Shell – How to wait for a file in the shell script

The 2 approaches I see here are more:

• setting a inline section or a POD-like documentation to display as help, or
• properly defining a .man file to add to your local man structure

I honestly don't see the point of having a separate file for that kind of help, unless you have a very big tool and the interface/GUI is already in different file(s).

So stay "plain and simple": all in one file regarding your command-line frontend.

You can still organize it as a chunk of inline-text or properly defined function which sole purpose is to display the help. So no ill done to the poor guy who will maintain your script in 10 years.

On a system, the only thing that is really persistent is a file. That's pretty much what you should use. Here's an solution using an init.d script.

Let's consider the following (simple) script, /etc/init.d/myupdate :

#! /bin/sh

### BEGIN INIT INFO
# Provides:          myupdate
### END INIT INFO

PATH=/sbin:/bin:/usr/sbin:/usr/bin

case "$1" in
    start)
        /path/to/update/script
        ;;
    stop|restart|reload)
        ;;
esac

If you activate it with update-rc.d myupdate defaults, the start action will be executed upon boot. Now, when your update script calls for a reboot:

touch /var/run/rebooting-for-updates
sudo reboot

With this solution, you can divide your update script into two parts :

before_reboot(){
    # Do stuff
}

after_reboot(){
    # Do stuff
}

if [ -f /var/run/rebooting-for-updates ]; then
    after_reboot
    rm /var/run/rebooting-for-updates
    update-rc.d myupdate remove
else
    before_reboot
    touch /var/run/rebooting-for-updates
    update-rc.d myupdate defaults
    sudo reboot
fi

It'll execute the before_reboot code section, create a file in /var/run, and reboot. Upon boot, the script will be called again, but since the file exists, after_reboot will be called instead of before_reboot.

Note that update-rc.d requires root privileges.

Without using a file (from Stephen Ostermiller's comment):

If you are familiar with the getopts utility, you may want to use options instead of files. In the init script, call the script with:

/path/to/update/script -r

And in your script, check for options instead of files. Call your script once without the option, and init.d will call it again on boot, this time with -r.

# Set AFTER_REBOOT according to options (-r).

if [ "x$AFTER_REBOOT" = "xyes" ]; then
    # After reboot
else
    # Before reboot
fi

You'll find more information about option handling here (for short options only). I also edited my script with calls to update-rc.d to keep this a one-time job (from another comment).