Add support as pre-commit hook

Changes:
* .pre-commit-hooks.yaml: defines this shellcheck repo as a hook for pre-commit
* .travis.yml: make sure .pre-commit-hooks.yams is in sync with release

.pre-commit-hooks.yaml

@@ -0,0 +1,7 @@
+# See https://pre-commit.com for more information
+-   id: shellcheck
+    name: shellcheck shell scripts
+    description: Validate shell scripts with shellcheck running in Docker
+    language: docker_image
+    entry: koalaman/shellcheck:v0.7.1
+    types: [shell]

.travis.yml

@@ -65,6 +65,17 @@ before_install: |
   test "$TRAVIS_BRANCH" = master && TAGS="$TAGS latest" || true
   test -n "$TRAVIS_TAG"          && TAGS="$TAGS stable $TRAVIS_TAG" || true
   echo "Tags are $TAGS"
+  set -e
+  # The version encoded in pre-commit and built version must be in sync. If not,
+  # the pre-commit version was forgotten to be updated.
+  if [ -n "$TRAVIS_TAG" ]; then
+    precommit_tag="$(awk -F: '/^ *entry:/ {print $NF}' .pre-commit-hooks.yaml)"
+    if [ "$TRAVIS_TAG" != "$precommit_tag" ]; then
+      echo "Wrong version number $precommit_tag in .pre-commit-hooks.yaml"
+      sleep 1  # give Travis time to grab the error message
+      exit 1
+    fi
+  fi
 
 # This is in global context and runs for every stage that doesn't override it.
 script:

README.md

@@ -211,6 +211,13 @@ docker run --rm -v "$PWD:/mnt" koalaman/shellcheck:stable myscript
 
 or use `koalaman/shellcheck-alpine` if you want a larger Alpine Linux based image to extend. It works exactly like a regular Alpine image, but has shellcheck preinstalled.
 
+From [pre-commit](https://pre-commit.com/), add this to your `.pre-commit-config.yaml`:
+
+    -   repo: https://github.com/koalaman/shellcheck
+        rev: ''  # Use the sha / tag you want to point at
+        hooks:
+        -   id: shellcheck
+
 Using the [nix package manager](https://nixos.org/nix):
 ```sh
 nix-env -iA nixpkgs.shellcheck