Add ShellCheck

Signed-off-by: yubiuser <github@yubiuser.dev>

.github/workflows/shellcheck.yml

@@ -0,0 +1,21 @@
+name: ShellCheck
+on:
+  pull_request:
+    types: [opened, synchronize, reopened, ready_for_review]
+
+jobs:
+  shellcheck:
+    if: github.event.pull_request.draft == false
+    runs-on: ubuntu-latest
+    steps:
+    -
+      name: Checkout repository
+      uses: actions/checkout@v4.2.2
+      with:
+        fetch-depth: 0 # Differential ShellCheck requires full git history
+
+    - name: Differential ShellCheck
+      uses: redhat-plumbers-in-action/differential-shellcheck@v5
+      with:
+        severity: warning
+        display-engine: sarif-fmt

.shellcheckrc

@@ -0,0 +1,6 @@
+external-sources=true # allow shellcheck to read external sources
+disable=SC3043 #disable SC3043: In POSIX sh, local is undefined.
+enable=useless-use-of-cat # disabled by default as of shellcheck 0.11.0
+enable=avoid-negated-conditions # avoid-negated-conditions is optional as of shellcheck 0.11.0
+enable=require-variable-braces
+enable=deprecate-which

padd.sh

@@ -1,9 +1,4 @@
 #!/usr/bin/env sh
-# shellcheck disable=SC1091
-
-# Ignore warning about `local` being undefinded in POSIX
-# shellcheck disable=SC3043
-# https://github.com/koalaman/shellcheck/wiki/SC3043#exceptions
 
 # PADD
 # A more advanced version of the chronometer provided with Pihole