Shell scripting for beginners: your first Bash script, step by step

A shell script sounds advanced. It isn't. A shell script is just a file full of commands you would otherwise type one by one. If you can type echo hello in a terminal, you can write a script.

Let's build one from nothing, and fix every error you will hit on the way.

1. Create the script

Open a new file with nano:

nano my-script

Type one command and save (Ctrl + O, Enter, then Ctrl + X to exit):

echo hello

No .sh extension needed. Unix doesn't care about extensions, though .sh is a nice hint for humans.

2. Run it, and fix "permission denied"

There's no x anywhere in the permissions, so nobody is allowed to execute it. Add execute permission for your user:

Add more lines and they run top to bottom:

echo hello
echo hello 2
ls

That's genuinely all a script is. Everything below is just making it smarter.

3. The shebang: say which shell runs it

The very first line of a script should be a shebang: #! (hash + bang) followed by the path of the interpreter that should run the file.

#!/bin/bash
echo hello

Find the path of any shell or interpreter with which:

Without a shebang, the script runs with whatever shell you happen to be in. That's fine until a teammate runs it from a different shell. The same trick works for other languages: #!/usr/bin/env python3 or #!/usr/bin/env node.

Curious which shell is active right now? echo $0 tells you. Type bash to start Bash inside Zsh, and exit to come back.

4. Variables

Declare with name=value (no spaces around =), read with $name:

#!/bin/bash
greet="hello"
echo "The greeting is $greet"
echo $greet

Without the $, echo greet just prints the word "greet". You already use variables like this every day: $USER, $HOME and $PATH are environment variables.

5. Command substitution: store a command's output

Wrap a command in $(...) and its output gets substituted in place:

#!/bin/bash
ls_output=$(ls)
echo "The command output is:"
echo "$ls_output"
 
echo "Today's date is $(date)"

date is a command, not a variable, so echo $date prints nothing. $(date) runs it.

6. User input: read and arguments

Ask while the script runs with read:

#!/bin/bash
echo "What is your name?"
read username
echo "Welcome $username"

Or pass arguments when you start it. They arrive as $1, $2 and so on:

#!/bin/bash
echo "Argument 1 is $1"
echo "Argument 2 is $2"

7. export and alias

export makes a variable available to every program and script you start from this session:

Without export, child processes like your script can't see it.

alias creates your own shortcut command:

alias ld='ls -ld'
ld my-script

What about if statements and loops?

Honest advice: almost nobody memorises Bash syntax for if, loops and string tricks. Keep a cheat sheet open. devhints.io/bash is excellent, and so is our free Linux command line cheat sheet.

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