A shell script is nothing but a set of commands.
The first step is to open the terminal and select a shell. Let’s go with the bash shell, the default, and widely used shell in most distributions.
Unlike the other commands we type in the shell, we first need to create a file using any text editor for the script. The file must be named with an extension .sh, the default extension for bash scripts. The following terminal shows the script file creation:
Create Script File
user@ubuntu:~$ nano first_script.sh
Every script should start from shebang. Shebang is a combination of some characters that are added at the beginning of a script, starting with #! followed by the name of the interpreter to use while executing the script. As we are writing our script in bash, let’s define it as the interpreter in the shebang.
first_script.sh
#!/bin/bash
We are all set to write our first script now. There are some fundamental building blocks of a script that together make an efficient script. Let’s learn and utilize these script constructs to write one script ourselves.
A variable stores a value inside it. Suppose you need to use some complex values, like a URL, a file path, etc., several times in your script. Instead of memorizing and writing them repeatedly, you can store them in a variable and use the variable name wherever you need it.
The script below displays a string on the screen: "Hey, what’s your name?” This is done by echo command. The second line of the script contains the code read name. read is used to take input from the user, and name is the variable in which the input would be stored. The last line uses echo to display the welcome line for the user, along with its name stored in the variable.
# Defining the Interpreter
#!/bin/bash
echo "Hey, what’s your name?"
read name
echo "Welcome, $name"
Now, save the script by pressing CTRL+X. Confirm by pressing Y and then ENTER.
To execute the script, we first need to make sure that the script has execution permissions. To give these permissions to the script, we can type the following command in our terminal:
Execution Permission to Script
user@ubuntu:~$ chmod +x first_script.sh
Now that the script has execution permissions use ./ before the script name to execute it. We use ./ before the script to run rather than typing the script name directly because ./ tells the shell to execute the file that is present in the current directory. If you don't define ./ before the script name, the shell will search the script in the PATH environment variable (that contains all the directories except the current one), and it will not find the defined script in any of those directories and generate an error. The below terminal shows the script in which we utilized the variables:
Script Execution