Using Other Languages

Recipes that start with #! are called shebang recipes, and are executed by saving the recipe body to a file and running it. This lets you write recipes in different languages:

polyglot: python js perl sh ruby nu

python:
  #!/usr/bin/env python3
  print('Hello from python!')

js:
  #!/usr/bin/env node
  console.log('Greetings from JavaScript!')

perl:
  #!/usr/bin/env perl
  print "Larry Wall says Hi!\n";

sh:
  #!/usr/bin/env sh
  hello='Yo'
  echo "$hello from a shell script!"

nu:
  #!/usr/bin/env nu
  let hello = 'Hola'
  echo $"($hello) from a nushell script!"

ruby:
  #!/usr/bin/env ruby
  puts "Hello from ruby!"

$ just polyglot
Hello from python!
Greetings from JavaScript!
Larry Wall says Hi!
Yo from a shell script!
Hola from a nushell script!
Hello from ruby!

On Unix-like operating systems, including Linux and MacOS, shebang recipes are executed by saving the recipe body to a file in a temporary directory, marking the file as executable, and executing it. The OS then parses the shebang line into a command line and invokes it, including the path to the file. For example, if a recipe starts with #!/usr/bin/env bash, the final command that the OS runs will be something like /usr/bin/env bash /tmp/PATH_TO_SAVED_RECIPE_BODY.

Shebang line splitting is operating system dependent. When passing a command with arguments, you may need to tell env to split them explicitly by using the -S flag:

run:
  #!/usr/bin/env -S bash -x
  ls

Windows does not support shebang lines. On Windows, just splits the shebang line into a command and arguments, saves the recipe body to a file, and invokes the split command and arguments, adding the path to the saved recipe body as the final argument. For example, on Windows, if a recipe starts with #! py, the final command the OS runs will be something like py C:\Temp\PATH_TO_SAVED_RECIPE_BODY.

Safer Bash Shebang Recipes

If you're writing a bash shebang recipe, consider adding set -euxo pipefail:

foo:
  #!/usr/bin/env bash
  set -euxo pipefail
  hello='Yo'
  echo "$hello from Bash!"

It isn't strictly necessary, but set -euxo pipefail turns on a few useful features that make bash shebang recipes behave more like normal, linewise just recipe:

• set -e makes bash exit if a command fails.

• set -u makes bash exit if a variable is undefined.

• set -x makes bash print each script line before it's run.

• set -o pipefail makes bash exit if a command in a pipeline fails. This is bash-specific, so isn't turned on in normal linewise just recipes.

Together, these avoid a lot of shell scripting gotchas.