How to run a shell script when a file or directory changes?

I use this script to run a build script on changes in a directory tree:

#!/bin/bash -eu
DIRECTORY_TO_OBSERVE="js"      # might want to change this
function block_for_change {
  inotifywait --recursive \
    --event modify,move,create,delete \
    $DIRECTORY_TO_OBSERVE
}
BUILD_SCRIPT=build.sh          # might want to change this too
function build {
  bash $BUILD_SCRIPT
}
build
while block_for_change; do
  build
done

Uses inotify-tools. Check inotifywait man page for how to customize what triggers the build.

You may try entr tool to run arbitrary commands when files change. Example for files:

$ ls -d * | entr sh -c 'make && make test'

or:

$ ls *.css *.html | entr reload-browser Firefox

or print Changed! when file file.txt is saved:

$ echo file.txt | entr echo Changed!

---

For directories use -d, but you've to use it in the loop, e.g.:

while true; do find path/ | entr -d echo Changed; done

or:

while true; do ls path/* | entr -pd echo Changed; done