= POSIX sh and shebangs

date: 2021-01-17

layout: post

lang: en

ref: posix-sh-and-shebangs

---

As I [keep moving][posix-awk-0] [towards POSIX][posix-awk-1], 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:
```shell
#!/usr/bin/env bash
set -Eeuo pipefail
cd "$(dirname "${BASH_SOURCE[0]}")"
```

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:
```shell
#!/bin/sh -eu
```

I moved the `-eu` options to the shebang line itself, striving for conciseness.
But as I changed callers from `./script.sh` to `sh script.sh`, things started to fail.
Some tests that should fail reported errors, but didn't return 1.

My first reaction was to revert back to `./script.sh`, but the POSIX bug I caught is a strong strain, and when I went back to it, I figured that the callers were missing some flags.
Specifically, `sh -eu script.sh`.

Then it clicked: when running with `sh script.sh`, the shebang line with the sh options is ignored, as it is a comment!

Which means that the shebang most friendly with POSIX is:

```shell
#!/bin/sh
set -eu
```

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.

[posix-awk-0]: {% link _tils/2020-12-15-awk-snippet-shellcheck-all-scripts-in-a-repository.md %}
[posix-awk-1]: {% link _tils/2021-01-12-awk-snippet-send-email-to-multiple-recipients-with-curl.md %}
