Watcher Script Using Unix Find.

watch.sh

#!/bin/bash

# Invoke like this:
# ./watch.sh my command here
# And it will run 'my command here' once, and then when it detects changes.

# TODO: Don't just search in the last second. Search for updates since the last
# completed build. Otherwise for big directories, midway through your search
# you've already taken 1s and you will miss updates.

COUNT=1
FILE_SET=""

echo RUNNING

daemon() {
    while true
    do
      COUNT=$((COUNT-1))
      DID_BUILD_BECAUSE_FILE_SET="false"
      if [ $COUNT == 0 ]; then
        # This command has no mtime flag. It gets the whole set.
        NEW_FILE_SET=$(find . -path \*/.git -prune -o -path \*/.hg -prune -o -path \*/node_modules -prune -o -path \*/_build -prune -o -not -name \*.merlin -not -name \*.opam -type f -name "*" -print)
        # This will trigger our initial build.
        if [ "${FILE_SET}" != "${NEW_FILE_SET}" ]; then
          # Don't print the file diffs for the first build.
          echo ""
          echo "[WATCH] Rebuilding Because Tracked File Set Changed:"
          diff  <(echo "$FILE_SET" ) <(echo "$NEW_FILE_SET")
          echo ""
          FILE_SET="${NEW_FILE_SET}"
          eval "$@"
          echo ""
          echo "[WATCH] Done Running Command:" "$@"
          echo ""
          DID_BUILD_BECAUSE_FILE_SET="true"
        fi
        COUNT=10
      fi
      # Already built due to finding a removed/new file. No need to build again.
      if [ "$DID_BUILD_BECAUSE_FILE_SET" != "true" ]; then
        # Find commands are like:
        # prune-command-that-evaluates-to-true -or another-prune-command -or actual-find-command print-or-exec
        # prune commands look like actual find commands followed by a -prune
        # You should always include a final print or exec to avoid parsing ambiguity!
        # https://stackoverflow.com/questions/1489277/how-to-use-prune-option-of-find-in-sh

        if [[ "$OSTYPE" == "linux-gnu" || "$OSTYPE" == "cygwin"  ]]; then
          # .016 is how you do -mtime -1s on linux.
          CHANGED_FILES=$(find . -path \*/.git -prune -o -path \*/.hg -prune -o -path \*/node_modules -prune -o -path -prune -o -path \*/_build -prune -o -not -name \*.merlin -not -name \*.opam -type f  -mmin .016 -name "*" -print)
        elif [[ "$OSTYPE" == "darwin"* ]]; then
          CHANGED_FILES=$(find . -path \*/.git -prune -o -path \*/.hg -prune -o -path \*/node_modules -prune -o -path -prune -o -path \*/_build -prune -o -not -name \*.merlin -not -name \*.opam -type f  -mtime -1s -name "*" -print)
        else
          echo "Unsupported Operating System: ${OSTYPE}"
        fi

        if [[ "$CHANGED_FILES" == "" ]]; then
          # Then nothing change in last -mtime. -mtime only reports files that have changed.
          true
        else
          echo ""
          echo "[WATCH] Rebuilding Due To File Changes:"
          echo "$CHANGED_FILES"
          echo ""
          echo ""
          echo "[WATCH] Running Command:" "$@"
          eval "$@"
          echo ""
          echo "[WATCH] Done Running Command:" "$@"
          echo ""
        fi
      fi
      sleep 0.6
    done
}

daemon "$@"

This script is awesome! My personal mod checks the gitignore and leaves gitignored directories out from the find, similar to how you leave out merlin and node modules, so that this works no matter the build files my directory has or dependencies.

For future reference, if you need to modify this to exclude other file names, change all of the three find commands like this:

# Original
find . -path \*/.git -prune -o -path \*/.hg -prune -o -path \*/node_modules -prune -o -path -prune -o -path \*/_build -prune -o -not -name \*.merlin -type f  -mmin .016 -name "*" -print
# New command - ignoring "*.foo" files
find . -path \*/.git -prune -o -path \*/.hg -prune -o -path \*/node_modules -prune -o -path -prune -o -path \*/_build -prune -o -not -name \*.merlin -not -name \*.foo -type f  -mmin .016 -name "*" -print

(maybe need to copy the -type f too? not sure.)

Updated with @kyldvs's changes. Someone could extend this so it doesn't hard code the list of ignored files/dirs, accepting them as inputs.