Question

I have a python script that’ll be checking a queue and performing an action on each item:

# checkqueue.py while True:   check_queue()   do_something() 

How do I write a bash script that will check if it’s running, and if not, start it. Roughly the following pseudo code (or maybe it should do something like ps | grep?):

# keepalivescript.sh if processidfile exists:   if processid is running:      exit, all ok  run checkqueue.py write processid to processidfile 

I’ll call that from a crontab:

# crontab */5 * * * * /path/to/keepalivescript.sh 

Answer 1

Avoid PID-files, crons, or anything else that tries to evaluate processes that aren’t their children.

There is a very good reason why in UNIX, you can ONLY wait on your children. Any method (ps parsing, pgrep, storing a PID, …) that tries to work around that is flawed and has gaping holes in it. Just say no.

Instead you need the process that monitors your process to be the process’ parent. What does this mean? It means only the process that starts your process can reliably wait for it to end. In bash, this is absolutely trivial.

until myserver; do     echo "Server 'myserver' crashed with exit code $?.  Respawning.." >&2     sleep 1 done 

Or to be able to stop it:

trap 'kill $(jobs -p)' EXIT; until myserver & wait; do     echo "ldap proxy crashed with exit code $?. Respawning.." >&2     sleep 1 done 

The above piece of bash code runs myserver in an until loop. The first line starts myserver and waits for it to end. When it ends, until checks its exit status. If the exit status is 0, it means it ended gracefully (which means you asked it to shut down somehow, and it did so successfully). In that case we don’t want to restart it (we just asked it to shut down!). If the exit status is not 0, until will run the loop body, which emits an error message on STDERR and restarts the loop (back to line 1) after 1 second.

Why do we wait a second? Because if something’s wrong with the startup sequence of myserver and it crashes immediately, you’ll have a very intensive loop of constant restarting and crashing on your hands. The sleep 1 takes away the strain from that.

Now all you need to do is start this bash script (asynchronously, probably), and it will monitor myserver and restart it as necessary. If you want to start the monitor on boot (making the server "survive" reboots), you can schedule it in your user’s cron(1) with an @reboot rule. Open your cron rules with crontab:

crontab -e 

Then add a rule to start your monitor script:

@reboot /usr/local/bin/myservermonitor 

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Alternatively; look at inittab(5) and /etc/inittab. You can add a line in there to have myserver start at a certain init level and be respawned automatically.

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Edit.

Let me add some information on why not to use PID files. While they are very popular; they are also very flawed and there’s no reason why you wouldn’t just do it the correct way.

Consider this:

1. PID recycling (killing the wrong process):

   • /etc/init.d/foo start: start foo, write foo‘s PID to /var/run/foo.pid
   • A while later: foo dies somehow.
   • A while later: any random process that starts (call it bar) takes a random PID, imagine it taking foo‘s old PID.
   • You notice foo‘s gone: /etc/init.d/foo/restart reads /var/run/foo.pid, checks to see if it’s still alive, finds bar, thinks it’s foo, kills it, starts a new foo.

2. PID files go stale. You need over-complicated (or should I say, non-trivial) logic to check whether the PID file is stale, and any such logic is again vulnerable to 1..

3. What if you don’t even have write access or are in a read-only environment?

4. It’s pointless overcomplication; see how simple my example above is. No need to complicate that, at all.

See also: Are PID-files still flawed when doing it 'right'?

By the way; even worse than PID files is parsing ps! Don’t ever do this.

1. ps is very unportable. While you find it on almost every UNIX system; its arguments vary greatly if you want non-standard output. And standard output is ONLY for human consumption, not for scripted parsing!
2. Parsing ps leads to a LOT of false positives. Take the ps aux | grep PID example, and now imagine someone starting a process with a number somewhere as argument that happens to be the same as the PID you stared your daemon with! Imagine two people starting an X session and you grepping for X to kill yours. It’s just all kinds of bad.

If you don’t want to manage the process yourself; there are some perfectly good systems out there that will act as monitor for your processes. Look into runit, for example.