Allow running this repo as a pre-commit hook

.dockerignore

@@ -1,6 +1,3 @@
-*
-!LICENSE
-!Setup.hs
-!ShellCheck.cabal
-!shellcheck.hs
-!src
+dist
+dist-newstyle
+.git

.pre-commit-hooks.yaml

@@ -0,0 +1,6 @@
+- id: shellcheck
+  name: shellcheck
+  description: Static analysis tool for shell scripts
+  types: [shell]
+  language: docker
+  entry: shellcheck

Dockerfile

@@ -0,0 +1,18 @@
+# This file builds ShellCheck for pre-commit.
+#
+# It may also be useful for local development, but it is notably NOT
+# used to build the official ShellCheck docker images.
+
+FROM ubuntu:20.04
+RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y cabal-install
+RUN cabal update
+
+# Install dependencies separately for more efficient iteration
+COPY ShellCheck.cabal /build/
+RUN cd /build && cabal install --dependencies-only
+
+# Now build the rest
+COPY . /build/
+RUN cd /build && cabal build shellcheck
+RUN find /build -type f -name shellcheck -perm /111 -exec cp -f {} /bin \;
+ENTRYPOINT ["/bin/shellcheck"]

test/distrotest

@@ -1,6 +1,9 @@
 #!/bin/bash
 # This script runs 'buildtest' on each of several distros
 # via Docker.
+
+# shellcheck disable=SC2016
+
 set -o pipefail
 
 exec 3>&1 4>&2
@@ -32,6 +35,33 @@ echo "Logging to $log" >&3
 exec >> "$log" 2>&1
 
 final=0
+
+echo "Trying to build pre-commit docker image"
+if ! docker build --tag precommit .
+then
+  final=1
+  echo "pre-commit image failed to build"
+else
+  if printf '%s\n' '#!/bin/sh' 'echo $1' | docker run -i precommit -
+  then
+    final=1
+    echo "pre-commit image succeeds with incorrect example"
+  fi
+
+  if ! printf '%s\n' '#!/bin/sh' 'echo "$1"' | docker run -i precommit -
+  then
+    final=1
+    echo "pre-commit image fails with correct example"
+  fi
+fi
+
+if [[ $final -ne 0 ]]
+then
+  echo >&3 "pre-commit image failure, see log"
+else
+  echo >&3 "pre-commit image succeeded"
+fi
+
 while read -r distro setup
 do
   [[ "$distro" = "#"* || -z "$distro" ]] && continue