Linux/UNIX shell commands

The UNIX shell is a command-line interface made for interacting with the OS. There are various commands to execute this.

Note: The current documentaiton is based on Ububtu and similar debian bases operating systems.

File systems

The linux file system is a tree structure with the root directory at the top. The root directory is denoted by /. The directories are separated by / and the files are separated by .. The file system is case sensitive. The file system is made up of the following:

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bin   dev  home  lib    lib64   lost+found  mnt  proc  run   snap  sys  usr
boot  etc  init  lib32  libx32  media       opt  root  sbin  srv   tmp  var

The bin folder comtains the binaries of the programs. The etc folder contains the configuration files. The home folder contains the user files. The lib folder contains the libraries. The root folder contains the root user files. The sbin folder contains the system binaries. The tmp folder contains the temporary files. The var folder contains the variable files. The home folder contains the user files. Everything a user executes is stored in the subfolders of the home folder. When the user creates an account on the linux operation system a new folder gets created in the home folder that contains all the information of the user.

Connect devices may be mounted in the /media directory, and can be found out by the lsblk command.

File management

ls: running this commmand will list all the files stored in the current directory or folder
ls -a: running this commmand will list all the files stored in the current directory or folder plus the hidden files
cd: change directory command will help you traverse through the file system. Running cd as a standalone command will take you to the home directory. Running cd .. will take you to the parent directory. Running cd <folder name> will take you to the folder name. Running cd / will take you to the root directory.
pwd: running this command will print the current working directory
mkdir <foldername>: running this command will create a new directory
touch <filename.fileextension>: running this command will create a new file
rm <filename.fileextension>: running this command will remove the file
rm -r <foldername>: running this command will remove the folder
rm -rf <foldername>: running this command will remove the folder and all the files in it including the hidden files
mv <path/file> <path/file>: running this command will move the file, if the path is same then it will rename the file.
cp <path/file> <path/file>: running this command will copy the file
cp -r <path/folder> <path/folder>: running this command will copy the folder
cat <file>: running this command will print the contents of the file
- cat <file> > <file>: running this command will copy the contents of the first file to the second file
- cat <file> >> <file>: running this command will append the contents of the first file to the second file
- cat <file> | grep <string>: running this command will print the lines of the file that contain the string
- cat <file> | grep -v <string>: running this command will print the lines of the file that do not contain the string
- cat <file> | grep -i <string>: running this command will print the lines of the file that contain the string irrespective of the case
- cat <file> | grep -n <string>: running this command will print the lines of the file that contain the string along with the line number
df and df -h can be used to check the occupied storage by the file system

File permissions

The file permissions are the permissions that are assigned to the files and the folders. The file permissions are assigned to the files and the folders by the following command

`1`	`chmod <permissions> <file>`

example

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chmod 777 <file>

This will give the file all the permissions. The file permissions are assigned by the following numbers

0: no permissions
1: execute
2: write
3: write and execute
4: read
5: read and execute
6: read and write
7: read, write and execute

This can also be assigned by the following letters

r: read
w: write
x: execute

The file permissions can be seen by the following command

ls -l

File ownership

The file ownership is the ownership of the file. The file ownership is assigned to the files and the folders by the following command

`1`	`chown <user>:<group> <file>`

example

`1`	`chown john:john <file>`

This will give the file to the user john and the group john.

About the file system

The file system is case sensitive
The file system is hierarchical
The file system is a tree structure
The file system is a multi-user system
The file system is a multi-tasking and multi-threaded system

Important commands to remember

man: running the man as the prefix to any command will give you the documentation of the whole binary/command
-h or --help: running any one of these commnds will give a short listing of the package
-v or --version: running any of of these commands will display the version of software you are using. This command is typically used to verify the installation of packages.

Installation of packages

Apps or packages can be installed in the system via the apt or the apt-get command. Here are the key things to remember.

