diff --git a/build_tools/git_version_gen.sh b/build_tools/git_version_gen.sh
index 791ac8ff7..b7faccbf4 100755
--- a/build_tools/git_version_gen.sh
+++ b/build_tools/git_version_gen.sh
@@ -1,11 +1,14 @@
-#!/bin/bash
+#!/bin/sh
 # Originally from the git sources (GIT-VERSION-GEN)
 # Presumably (C) Junio C Hamano <junkio@cox.net>
 # Reused under GPL v2.0
 # Modified for fish by David Adam <zanchey@ucc.gu.uwa.edu.au>
 
+# Obtain directory containing this script in POSIX-compatible manner
+# See https://stackoverflow.com/a/43919044/17027 (public domain)
+a="/$0"; a="${a%/*}"; a="${a:-.}"; a="${a#/}/"; BASEDIR=$(cd "$a"; pwd)
 # Find the fish git directory as two levels up from this directory.
-GIT_DIR=$(dirname $(dirname "${BASH_SOURCE[0]}"))
+GIT_DIR=$(dirname $(dirname "$a"))
 
 FBVF=FISH-BUILD-VERSION-FILE
 DEF_VER=unknown