Terminal Tips: bash cron script to keep an app running

A better .plist example:





Label
com.example.exampled
ProgramArguments

exampled

KeepAlive




http://developer.apple.com/library/mac/#documentation/Darwin/Reference/ManPages/man5/launchd.plist.5.html

AppleTV will not stream if iTunes is not running. I use a variant of this script to keep it running if I inadvertently quit the program.

Great comments!
Do you guys know how to shorten terminal command, and give it a letter?

For instance I often use whois command via terminal and would like to type w and than domain name to get whois output.

Thank you!

//Rob

If you'd like to try out Dropbox, use this link to get some extra quota:

http://www.dropbox.com/referrals/NTI0NDI4OTE5

@Many - I stand corrected about the need to reboot.

Thanks for the insights. My knowledge of launchd was obviously outdated.

"Lingon is not wrong about needing to restart or logout, I've tested that for myself."

So apparently you've never tried 'launchctl load plistfile'?

Thanks to patg for the other examples.

I'll now transition from 'random commenter on the internet' back to 'random reader of the internet'.

Seems like my last couple of comments got lost. Here's a launchd plist without any absolute paths:


<?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"&gt;
<plist version="1.0">
<dict>
<key>Label</key>
<string>local.keep_dropbox_running</string>
<key>ProgramArguments</key>
<array>
<string>open</string>
<string>-W</string>
<string>-a</string>
<string>Dropbox</string>
</array>
<key>KeepAlive</key>
<true />
<key>RunAtLoad</key>
<true />
</dict>
</plist>

Should be:

<?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"&gt;
<plist version="1.0">
<dict>
<key>Label</key>
<string>local.keep_dropbox_running</string>
<key>ProgramArguments</key>
<array>
<string>open</string>
<string>-W</string>
<string>-a</string>
<string>Dropbox</string>
</array>
<key>KeepAlive</key>
<true />
<key>RunAtLoad</key>
<true />
</dict>
</plist>

There also happens to be an absolute path right here:
*/5 * * * * /Users/luomat/bin/keep-my-app-running.sh



Here's a launchd plist that should achieve the same without any absolute paths:

<?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"&gt;
<plist version="1.0">
<dict>
<key>Label</key>
<string>local.keep_dropbox_running</string>
<key>ProgramArguments</key>
<array>
<string>/usr/bin/open</string>
<string>-W</string>
<string>-a</string>
<string>Dropbox</string>
</array>
<key>KeepAlive</key>
<true />
<key>RunAtLoad</key>
<true />
</dict>
</plist>

The Program key in a launchd plist has the same semantics as execvp. From the launchd.plist manpage:

Program
This key maps to the first argument of execvp(3). If this key is missing,
then the first element of the array of strings provided to the
ProgramArguments will be used instead.

According to the execvp manpage, a call to execvp uses the PATH environment variable if it is set. Otherwise, it searches /usr/bin and /bin. The absolute path is necessary because the directory containing the actual executable for a given application (usually /Contents/MacOS/) is unlikely to be in a user's PATH. Even if it were, it's possible that a user's PATH hasn't been set when launchd loads a given job (e.g. if .bash_profile is not read until after all launchd jobs have been loaded).

On another note, a big downside of the script above is that it cannot check the exit status of the application it is keeping alive. In other words, you cannot reliably quit an application (if you need to) without disabling the cron job.

Here are a couple of scripts that generate/install launchd jobs to relaunch a given application or executable if it exits unsuccessfully:

http://www2.uic.edu/~pgavli2/keep-running.sh
http://www2.uic.edu/~pgavli2/gen-keepalive-plist.sh

So long as an application is guaranteed to exit with a non-zero status when it crashes, these jobs should operate properly.

"Lingon recommends", "Select an app in Lingon", etc.

Apparently you think 'Lingon = launchd". Lingon is an abandoned third-party GUI for editing launchd plists. It's a nice tool, but it's NOT launchd.

"The only way to manage it via ssh would be to edit XML plist by hand. Can it be done? Yes. It is fraught with peril and easy to screw up? Yes."

And you've never screwed up a crontab entry? Tell me, without looking, which column is for 'day of the week'?

Sheesh, talk about zealotry...