There is a pool where all the system binaries are stored
These system binaries need to be updated via the following command before installation of every program
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sudo apt update
Binares and packages can be updated in the system by running this command
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sudo apt upgrade
Packages can be installed by this command
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sudo apt install <package name>
Packages can be removed by this command
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sudo apt remove --purge <package name>
and then removing any unused dependencies by running this command
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sudo apt autoremove
Packages can be searched by this command
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sudo apt search <package name>

DPKG

Dpkg is a package manager for Debian based systems. It is used to install, remove, and provide information about .deb packages. Dpkg is a low level tool that interacts with the .deb packages. Some packages may not be available in the apt pool, then the package can be installed with the dpkg command. .deb can be installed with the software manager in ubuntu via double clicking but with cli we can use this command to install the package

`1`	`sudo dpkg -i <package name>`

The package can be removed by this command

`1`	`sudo dpkg -r <package name>`

The package can be removed along with the configuration files by this command

`1`	`sudo dpkg -P <package name>`

Shells

Shells are the command line interpreters that run the commands. There are various shells available in the linux operating system. The default shell in the linux operating system is the bash shell. However the benifit of the linux family is customisablity and hence there are various shells available. The zsh shell is one of the most popular shells. To install this you can follow this link.

Things to show off to your non-linux friends

cmatrix: running this command will show you the matrix effect

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sudo apt update
sudo apt install cmatrix
cmatrix

sl: running this command will show you the train running on the terminal
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sudo apt update sudo apt install sl sl

cowsay: running this command will show you a cow saying something

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sudo apt update
sudo apt install cowsay
cowsay <message>

Processes

When a linux operating system is run then many processes are created, these usually run at the user level rather running at the kernal level.
Processes can be run via the ps

ps

Output

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    PID TTY          TIME CMD
    372 pts/0    00:00:00 zsh
    470 pts/0    00:00:00 ps

An alternative to running the ps command is running the top or htop command. Htop is not natively installed in many systems and can be installed via sudo apt install htop in debian based systems.
To see every process on the system

ps -e

Users

Users are the people who use the system. In linux based operating systems there may be multiple users (can be upto hundereds in a shared system like a server). The users in linux have a unique user id (UID) and a group id (GID). The UID is a number that is assigned to the user and the GID is a number that is assigned to the group. The UID and GID are stored in the /etc/passwd file. Every users has a home directory that is stored in the /home directory. The user may have different privalages that are assigned when the users are created and added to the system.

Adding a user
A user can be added to the system by running the following command
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sudo adduser <username>
Deleting a user
A user can be deleted from the system by running the following command
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sudo deluser <username>
Changing the password of a user
A user can change the password of the user by running the following command
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sudo passwd <username>

Shell scripting

Shell scripting is a way to automate the tasks that are done on the command line. Shell scripts are written in the bash shell. The shell scripts are saved with the .sh extension. The shell scripts can be run by the following command

`1`	`bash <filename.sh>`

The shell scripts can be run in the background by the following command

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./<filename.sh> &

The shell scripts can be run in the background and the output can be redirected to a file by the following command

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./<filename.sh> &> <filename.txt> &

How to create a shell scripting file

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#!/bin/bash
echo "Hello World"

Basic operations

Variables
Variables can be created by the following command
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<variable name>=<value>
Variables can be accessed by the following command
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echo $<variable name>
Demonstration
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#!/bin/bash NAME="John Doe" echo "Hello $NAME"

User input
User input can be taken by the following command

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read <variable name>

Demonstration

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#!/bin/bash
echo "Enter your name"
read NAME
echo "Hello $NAME"

If else
If else can be used to run a command if a condition is true or false

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if <condition>
then
	<command>
else
	<command>
fi

Demonstration

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#!/bin/bash
echo "Enter your name"
read NAME
if [ "$NAME" == "John Doe" ]
then
	echo "Hello $NAME"
else
	echo "You are not John Doe"
fi

Loops
Loops can be used to run a command multiple times

For loop
For loop can be used to run a command multiple times

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for <variable name> in <list>
do
	<command>
done

Demonstration

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#!/bin/bash
for i in 1 2 3 4 5
do
	echo "Hello $i"
done

While loop
While loop can be used to run a command multiple times

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while <condition>
do
	<command>
done

Demonstration

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#!/bin/bash
i=1
while [ $i -le 5 ]
do
	echo "Hello $i"
	i=$(( i+1 ))
done

