POSIX sh and shebangs

Posted on

As I keep moving towards POSIX, I’m on the process of migrating all my Bash scripts to POSIX sh.

As I dropped [[, arrays and other Bashisms, I was left staring at the first line of every script, wondering what to do: what is the POSIX sh equivalent of #!/usr/bin/env bash? I already knew that POSIX says nothing about shebangs, and that the portable way to call a POSIX sh script is sh script.sh, but I didn’t know what to do with that first line.

What I had previously was:

1
2
3
 
#!/usr/bin/env bash
set -Eeuo pipefail
cd "$(dirname "${BASH_SOURCE[0]}")"

plaintext

Obviously, the $BASH_SOURCE would be gone, and I would have to adapt some of my scripts to not rely on the script location. The -E and -o pipefail options were also gone, and would be replaced by nothing.

I converted all of them to:

1
2
 
#!/bin/sh
set -eu

plaintext

  1. when running via ./script.sh, if the system has an executable at /bin/sh, it will be used to run the script;

  2. when running via sh script.sh, the sh options aren’t ignored as previously.

TIL.