• A Unix shell is a command language interpreter, the primary purpose of which is to translate commands typed in the terminal into system actions. Hence, the shell itself is a program through which other programs are invoked.
• Standard Unix commands can be combined to use build our own commands. This is achieved by writing a shell script which contains a sequence of commands; this file is then executed as a single command.
• Writing shell scripts can be helpful in automating complex yet periodic tasks.

Knowing Supported Shells and their location

• To know which shells are supported by the Linux system you can use cat /etc/shells command. A sample output of this is -

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

• To know where a shell is located you can use which shell-name command. A sample output of this is -

  $ which bash
  /bin/bash
  

Creating a "Hello world" shell script

<aside> 💡 Giving the .sh extension to a shell is optional. The file can be executed as a shell script without the .sh extension. Using an extension will help editors to provide linting capabilites.

</aside>

<aside> ⚠️ Sometimes when you try to execute a shell script you might face an error saying Permission denied. When this happens, use the chmod command to give the execute permission to the user or to one who wants to execute the script.

</aside>

$ touch hello-world.sh
$ code hello-world.sh

#! /bin/bash
echo "Hello world"

$ ./hello-world.sh
Hello world

• The first line (!# /bin/bash) known as hash bang helps the interpreter know which shell is being used.