Run bash script as daemon

I have a script, which runs my PHP script each X times:

#!/bin/bash
while true; do
/usr/bin/php -f ./my-script.php
echo "Waiting..."
sleep 3
done

How can I start it as daemon?

To run it as a full daemon from a shell, you'll need to use setsid and redirect its output. You can redirect the output to a logfile, or to /dev/null to discard it. Assuming your script is called myscript.sh, use the following command:

setsid myscript.sh >/dev/null 2>&1 < /dev/null &

This will completely detach the process from your current shell (stdin, stdout and stderr). If you want to keep the output in a logfile, replace the first /dev/null with your /path/to/logfile.

You have to redirect the output, otherwise it will not run as a true daemon (it will depend on your shell to read and write output).

A Daemon is just program that runs as a background process, rather than being under the direct control of an interactive user...

[The below bash code is for Debian systems - Ubuntu, Linux Mint distros and so on]

The simple way:

The simple way would be to edit your /etc/rc.local file and then just have your script run from there (i.e. everytime you boot up the system):

sudo nano /etc/rc.local

Add the following and save:

#For a BASH script
/bin/sh TheNameOfYourScript.sh > /dev/null &

The better way to do this would be to create a Daemon via Upstart:

sudo nano /etc/init/TheNameOfYourDaemon.conf

add the following:

description "My Daemon Job"
author "Your Name"
start on runlevel [2345]    
   
pre-start script
  echo "[`date`] My Daemon Starting" >> /var/log/TheNameOfYourDaemonJobLog.log
end script
    
exec /bin/sh TheNameOfYourScript.sh > /dev/null &

Save this.

Confirm that it looks ok:

init-checkconf /etc/init/TheNameOfYourDaemon.conf

Now reboot the machine:

sudo reboot

Now when you boot up your system, you can see the log file stating that your Daemon is running:

cat  /var/log/TheNameOfYourDaemonJobLog.log

• Now you may start/stop/restart/get the status of your Daemon via:

restart: this will stop, then start a service

sudo service TheNameOfYourDaemonrestart restart

start: this will start a service, if it's not running

sudo service TheNameOfYourDaemonstart start

stop: this will stop a service, if it's running

sudo service TheNameOfYourDaemonstop stop

status: this will display the status of a service

sudo service TheNameOfYourDaemonstatus status

You can go to /etc/init.d/ - you will see a daemon template called skeleton.

You can duplicate it and then enter your script under the start function.

Another cool trick is to run functions or subshells in background, not always feasible though

name(){
  echo "Do something"
  sleep 1
}

# put a function in the background
name &
#Example taken from here
#https://bash.cyberciti.biz/guide/Putting_functions_in_background

Running a subshell in the background

(echo "started"; sleep 15; echo "stopped") &

Some commentors already stated that answers to your question will not work for all distributions. Since you did not include CentOS in the question but only in the tags, I'd like to post here the topics one has to understand in order to have a control over his/her proceeding regardless of the distribution:

1. what is the init daemon (optional)
2. what is the inittab file (/etc/inittab)
3. what does the inittab file do in your distro (e.g. does it actually run all scripts in /etc/init.d ?)

For your problem, one could start the script on sysinit by adding this line in /etc/inittab and make it respawn in case it terminates:

# start and respawn after termination
ttyS0::respawn:/bin/sh /path/to/my_script.sh

The script has to be made executable in advance of course:

chmod +x /path/to/my_script.sh

Hope this helps