How to act upon condition of whether a file updated via Rsync was updated

Question

I think this is simple but I can't find anything in my googling, maybe I'm searching the wrong terms.

I have a machine that syncs itself to another machine. If there are changes to a file on the remote machine, then I need to run additional scripts on the local machine. But, I can't seem to figure out how to set a condition on an rsync command allowing it to only run additional scripts if the file has been changed/updated.

Thanks a bunch in advance!

Answer 1

There isn't an obvious answer, like an option that lets you specify a script to be run if changes were found. However, you could use the --stats flag to generate statistics about what happened, parse them, and make an educated guess.

$ rsync -avP --stats src dest 
sending incremental file list
...

Number of files: 10
Number of files transferred: 2
Total file size: 27 bytes
Total transferred file size: 27 bytes
Literal data: 27 bytes
Matched data: 0 bytes
File list size: 253
File list generation time: 0.001 seconds
File list transfer time: 0.000 seconds
Total bytes sent: 411
Total bytes received: 82

sent 411 bytes  received 82 bytes  986.00 bytes/sec
total size is 27  speedup is 0.05

You could capture that and look for the Number of files transferred: 2

$ rsync -avP --stats src dest | grep 'Number of files transferred:' 
Number of files transferred: 0
$ touch src/another
$ rsync -avP --stats src dest | grep 'Number of files transferred:'
Number of files transferred: 1

Or, to be more paranoid, capture the output and report back either a number of files or if the line wasn't found (in case the way the stats are reported ever changes).

#!/bin/bash

set -e

tmpfile=$(mktemp)

rsync -avP --stats src dest | grep 'Number of files transferred:' >"$tmpfile"

if [[ $(wc -l < "$tmpfile") == "1" ]]; then
  changedfiles=$(awk '{ print $5 }' <"$tmpfile")
else
  changedfiles="ERROR"
fi
rm -f "$tmpfile"

echo CHANGEDFILES="$changedfiles"