Monitor a folder for changes, and run a command when a change is detected

Question

Is there a simple way to automatically run a (Terminal) command every time a file is changed in a specific folder?

This should be possible via the command line, or a system built-in application, no third-party applications.

Any ideas?

Answer 1

You could use Folder Actions, which lets you execute (Automator) scripts whenever contents in a folder change. As far as I know, Automator has a template that lets you easily create a new folder action and attach it to the desired folder. And by adding a "Run Shell Script" action, you should just get the desired effect.

Answer 2

Save a property list like this as ~/Library/LaunchAgents/test.plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC -//Apple Computer//DTD PLIST 1.0//EN
http://www.apple.com/DTDs/PropertyList-1.0.dtd>
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>test</string>
    <key>ProgramArguments</key>
    <array>
        <string>say</string>
        <string>yy</string>
    </array>
    <key>WatchPaths</key>
    <array>
        <string>~/Desktop/</string>
    </array>
</dict>
</plist>

The agent can be loaded with launchctl load ~/Library/LaunchAgents/test.plist or by logging out and back in. Applying changes requires unloading and loading the plist.

Tilde expansion (~/) works in WatchPaths by default. EnableGlobbing adds wildcard and tilde expansion for ProgramArguments, but it doesn't affect Program or WatchPaths.

If a watched file is modified within ten seconds from the last invocation, a message like this is added to system.log:

com.apple.launchd.peruser.501[146]: (test) Throttling respawn: Will start in 7 seconds

One way to get rid of the messages is to add something like sleep 10 to the end of scripts. Setting ThrottleInterval to 10 doesn't help.

Changes in subfolders of watched folders aren't detected.

See man launchd and man launchd.plist for more information.

Answer 3

Automator has a type of workflow called a "Folder Action" which automatically runs when something is added to a folder. Create one, then use the action "Run Shell Script."

Answer 4

You might want to try something like this:

touch /tmp/dirb.tmp
while true do 
  ls /thedirtocheck > /tmp/dira.tmp 
  diff /tmp/dira.tmp /tmp/dirb.tmp || echo "something changed" 
  cp /tmp/dira.tmp /tmp/dirb.tmp 
  sleep 100 
done