How to make sure an application keeps running on Linux

I have used a simple script with cron to make sure that the program is running. If it is not, then it will start it up. This may not be the perfect solution you are looking for, but it is simple and works rather well.

#!/bin/bash
#make-run.sh
#make sure a process is always running.

export DISPLAY=:0 #needed if you are running a simple gui app.

process=YourProcessName
makerun="/usr/bin/program"

if ps ax | grep -v grep | grep $process > /dev/null
then
    exit
else
    $makerun &
fi

exit

Then add a cron job every minute, or every 5 minutes.

Monit is perfect for this :)

You can write simple config files which tell monit to watch e.g. a TCP port, a PID file etc

monit will run a command you specify when the process it is monitoring is unavailable/using too much memory/is pegging the CPU for too long/etc. It will also pop out an email alert telling you what happened and whether it could do anything about it.

We use it to keep a load of our websites running while giving us early warning when something's going wrong.

-- Your faithful employee, Monit

Notice: Upstart is in maintenance mode and was abandoned by Ubuntu which uses systemd. One should check the systemd' manual for details how to write service definition.

Since you're using Ubuntu, you may be interested in Upstart, which has replaced the traditional sysV init. One key feature is that it can restart a service if it dies unexpectedly. Fedora has moved to upstart, and Debian is in experimental, so it may be worth looking into.

This may be overkill for this situation though, as a cron script will take 2 minutes to implement.

#!/bin/bash
if [[ ! `pidof -s yourapp` ]]; then
    invoke-rc.d yourapp start
fi

I have used from cron "killall -0 programname || /etc/init.d/programname start". kill will error if the process doesn't exist. If it does exist, it'll deliver a null signal to the process (which the kernel will ignore and not bother passing on.)

This idiom is simple to remember (IMHO). Generally I use this while I'm still trying to discover why the service itself is failing. IMHO a program shouldn't just disappear unexpectedly :)

Put your run in a loop- so when it exits, it runs again... while(true){ run my app.. }

It's a job for a DMD (daemon monitoring daemon). there are a few around; but I usually just write a script that checks if the daemon is running, and run if not, and put it in cron to run every minute.

first of all, how do you start this app? Does it fork itself to the background? Is it started with nohup .. & etc? If it's the latter, check why it died in nohup.out, if it's the first, build logging.

As for your main question: you could cron it, or run another process on the background (not the best choice) and use pidof in a bashscript, easy enough:

if [ `pidof -s app` -eq 0 ]; then
    nohup app &
fi

You could make it a service launched from inittab (although some Linuxes have moved on to something newer in /etc/event.d). These built in systems make sure your service keeps running without writing your own scripts or installing something new.

Check out 'nanny' referenced in Chapter 9 (p197 or thereabouts) of "Unix Hater's Handbook" (one of several sources for the book in PDF).

I think a better solution is if you test the function, too. For example, if you had to test an apache, it is not enough only to test, if "apache" processes on the systems exist.

If you want to test if apache OK is, then try to download a simple web page, and test if your unique code is in the output.

If not, kill the apache with -9 and then do a restart. And send a mail to the root (which is a forwarded mail address to the roots of the company/server/project).

One can install minutely monitoring cronjob like this:

crontab -l > crontab;echo -e '* * * * * export DISPLAY=":0.0" && for app in "eiskaltdcpp-qt" "transmission-gtk" "nicotine";do ps aux|grep -v grep|grep "$app";done||"$app" &' >> crontab;crontab crontab

disadvantage is that the app names you enter have to be found in ps aux|grep "appname" output and at same time being able to be launched using that name: "appname" &