Comparisons
Comparisons can be used to compare two values
- String comparison
  String comparison can be used to compare two strings
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  if [ "$<variable name>" == "<string>" ] then <command> fi
- Number comparison
  Number comparison can be used to compare two numbers
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  if [ $<variable name> -eq <number> ] then <command> fi
- File comparison
  File comparison can be used to compare two files
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  if [ -f <file name> ] then <command> fi
Greater than or less than
Greater than or less than can be used to compare two numbers
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if [ $<variable name> -gt <number> ] then <command> fi
Greater than or equal to or less than or equal to
Greater than or equal to or less than or equal to can be used to compare two numbers
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if [ $<variable name> -ge <number> ] then <command> fi

Startup scripts

Startup scripts are the scripts that are run when the system is started. The easiest way to automate the startup scripts is to use crontab. Crontab is a time based job scheduler. Crontab can be used to run the scripts at the startup of the system. The crontab can be edited by the following command

`1`	`crontab -e`

A startup script in crontab may look like this

`1`	`@reboot /path/to/script.sh`

Alising

Alising is a way to create a shortcut for a command. Alising can be done by the following command

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alias <alias name>="<command>"

For example I use python3 a lot an I find it hard typing python3 everytime so I can create an alias py for python3 by running the following command

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alias py="python3"

The alias can be removed by the following command

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unalias <alias name>

Aliases can be saved in the .bashrc file. The .bashrc file is a file that is run when the terminal is opened. The .bashrc file is stored in the home directory. The .bashrc file can be edited by the following command

`1`	`nano ~/.bashrc`

Then you may name the alias in the file like this

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alias py="python3"

Foreground and backgorund processes

Processes can be run in the foreground or the background. The foreground processes are the processes that are run in the terminal. The background processes are the processes that are run in the background. The background processes are run by the following command

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<command> &

The background processes can be stopped by the following command

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kill %<job number>

In a linux system with just one terminal the simplest way to run a process in the background is to run the process in the background by the following command

`1`	`<some command>`

Then press ctrl + z to stop the process and then run the following command

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bg

Then the process will run in the background. The process can be stopped by the following command

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kill %<job number>

Users and groups

Definitions

Users: Users are the people who use the system. In linux based operating systems there may be multiple users (can be upto hundereds in a shared system like a server). The users in linux have a unique user id (UID) and a group id (GID). The UID is a number that is assigned to the user and the GID is a number that is assigned to the group. The UID and GID are stored in the /etc/passwd file. Every users has a home directory that is stored in the /home directory. The user may have different privalages that are assigned when the users are created and added to the system.
Groups: Groups are the collection of users. The groups are used to assign the permissions to the users. The groups are stored in the /etc/group file

Commands

Users can be added by the following command
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sudo adduser <username>
Users can be deleted by the following command
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sudo deluser <username>
Users can be added to a group by the following command
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sudo usermod -aG <groupname> <username>
Users can be removed from a group by the following command
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sudo deluser <username> <groupname>
Groups can be added by the following command
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sudo addgroup <groupname>
Groups can be deleted by the following command
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sudo delgroup <groupname>

Services in linux

In Linux, a service is a type of software that runs in the background and performs certain tasks without user intervention. These tasks could be anything from handling requests on a server, monitoring system events, or running scheduled tasks.

Services in Linux are also known as “daemons”. They start at system boot and continue to run until the system is shut down.

You can manage services in Linux using system commands like systemctl, service, or init scripts depending on your Linux distribution and the init system it uses (System V, Upstart, or systemd). For example, you can start, stop, restart, enable, or disable services.

Here’s an example of how you might use systemctl to manage a service:

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# To start a service
sudo systemctl start serviceName

# To stop a service
sudo systemctl stop serviceName

# To restart a service
sudo systemctl restart serviceName

# To enable a service to start on boot
sudo systemctl enable serviceName

# To disable a service from starting on boot
sudo systemctl disable serviceName

Creating a service

Creating a custom service in Linux involves writing a service unit file. This file tells systemd how to manage your service. Here’s a basic example of how to create a custom service using systemd, which is the init system used by most modern Linux distributions.

