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

Question

I want to run a shell script when a specific file or directory changes.

How can I easily do that?

Answer 1

Use inotify-tools.

The linked Github page has a number of examples; here is one of them.

#!/bin/sh

cwd=$(pwd)

inotifywait -mr \
  --timefmt '%d/%m/%y %H:%M' --format '%T %w %f' \
  -e close_write /tmp/test |
while read -r date time dir file; do
       changed_abs=${dir}${file}
       changed_rel=${changed_abs#"$cwd"/}

       rsync --progress --relative -vrae 'ssh -p 22' "$changed_rel" \
           [email protected]:/backup/root/dir && \
       echo "At ${time} on ${date}, file $changed_abs was backed up via rsync" >&2
done