Add TIL on POSIX sh shebangs

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+---
+
+title: 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;
+1. 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 %}