What is a Shell in Linux?

Shell is an interface that directly interacts with the kernel by accepting a set of commands that is submitted by the user or process.

There are many types of shells like fish, Zsh, Csh, etc., exists. Bash is the most popular one.

Bash (Bourne Again Shell) is derived from SH (Bourne Shell).

The Bourne shell is designed to comply with POSIX standards and many popular shells like Bash, Dash, Zsh, etc., are derived from the Bourne shell. As time flies, a more rich set of features has been added to bash and which made it not comply with POSIX standards.

Initially, bourne shell(sh) is symlinked to bourne again shell(bash,) but now it is symlinked to dash shell.

So if you are trying to write scripts using bourne shell(sh) thinking it is pointed to bash then it may not be correct.

Check before writing your scripts on how bourne shell is implemented in your machine.

$ ls -ln /usr/bin/bash /usr/bin/sh
-rwxr-xr-x 1 0 0 1404744 Mar 19  2021 /usr/bin/bash
lrwxrwxrwx 1 0 0       4 Mar 24  2021 /usr/bin/sh -> dash

You can run the following commands to check information about your shell environment.

To check the list of installed shells in your operating system run the following command. The list of installed shells may vary according to your operating system.

$ cat /etc/shells
# /etc/shells: valid login shells
/bin/sh
/bin/bash
/usr/bin/bash
/bin/rbash
/usr/bin/rbash
/bin/dash
/usr/bin/dash
/usr/bin/sh
/usr/bin/tmux

To check the current shell run the following command:

$ echo $SHELL
/usr/bin/bash

To check the version of the bash shell, run the following command:

$ bash --version

Sample output:

GNU bash, version 5.1.4(1)-release (x86_64-pc-linux-gnu)
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

File Extensions

Linux treats everything as a file and it is not mandatory to add extensions(.sh or .bash) for your script.

Though it is not mandatory, I would suggest you add an extension to your shell script, so you can come to know what type of file you are dealing with.

When creating files in Linux try to avoid space between files and add underscore as the separator.

The reason is when there is a space between file names you either have to add double quotes or use the backslash.

 $ touch /home/${USER}/Documents/"first script.sh"  

 $ ls -l /home/${USER}/Documents/first\ script.sh -rw-rw-r-- 1 karthick karthick 0 Oct 21 22:55 '/home/karthick/Documents/first script.sh'  

$ ls -l /home/${USER}/Documents/"first script.sh" -rw-rw-r-- 1 karthick karthick 0 Oct 21 22:55 '/home/karthick/Documents/first script.sh'

What is Shebang?

Shebang is the first line you write when you create the scripts. Shebang points to the absolute path for the interpreter.

For example, if you are writing bash scripts, the shebang line should be as follows.

#!/usr/bin/bash 

Or,

#!/bin/bash

You can also find the interpreter path by running the type command.

$ type -a bash
bash is /usr/bin/bash
bash is /bin/bash

Shebang is not tied with bash scripting only.

Even if you create programs using Python, Perl, or any other language, you have to write shebang pointing to the interpreter.

#!/usr/bin/python3
#!/usr/bin/perl

It is also possible to create and run the script without shebang, but as a good practice always try to include shebang as the first line in your script.

Absolute Path and Relative Path

Before running your first script, you should understand what is the difference between absolute and relative path.

Absolute Path - Points to the full path for the file starting from the top of the hierarchy(/).

For example, if you have the script named "first_script.sh" under your desktop then the absolute path will be

"/home/username/Desktop/first_script.sh".

Relative Path - In the relative path, the file is present in the current working directory. In every directory, you can find two directories(. and ..).

Single dot points to the current directory you are in.

Double dot points to one directory above your current directory.

In other words, single dot represents current directory and double dots represent parent directory.

Steps to Create and Run Your First Bash Script

1. Create a new file with the .sh extension.

You can create the file directly from text editors and save it. I am using the touch command to create the file.

$ touch /home/${USER}/first_script.sh

2. Grant execute permission to the script.

$ chmod u+x /home/${USER}/first_script.sh

3. Copy paste below piece of code and save it.

#!/usr/bin/bash

echo "Howdy. This is your first step in learning bash scripting"

4. Run the script. There are different ways to trigger the script. As mentioned in the previous section, either you can call the script using the absolute path or relative path.

# Trigger using the full path.
$ /home/${USER}/first_script.sh

In the relative path, you have to add a dot .) and forward slash(/) to point to the current directory followed by the script name.

# Trigger using a relative path.
$ cd /home/${USER}/
$ ./first_script.sh

Importance of Comments

Comments are very important in bash scripts. It will help you or someone you collaborate with to understand what exactly the script is created for and how it works.

In bash, you can use the # symbol (hash) to create comments. Other than the shebang line any line you prefix with # symbol will be treated as comments by the bash interpreter.

Take a look at the below image. Don’t focus on what the program does and just focus on comments sections.

1. Included the script-related information like author, script creation date, version, etc. According to your requirement, you can add more information.

   
   

2. Block comments will help you understand the code that comes after the comment.

   
   

3. Inline comments are added at the same line as the statement. It tells you what that particular line is doing. Here my inline comment shows it is calling the dummy function.

   
   

Down the line when you write a lot of bash scripts you will come to know the best practices for writing comments.

Conclusion

This is an introduction article for bash scripting where important topics like file extensions, shebang, and comments are covered to make you write your first shell script.

Bash scripting is something you cannot neglect if you are working in a Linux environment.

If you already have programming experience, learning bash will not be a hurdle.