Safer Bash Shebang Recipes

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

  #!/usr/bin/env bash
  set -euxo pipefail
  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.

Shebang Recipe Execution on Windows

On Windows, shebang interpreter paths containing a / are translated from Unix-style paths to Windows-style paths using cygpath, a utility that ships with Cygwin.

For example, to execute this recipe on Windows:

  echo "Hello!"

The interpreter path /bin/sh will be translated to a Windows-style path using cygpath before being executed.

If the interpreter path does not contain a / it will be executed without being translated. This is useful if cygpath is not available, or you wish to pass a Windows-style path to the interpreter.