:

chmod 640 file

Note: The optional argument [-R] is used to recursively modify permissions of all the files inside a directory.

Why/When/How do we use chmod? Suppose you are an admin and a staff member (who is part of the staff group) wants to edit file.txt, but for some odd reason they can’t. To investigate they run ls -l and this is the result: -rw-r--r-- 1 danielmenjivar staff 0 Jun 23 11:56 file.txt. The staff members ask you give them read-and-write permission. You don’t want the public to be able to read-and-write and there is no need to give them ownership of file.txt. To do this, all you have to do is run: chmod 664 ./file.txt, where 6 is read-and-write for yourself (no reason to give up these permissions for yourself, let’s assume you are me danmenjivar, the owner), 6 is read-and-write for the staff group, and 4 is read only for the public (you don’t want any shenanigans happening). Every column gets a number, for more details on chmod check out the man page on it.

Find and Grep

Find

cmd	description
find <directory> [<type>] <criteria>	search for files

• <directory> is the path of the directory you want to begin searching at

• <type> is an optional flag, omitting this flag will search for both files and directories

  • -type f will only search for files
  • -type d will only search for directories

• a search <criteria> such as filename, file permissions, or file size

  • -[i]name <filename> search by name with the optional [i] flag to ignore case sensitivity
  • -perm mode search by permissions using a mode (e.g. 0664)
  • -size [+|-]size you can search for files greater, lesser, or of exact size (e.g. +1M, -1M, 1M)
  • Note: you can negate a search criteria by placing the -not option in front of the criteria.

• Find is recursive by default, to disable this use -maxdepth <amount> where <amount> is the numeric level (e.g. 1) to limit your search.

Grep

cmd	description
grep [-n] [-i] <keyword> <directory>	search contents of files

• <directory> is the path of the directory you want to begin searching at
• <keyword> is the search string, i.e. what you’re looking for
• [-i] optional flag, ignores case sensitivity
• [-n] optional flag, appends the line numbers to each result

Grep+Find

cmd	description
find <directory> [<type>] <criteria> -exec grep [-n] [-i] <keyword> <directory> {} +	search contents of some particular files
find <directory> [<type>] <criteria> \| xargs grep [-n] [-i] <keyword> <directory>	search contents of some particular files

• -exec joins the find and grep commands
• {} + is used to delimit the end of the -exec
• xargs is used to build an execution pipeline from standard input

When/How/Why? Suppose you want to search for the contents of some php files in your current directory. The find command for searching php files would be: find . -type -iname "*.php". To search the contents of these files you can pair find with grep using the -exec command. All -exec does is run your grep command after running find. To signal when the exec is over, we append {} + to the end of the grep command. The resulting command would look like this: find . -type -iname "*.php" -exec grep -i -n "function" {} +.

Redirecting Output of a Command

cmd	description
<cmd> \| <cmd>	pipes transfers output from the command on its left into the standard input of the command on its right
<command> > <output>	redirect the output of the command on the left to the file on the right (e.g. ls > out.txt) [2
<cmd> >> <output>	redirect output of a command on the left and append it to the end of the file on the right
<cmd> < <input>	read from a file
<command> \| tee <output>	redirecting output of a command while seeing results in real-time (e.g. ls | tee out.txt)

Note: Apart from redirecting between apps & files, we can also redirect between streams by specifying a POSIX stream ID. The > by itself is redirecting standard output (i.e. stream ID 1). For example, we can redirect standard error like this: <cmd> 2> <output> where 2 indicates the POSIX stream ID. To redirect both standard output and error we’d use <cmd> &> <output>. Note that > and 1> are identical since the default behavior of redirect is to do so on standard output.

POSIX stream ID	description
0	standard input
1	standard output
2	standard error

Manage Processes (top)

cmd	description
top	shows in real-time the top of the list of running applications
ps aux	captures a list of running applications (not real-time)
pgrep <command>	lists the pid’s associated with a command
kill <pid>	kill a process instance
killall <command>	kill all instances of a process

Notes:

• top:

  • PID is the process id, used to manage processes
  • USER who the process is running for
  • TIME+ how long the process has been running for
  • COMMAND the command associated with the process

• ps aux:

  • You can use grep to search for processes like this: ps aux | grep firefox

• pgrep:

  • the pid’s are listed in chronological order

Tip: You can run commands in the background by post fixig the & command. This can be useful for opening applications (e.g. firefox &) or running several commands inside one ssh shell.

Services

Services are background processes.

cmd	description
service <name> start	start a service
service <name> stop	stop a service
service <name> restart	restart a service
systemctl start <name>	start a service
systemctl stop <name>	stop a service

Scheduling Tasks (crontab/cronjobs)

Cronjobs are processes used to automate tasks (e.g. running backups, updates, etc.)

cmd	description
crontab -e	start a ctrontab & save it to /etc

Cronjob Structure

m h dom dow command

abbreviation	meaning	values
m	minutes	0-59
h	hours	0-23
dom	day of month	1-31
mon	month number	1-12
dow	day of week	0-6
command	to perform

Managing Users

cmd	description
adduser <username>	add new user
adduser <username> sudo | add user to sudo group
deluser <username>	remove user
passwd <username>	change a password for a user account
groupadd <groupname>	create a new group
adduser <username> <groupname>	add user to group
deluser <username> <groupname>	remove user from group

History: Running Past Commands

cmd	description
!!	“bang bang”, shorthand for last command
history	displays the last couple commands ran
!<line>	run past command based on a command number

Tip: Ran a command that needs sudo without it? Just run sudo !!.

Fun Terminal Commands

cmd	description
figlet	turn boring text into fancy big text
fortune	generate random messages
cowsay	because everything is better when a cow says it
cowsay -f moofasa	turn cow into ’moofasa’
lolcat [-a]	rainbow print concatenation [animate]
sl	steam train coming through
cmatrix	become the chosen one