How to Write a Bash Script to Restart a Process If It Dies

How do I write a bash script to restart a process if it dies?

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

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.

How do I write a bash script to restart a process if it exits gracefully?

A simple way:

while sleep 1; do 
    echo "success"
done

Seems to work fine for me.

Replace sleep 1 with the command to start your process.

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edit: this is an answer for the question in the title, I'm not sure what /etc/init or the code you gave has to do with the question

Shell script: How to restart a process (with pipe) if it dies

The until loop itself can be piped into logger:

until myserver 2>&1; do
    echo "..."
    sleep 1
done | /usr/bin/logger -p local0.info &

since myserver inherits its standard output and error from the loop (which inherits from the shell).

How to restart a process in bash or kill it on command?

From the manual:

If Bash is waiting for a command to complete and receives a signal for which a trap has been set, the trap will not be executed until the command completes. When Bash is waiting for an asynchronous command via the wait builtin, the reception of a signal for which a trap has been set will cause the wait builtin to return immediately with an exit status greater than 128, immediately after which the trap is executed.

Emphasis is mine.

So in your case, while your command is executing, Bash will wait until it ends before it triggers the trap.

To fix this, you need to run your program as a job, and wait for it. If your program never exits with a return code greater than 128, you could simplify the following code, but I'm not making this assumption:

#!/bin/bash

desc="Foo Manager"
to_exec=( python "/myPath/bin/FooManager.pyc" )

trap 'trap_triggered=true' SIGHUP SIGINT SIGTERM

trap_triggered=false
while ! $trap_triggered; do
   "${to_exec[@]}" &
   job_pid=$!
   wait $job_pid
   job_ret=$?
   if [[ $job_ret = 0 ]]; then
      echo >&2 "Job ended gracefully with no errors... quitting..."
      break
   elif ! $trap_triggered; then
      echo >&2 "Server $desc crashed with exit code $job_ret. Restarting..."
   else
      printf >&2 "Received fatal signal... "
      if kill -0 $job_pid >&/dev/null; then
          printf >&2 "killing job $job_pid... "
          kill $job_pid
          wait $job_pid
      fi
      printf >&2 "quitting...\n"
   fi
done

Notes.

1. I used lowercase variable name, since uppercase are considered bad practice: they can clash with Bash's reserved names, or environmental variables.
2. I didn't use a string to store the command, but an array. With a string, you'll have a lot of problems if you want to have funny characters like spaces passed as arguments. With a properly quoted array, you won't have any problems. (Some would argue that it would be even better to use a function.)

How do I write a bash script to restart a service if it dies?

I think it will be better to manage your process with supervisord, or other process control system.

How can a bash script restart a process on non-0 exit while sending signals to child

Don't write a shell script. Use systemd, supervisor, docker or any available service manager to manage the docker/script process directly. This is the job service managers were built to do, they live for it.

A systemd service would run docker run {image} python test.py and you would need to set it to run indefinitely.

A systemd config would look like:

[Unit]
Description=My Super Script
Requires=docker.service
After=docker.service

[Service]
ExecStart=/bin/docker run --name={container} --rm=true {image} python test.py
ExecStop=/bin/docker stop --time=10 {container}
TimeoutStopSec=11
KillMode=control-group

Restart=on-failure
RestartSec=5
TimeoutStartSec=5

[Install]
WantedBy=multi-user.target

The Restart=on-failure setting matches your requirement of only restarting the process when a non 0 exit code is returned so you can still kill the process underneath systemd, if required.

If you want to run and manage your python process inside an already running container, it might be easier to run supervisord as the main container process and have it manage python test.py. Supervisor is not as feature complete as systemd but it can do all the basic service management tasks.

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