Bash Script for Beginners: A Step-by-Step Guide

Bash scripting is a useful and effective tool for system administration and development. It may appear extremely horrifying the first time you do it, but hopefully this guide will help relieve your anxiety.

Bash is a Unix shell, which is a command line interface (CLI) for interacting with an operating system (OS). Any command that you can run from the command line can be used in a bash script. Scripts are used to run a series of commands. Bash is available by default on Linux and macOS operating systems.

This is not intended to be a comprehensive resource to bash scripting, but rather a step-by-step guide to creating your first script and learning some basic bash syntax.

Note: Newer macOS installations (from Catalina) come installed with zsh (Z shell) as the new default, but everything in this article will still be applicable.

PREREQUISITES

• A basic command line knowledge is required.

This guide was created on macOS, and will be using /Users/you as the default user directory for all examples. However, the concepts here will apply to any Unix-like operating system, including macOS and various Linux distributions.

Table of Content

In this tutorial, we’re going to:

• Create a bash script
• Learn about:
  • Strings
  • Variables
  • Shell execution
  • User input
  • Comparison
  • Conditions
  • Loops
  • Arrays

Create Your First Script

Making a bash script is a lot simpler than you might think.

Create a file called first-script, using the touch command.

$ touch first-script

Edit the file with the program of your choice. Within the file, print a string that says “My First Bash Script!’ using echo.

echo "My First Bash Script"

$ bash first-script

That’s it, you’ve created your first script!

Executable Scripts

So far, you’ve learned how to run a script from the command line, prefixed with the bash interpreter. However, if you want to run the script by name alone, it won’t work. Try to run the file simply by typing the name of the file and pressing enter. Note that we’re prefixing the file with ./, which means a file in the current directory.

$ ./first-script

In order to run a file directly, we’ll need to change the permissions to allow the script to be executable for the user. chmod is a command that changes permissions on a file, and +x will add execute rights to the script.

$ chmod +x first-script

In order to interpret the file as an executable, you’ll also have to add the shebang (#!) at the top of the script. In Unix-like systems, a text file with a shebang is interpreted as an executable file. You can confirm where the bash interpreter is located with which bash.

$ which bash

We’ll add #!/bin/bash to the top of the script.

#!/bin/bash

echo "My First Bash Script"

$ ./first-script

Note: In order to run a bash script without specifying the directory (using ./, for example) you would have to add the directory of the script to the PATH by running export PATH=$PATH:/path/to/script/directory. However, this is generally not necessary for personal scripts.

Strings

A simple string in Bash does not require double quotes – you can write it directly.

echo My first bash script

A single or double quote will expect a closing match, so in order to use one in such a string, you would need to escape the quote

echo I\'m a script

However, if you want to use a single or double quote in a string without escaping characters, you can do so by wrapping your string in quotes.

echo 'A single quoted "string"'
echo "A double quoted 'string'"

With the -e flag, bash will interpret strings with backslash-escaped characters, such as \n for newline. This also requires a quoted string.

echo -e "This string has a \nnew line"

Double quoted strings are also important for use with variables, as we’ll see in the next section.

Variables

A variable is declared without a dollar sign ($), but has one when invoked. Let’s edit our my first bash script example to use a variable for the entity being greeted, which is bash script.

#!/bin/bash

filename="bash script"

echo "My first $filename"

Double quoted strings are required for interpolating variables. Within a single quoted string, the dollar sign would be interpreted literally

echo 'My first $filename'

echo "My first ${filename}!"

echo "Hello, $(whoami)!"

#!/bin/bash

echo 'Who are you?'

read who

echo "Hello, $who!"

Comparison

Operators are slightly different in bash than what you might be used to.

In order to compare numbers, you will use the operators in the number comparison column, such as -lt for less than.

In order to compare strings, you will use the operators in the string comparison column, such as < for less than.

This is the opposite of what you might expect, but it’s the way it works in bash.

You can also use -z to test for emptiness on a string.

Conditions

if statements use the if, then, else, and fi keywords. The condition goes in square brackets.

#!/bin/bash

echo 'How old are you?'

read age

if [ $age -gt 20 ]
then
    echo 'You can drink.'
else
    echo 'You are too young.'
fi

Loops

Bash uses for, while, and until loops. In this example, I’ll use the for...in loop to get all the files in a directory and list them.

#!/bin/bash

files=/Users/you/dev/*

for file in $files
do
  echo $(basename $file)
done

Arrays

An array in bash is defined inside parentheses. There are no commas between the items of the array.

anime=('Naruto' 'One Pieace' 'Black Clover' 'Attack on title')

To access an item from an array, you’ll use square brackets ([]). Arrays are 0-indexed in bash. It is also necessary to use the parameter expansion syntax.

echo ${anime[3]}

Conclusion

I hope that this article has been helpful in getting you started with bash scripting. The idea of a script having complete access to everything on my computer was initially frightening to me, but once I got used to it, I realized how useful and efficient it can be.

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