In this guide, check out what shebang bash is and how to use it.
Shebang Bash:
In bash scripting, Shebang is a way to declare what interpreter the operating system will use to parse the rest of the file. The Shebang is characterized by the set of characters “#!” (without quotes).
Here’s a quick breakdown of the Shebang interpreter directive.
For example, if a script is to be parsed using the Bash shell, then the Shebang interpreter directive would look like this.
The Shebang interpreter directive has certain properties.
- It must be the first line of the script.
- It must start with a shebang (#!).
- There may or may not be whitespace after the shebang (#!).
- The interpreter will be the full path to a binary file.
- There may or may not be interpreter arguments.
Here’s a shortlist of some of the most common Shebang interpreter directives.
- #!/bin/bash: The script will be parsed using bash.
- #!/usr/bin/python: The script will be parsed using the python binary.
- #!/usr/bin/env perl: The script will be parsed using the perl executable. The location of the perl executable will be provided by the env command.
Using Shebang Bash:
Scripts can have no Shebang bash. Any such script will be parsed using the default interpreter. For example, bash is the default interpreter for bash and sh for zsh. While most of the UNIX/Linux systems have bash as the default, users have options to use others. In such a scenario, without declaring the interpreter, the script may not perform its preferred task.
There are two methods of using the Shebang directive to declare the interpreter. The first one is to declare the file path to the executable.
Another method is to use the env utility to locate the executable.
The benefit of using the env utility is, it will look for the executable under the $PATH environment variable of the current user. In this example, env will look for the bash. If there is more than one bash executable declared in the $PATH variable, then the first one will be used.
As mentioned, Shebang bash also supports executable arguments. For example, to use bash with debug mode, the Shebang interpreter directive would look like this.
When it comes to using env for the Shebang bash, adding an executable argument requires using the “set” option. For example, the following one will use bash with debug mode enabled.
$ set -x
Script Example:
We’ve discussed the basics of Shebang bash. It’s time to put it into practice. Let’s have a look at the implementation of Shebang bash.
Launch the text editor of your choice and type the following script:
$ echo "hello_world_bash"
Save the file. To run the script, it has to be marked as an executable. Run the following command:
Run the script:
Not very difficult, right? Now, let’s try using the different Shebang bash expressions. Try the ones given below:
$ set -x
$ echo "hello_world_bash_debug"
Overriding the Shebang Bash:
While the Shebang bash defines the interpreter to use, in certain situations, you may want to use a different interpreter. It’s possible to override the script-defined interpreter by explicitly specifying the interpreter to the shell.
For example, have a look at the following script:
$ echo "hello_world_sh"
By default, it would be run using sh. To run it using bash, use the following method:
Note that for normal use cases just using the default sh or bash interpreter is good enough and overriding is not relevant.
Final Thoughts:
Shebang bash is a very simple concept. For bash scripting, it’s very important to understand and implement it.
Interested more in Bash scripting? Check out this beginner’s guide on how to write a simple bash script.
Happy computing!