Create a new service file in /etc/systemd/system with a .service extension. For example, my_service.service. Use sudo because this is a system directory.

`1`	`sudo nano /etc/systemd/system/my_service.service`

In the service file, define the service with [Unit], [Service], and [Install] sections.

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[Unit]
Description=My Custom Service
After=network.target

[Service]
ExecStart=/usr/bin/my_service
Restart=always
User=username
Group=groupname
Environment=PATH=/usr/bin:/usr/local/bin
Environment=NODE_ENV=production
WorkingDirectory=/home/username/my_service

[Install]
WantedBy=multi-user.target

Replace /usr/bin/my_service with the path to the script or executable that your service will run. Replace username and groupname with the user and group that should own the process. Replace /home/username/my_service with the working directory of your service.

After saving and closing the file, reload the systemd manager configuration.

`1`	`sudo systemctl daemon-reload`

Enable your service so that it starts on boot.

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sudo systemctl enable my_service

Start your service.

`1`	`sudo systemctl start my_service`

Check the status of your service.

`1`	`sudo systemctl status my_service`

Remember to replace my_service with the name of your service.

Replace serviceName with the name of the service you want to manage.

Extra commands

Getting user list
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cat /etc/passwd
Get only the usernames
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awk -F':' '{ print $1}' /etc/passwd

Other important services

Transmission: Transmission is a bittorrent client that can be used to download torrents. Transmission can be installed by the following command
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sudo apt update sudo apt install transmission-qt
Known issues of previous transmission versions Some the the previously installed versions may support normal torrent transmissions but may fail in magnet links. This can be fixed by installing the following package
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sudo apt remove transmission-gtk sudo apt install transmission-qt

Yoy may check the references from here.

Apache: Apache is a web server that can be used to host websites. Apache can be installed by the following command
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sudo apt update sudo apt install apache2
Nginx: Nginx is a web server that can be used to host websites. Nginx can be installed by the following command
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sudo apt update sudo apt install nginx

VIM: Vim is a text editor that can be used to edit text files. Vim can be installed by the following command. There may be a chance that it is preinstalled. To check wheter its preinstalled the following command may be used.

`1`	`vim --version`

Any output like the following means that it is already installed

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VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Mar 14 2024 09:05:11)
Included patches: 1-579, 1969, 580-1848, 4975, 5016, 5023, 5072, 2068, 1849-1854, 1857, 1855-1857, 1331, 1858, 1858-1859, 1873, 1860-1969, 1992, 1970-1992, 2010, 1993-2068, 2106, 2069-2106, 2108, 2107-2109, 2109-3995, 4563, 4646, 4774, 4895, 4899, 4901, 4919, 213, 1840, 1846-1847, 2110-2112, 2121

If the output is mentioning it is not installed then this command may be used.

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sudo apt update
sudo apt install vim

You may check the commands here

Misc commands

vcgencmd measure_temp: running this command will give you the temperature of the raspberry pi. Using this with the watch command will give you the temperature in real time (watch vcgencmd measure_temp).
history: running this command will give you the history of all the commands you have run in the terminal.

Linux/UNIX shell commands#

File systems#

File management#

File permissions#

File ownership#

About the file system#

Important commands to remember#

Installation of packages#

DPKG#

Shells#

Things to show off to your non-linux friends#

Processes#

Users#

Shell scripting#

How to create a shell scripting file#

Basic operations#

Startup scripts#

Alising#

Foreground and backgorund processes#

Users and groups#

Definitions#

Commands#

Services in linux#

Creating a service#

Extra commands#

Other important services#

Misc commands#

Linux/UNIX shell commands

File systems

File management

File permissions

File ownership

About the file system

Important commands to remember

Installation of packages

DPKG

Shells

Things to show off to your non-linux friends

Processes

Users

Shell scripting

How to create a shell scripting file

Basic operations

Startup scripts

Alising

Foreground and backgorund processes

Users and groups

Definitions

Commands

Services in linux

Creating a service

Extra commands

Other important services

Misc commands