launchd-keepalive

Mac OS X plist files for use with launchd demonstrating how to use KeepAlive

• at.obdev.LittleSnitchUIAgent.plist is a basic 'KeepAlive' plist.

Note the

	<key>KeepAlive</key>
	<true/>

And

	<key>RunAtLoad</key>
	<true/>

Together, those tell launchd to run the program listed in ProgramArguments as soon as the user logs in, and keep it running as long as they are logged in, no matter what:

	<?xml version="1.0" encoding="UTF-8"?>
	<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
	<plist version="1.0">
	<dict>
		<key>KeepAlive</key>
		<true/>
		<key>Label</key>
		<string>at.obdev.LittleSnitchUIAgent</string>
		<key>ProgramArguments</key>
		<array>
			<string>/Library/Little Snitch/Little Snitch Agent.app/Contents/MacOS/Little Snitch Agent</string>
		</array>
		<key>RunAtLoad</key>
		<true/>
	</dict>
	</plist>

Now if Little Snitch Agent is terminated for any reason (i.e. crash or kill) it will automatically restart.

At first you might be tempted to create several plists like this to keep all of your favorite apps running all of the time.

Don't do that.

Think about what happens if an app you have designated as KeepAlive needs to be updated. What happens? The app tells you it has an update, you download it, the app quits and then -- uh, it restarted. Did it have a chance to finish updating? Are you sure?

Occasionally, KeepAlive apps can interfere with rebooting or logging out.

In most cases, a better solution is to use KeepAlive together with SuccessfulExit.

KeepAlive and SuccessfulExit

For example com.tjluoma.keeprunning.moveaddict.plist shows how to create a plist to keep an app running all of the time unless it exits cleanly (i.e. the user told it to quit). It uses KeepAlive but adds a check called SuccessfulExit. If an app exits "successfully" (technically, with an exit code = 0) then the app will not be automatically restarted (kept alive). However, if the app crashes (exit code not equal to 0) then it will be automatically restarted. Here is the relevant bit of launchd code:

	<key>KeepAlive</key>
	<dict>
		<key>SuccessfulExit</key>
		<false/>
	</dict>

This is handy if you have an app which occasionally crashes, especially if that app is one which runs in the background or the menu bar where you might not notice immediately.

KeepAlive and NetworkState

Another option is KeepAlive and NetworkState as shown in com.tjluoma.keeprunning.mail.plist which tells Mail.app to always keep running as long as we have a network connection.

Here is the portion of the plist which specifically deals with the "is the network up?" part:

	<key>KeepAlive</key>
	<dict>
		<key>NetworkState</key>
		<true/>
	</dict>

Important note #1: launchd considers the "network" to be "up" if you have an IP address. However, it is possible that your local network could be up but your connection to the Internet is down. For example, right now my ISP is offline, but I am connected to my local Wi-Fi network, so as far as launchd is concerned, the "network" is up. Just remember that "Network is up" does not necessarily mean "Internet is up/accessible."

Important note #2: It is also important to remember that launchd will not quit an app just because the network connection goes down. The only time the KeepAlive would be used is if the app quits, at which point launchd will check and say "Is the network up?" and if the answer is yes, it will relaunch the app. If the answer is no, it will not relaunch the app.

Installation

Unless otherwise noted, plists should be placed in the "$HOME/Library/LaunchAgents" (where "$HOME" represents the path to your home directory, for example: /Users/sjobs/Library/LaunchAgents/

How to start a new launchd plist

By default, launchd will load the '.plist' files from "$HOME/Library/LaunchAgents" when you log in. If you want to start a new .plist, you will have to tell launchd to load it. To do this, launch Terminal.app and type:

	cd "$HOME/Library/LaunchAgents"

And then:

	launchctl load com.tjluoma.keeprunning.mail.plist

(change 'com.tjluoma.keeprunning.mail.plist' to the filename of whichever plist you want to load)

How to tell launchd that you have changed an existing plist

If you have changed an existing plist and want the changes to be recognized right away, you have to unload it, and then reload it:

	cd "$HOME/Library/LaunchAgents"

	launchctl unload com.tjluoma.keeprunning.mail.plist

	launchctl load com.tjluoma.keeprunning.mail.plist

How to uninstall / remove a plist

Again, in Terminal.app, do:

	cd "$HOME/Library/LaunchAgents"

	launchctl unload com.tjluoma.keeprunning.mail.plist

And then either move it to the trash:

	mv com.tjluoma.keeprunning.mail.plist ~/.Trash/

Or delete it:

	rm -i com.tjluoma.keeprunning.mail.plist