Shell Script Launching Child Processes

backgrounddaemonprocessshell

Disclaimer: I'm totally new to shell scripting, but have quite a bit of experience in other languages like PHP and Obj-C.

I'm writing my first daemon script. Here are the goals:

I want it to run in the background
I want it to be triggered by an init.d script that includes start/stop/restart commands
I want each process in a loop to trigger its own subprocess.
When the parent process kicked off by the init.d script is killed, I want the subprocesses to die as well.

Essentially, I'm looking for the same kind of behavior that appears to be very common among software like apache, spamd, dovecot, etc. But, based on my research, I haven't found a single, simple answer as to how this kind of thing is achieved.

Any help is greatly appreciated.

Best Answer

You will find lots of very good information by reading Process Management.

Also, take a look at your existing init.d scripts and use them as models as you write your own. Keep in mind though that some of them could stand improving. So try not to pick up any bad habits.

You'll likely want to write your scripts in the Bourne shell (sh) rather than Bash, but you can learn a lot that's applicable to both by reading Bash FAQs and Bash Pitfalls.

Related Solutions

Linux – Shell command to monitor changes in a file

Do you mean

tail -f logfile.log

(Man page for tail)

Shell Script to Execute When a Daemon Dies – Scripting Guide

monit is great in this case - it runs on localhost, so you don't need a network connection to restart your daemon (in case it fails, or daemon is responsible for networking). It also has small footprint on the system, and you can use it to monitor your other daemons/disk space/etc. as well.

Create a start/stop script (similar to those in /etc/init.d/ and create symlinks for it in runlevels that your system uses for normal operation, ensuring that your daemon will start at reboot and stop at shutdown properly. If your daemon doesn't have pidfile, create one using start-stop-daemon script.

After that, install monit and create configuration for your daemon, something like this:

check process daemond with pidfile /var/run/daemond.pid
    start program "/etc/init.d/daemond start"
    stop program "/etc/init.d/daemond stop"
    if failed port 1234 type TCP for 5 times within 10 cycles then restart
    if 3 restarts within 5 cycles then alert

This configuration will ensure that if daemon stops responding on tcp port 1234, or will stop running, it will be restarted using the init script. monit will also send you an alert through email, or do other things, depending on how you configure it. Just check out monit(1) manpage.

Best Answer

Related Solutions

Linux – Shell command to monitor changes in a file

Shell Script to Execute When a Daemon Dies – Scripting Guide

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