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+= POSIX sh and shebangs
+
+:awk-1: link:../../../2020/12/15/shellcheck-repo.html
+:awk-2: link:../12/curl-awk-emails.html
+
+As I {awk-1}[keep moving] {awk-2}[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:
+
+[source,sh]
+----
+#!/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:
+
+[source,sh]
+----
+#!/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:
+
+[source,sh]
+----
+#!/bin/sh
+set -eu
+----
+
+. when running via `./script.sh`, if the system has an executable at `/bin/sh`,
+  it will be used to run the script;
+. when running via `sh script.sh`, the sh options aren't ignored as previously.
+
+TIL.