Bash Shebang

Introduction

If you're learning Bash scripting by reading other people's code, you've probably observed that the scripts start with the #! characters and the path to the Bash interpreter on the first line.

Shebang is a string of characters (#!) that tells the operating system which interpreter to use to parse the rest of the file.

In this tutorial, you will understand Bash Shebang. We will also address a few FAQs on Bash Shebang.

Shebang Interpreter Directive

The Shebang interpreter instruction is written as follows:

#!interpreter [arguments]

The directive must be the script's first line.
The directive must start with shebang #!.
The use of white space after the shebang letters is optional.
The whole path of a binary file (for example, /bin/sh, /bin/bash) is called the interpreter.
Arguments to the interpreter are optional.

Examples:

#!/bin/bash - Parses the file with bash.
#!/usr/bin/env perl - Finds the path of the perl executable using the env command.
#!/usr/bin/python The python binary is used to run the file.

Using Shebang in Bash Script

If no shebang is given and the person running the Bash script is using another Shell, the script will be interpreted using that Shell's default interpreter. For example, bash's default interpreter is bash, and zsh default interpreter is sh. To ensure that your script is always evaluated with Bash, use the shebang command to specify the executable path.

The Shebang directive can be used in two ways to set the interpreter.

Using the bash binary's absolute path:

#!/bin/bash

Using the env command:

#!/usr/bin/env bash

The second approach has the advantage of looking for the bash executable in the user's $PATH environment variable. If there are many paths to bash, the script will use the first one.

Pass it to the interpreter when using the first option to add an option to the Bash shell supply. #!/bin/bash -x, for example, can be used to run the script in debug mode. If you're using the env method, you'll need to declare the option with set. After the shebang line, add set -x to enable the debug mode.

Example Script

Let's make a basic script that prints "Hello, World" using shebang. Copy and paste the following line into your text editor:

nano hello_world

#!/bin/bash

echo "Hello, World"

To run the script without having to provide the interpreter from the command line, make the file executable:

chmod +x hello_world

If you type ./ followed by the script name, you may now run it:

./hello_world

Output

Hello, World

Overriding the Shebang

If you want to override the interpreter specified in the Shebang line for some reason, you must run the script explicitly specifying the desired shell.

For example, to use the bash shell to run a script that has #!/bin/sh provided in the Shebang line, type:

bash hello_world

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Overriding the shell interpreter is not a good idea because it may cause the script to behave unexpectedly.

FAQs on Bash Shebang

Why is a Bash Shebang important?

By using a Bash Shebang, you can ensure that your Bash script is executed by the right interpreter, even if the script is not explicitly invoked by typing bash scriptname on the command line.

What is the proper syntax for a Bash Shebang?

The proper syntax for a Bash Shebang is #! /bin/bash, where /bin/bash is the path to the Bash interpreter.

Can I use other interpreters besides Bash in a shebang?

Yes, you can use other interpreters by replacing /bin/bash with the desired interpreter's path. For example, #! /bin/python specifies Python as the interpreter.

Can I specify command-line arguments in a shebang?

Yes, you can include command-line arguments in a shebang. For instance, #! /bin/bash -e includes the -e option, which enables the script to exit immediately on error.

Do I always need to use a shebang in my Bash scripts?

No, a shebang is not mandatory. However, it is considered good practice to use one to ensure proper execution and avoid potential issues when the script is run.

What happens if the shebang points to an invalid interpreter?

If the shebang points to an invalid interpreter, an error will occur when trying to execute the script. Make sure the interpreter's path is correct.

Can I use a relative path in a shebang?

It is recommended to use an absolute path in a shebang, but you can use a relative path. However, the relative path might not work if the script is executed from a different directory.