How to Create and Use Bash Scripts
Tags: bash, scripts, terminal, zsh
Published: 2020-07-31 05:35:59

<!-- TOC -->

- [How to Create and Use Bash Scripts](#how-to-create-and-use-bash-scripts)
  - [Prerequisites](#prerequisites)
  - [Goals](#goals)
  - [Create Your First Script](#create-your-first-script)
  - [Executable Scripts](#executable-scripts)
  - [Strings](#strings)
  - [Variables](#variables)
  - [Shell Execution](#shell-execution)
  - [User Input](#user-input)
  - [Comparison](#comparison)
  - [Conditions](#conditions)
  - [Loops](#loops)

<!-- /TOC -->

Bash scripting is an extremely useful and powerful part of system administration and development. It might seem extremely scary the first time you do it, but hopefully this guide will help ease the fear.

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 meant to be an extensive guide to bash scripting, but just a straightforward guide to getting started with making your first script, and learning some basic bash syntax.

> **Note:** New 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/Username` 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.

## Goals

In this tutorial, we're going to:

- Create a bash script
- Learn about:
  - [Strings](#strings)
  - [Variables](#variables)
  - [Shell execution](#shell-execution)
  - [User input](#user-input)
  - [Comparison](#comparison)
  - [Conditions](#conditions)
  - [Loops](#loops)
  - [Arrays](#arrays)

## Create Your First Script

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

Create a file called `hello-world`, using the `touch` command.

```sh
$ touch hello-world
```

Edit the file with the program of your choice. Within the file, print a string that says "Hello, world!' using `echo`.

`hello-world`:

```sh
echo "Hello, world!"
```

Now from the command line, run the script using the `bash` interpreter:

```sh
$ bash hello-world
```

You'll see the script has run successfully from the output.

```sh
> Hello, world!
```

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.

```sh
$ ./hello-world
```

```sh
> bash: ./hello-world: Permission denied
```

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.

```sh
$ chmod +x hello-world
```

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.

```sh
$ which bash
```

```sh
> /bin/bash
```

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

```sh
#!/bin/bash

echo "Hello, world!"
```

> **Note:** You may also see `#!/usr/bin/env bash` instead, which can be used if you don't know the exact path for bash.

Now you can run hello-world directly.

```sh
$ ./hello-world
```

```sh
> Hello, world!
```

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.

```sh
echo Just a regular string
# Just a regular string
```

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.

```sh
echo I\'m a string
# I'm a string
```

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.

```sh
echo 'A single quoted "string"'
echo "A double quoted 'string'"
# A single quoted "string"
# 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.

```sh
echo -e "This string has a \nnew line"
# This string has a
# new 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 hello-world example to use a variable for the entity being greeted, which is World.

`hello-world`:

```sh
#!/bin/bash

who="World"

echo "Hello, \$who!"
# Hello, World!
```

Note that `who = "World"` with a space between the assignment is not valid - **_there must not be a space between variable and value_**.

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

```sh
echo 'Hello, $who!'
# Hello, $who!
```

Another way you might see variables written is surrounded by curly brackets along with the dollar sign, which is known as **parameter expansion**.

```sh
echo "Hello, \${who}!"
```

This syntax is necessary for anything more complex you might do with a variable, such as getting one item from an array.

## Shell Execution

If you would like to use the output of a shell execution within a string, you can do so with a dollar sign followed by parentheses, `\$()`. For example the `whoami` command will print out your current user. To use it within a string, wrap `whoami` in the shell execution syntax:

```sh
echo "Hello, \$(whoami)!"
# Hello, username!
```

## User Input

We declared a variable in the last example, but we can also have the user set the value of a variable dynamically. For example, instead of just having the script say Hello, World!, we can make it ask for the name of the person calling the script, then output that name. We'll do this using the read command.

`hello-world`:

```sh
#!/bin/bash

echo 'Who are you?'

read who

echo "Hello, \$who!"
```

And then try running the script:

```sh
> Who are you?
$ Connor
> Hello, Connor!
```

## Comparison

> NOTE: 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.

| Number Comparison | String Comparison | Description           |
| :---------------: | :---------------: | --------------------- |
|       `-eq`       |       `==`        | Equal                 |
|       `-ne`       |       `!=`        | Not equal             |
|       `-gt`       |        `>`        | Greater than          |
|       `-ge`       |       `>=`        | Greater than or equal |
|       `-lt`       |        `<`        | Less than             |
|       `-le`       |       `<=`        | Less than or equal    |

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.

`check-id`:

```sh
#!/bin/bash

echo 'How old are you?'

read age

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

If we run the script, we'll get something like this:

```sh
> How old are you?
$ 30
> You can drink.
```

## 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.

`list-files`:

```sh
#!/bin/bash

files=/Users/Username/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 (unless specified within the script, using a parameter called `IFS=`).

```sh
$ beatles=('John' 'Paul' 'George' 'Ringo')
```

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.

```sh
echo \${beatles[3]}
Ringo
```