Watch for file or directory changes and run command

Sep 25, 2025

Using entr and fd (or find), the following script will watch for file or directory changes and run a command.

Usage:

fwatch file_name: watches the file, if it’s an executable, run it, otherwise cat it
fwatch file_name command: watches the file and runs the command
fwatch dir_name command: watches the directory and runs the command (will not work for files added after the script is started)
you can expand the “magic” section to add more file extension preset commands

#!/bin/bash
#
# watch for file or directory changes and run command

if ! command -v entr > /dev/null; then
  echo "entr command not found."
  exit 1
fi

f=$1
shift

handle_file() {
  # command is specified, use it
  # NOTE: when running a script e.g `./script arg1`,
  # you have to provide it twice: `fwatch script ./script arg1`
  if (($# > 0)); then
    command ls "${f}" | entr -c "$@"
    return
  fi

  # no command has been specified, use magic
  if [[ -x ${f} ]]; then # file is executable, run it
    command ls "${f}" | entr -c ./"${f}"
  else
    case "${f}" in
      *.hurl) cmd="hurl" ;;
      *) cmd="bat -pp" ;; # unknown filetype, cat it
    esac
    # shellcheck disable=SC2086
    command ls "${f}" | entr -c ${cmd} "${f}"
  fi
}

handle_dir() {
  if ! command -v fd > /dev/null; then
    echo "fd command not found."
    exit 1
  fi
  fd . "${f}" | entr -c "$@"
}

if [[ -f ${f} ]]; then
  handle_file "$@"
elif [[ -d ${f} ]]; then
  handle_dir "$@"
fi

You can find the most up to date version of this script here.