ci: add shellcheck job and script

Add FIXME for OS/400 scripts.

Cherry-picked from #1358

.github/workflows/ci.yml

@@ -26,6 +26,14 @@ jobs:
       - name: 'checksrc'
         run: ./ci/checksrc.sh
 
+  shellcheck:
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v4
+      - name: 'shellcheck'
+        run: ./ci/shellcheck.sh
+
   spellcheck:
     runs-on: ubuntu-latest
     steps:

ci/shellcheck.sh

@@ -0,0 +1,12 @@
+#!/bin/sh
+# Copyright (C) Viktor Szakats
+#
+# SPDX-License-Identifier: curl
+
+# FIXME: os400/* scripts
+
+shellcheck --version
+# shellcheck disable=SC2046
+shellcheck --exclude=1091 \
+  --enable=avoid-nullary-conditions,deprecate-which \
+  $(grep -l -E '^#!(/usr/bin/env bash|/bin/sh|/bin/bash)' $(git ls-files | grep -v -F 'os400/'))