Running Scripts at Startup – init.d vs cron @reboot Explained

croninit-scriptscriptingstartupsysvinit

I am currently trying to understand the difference between init.d and cron @reboot for running a script at startup/booting of the system.

The use of @reboot (this method was mentioned in this forum by hs.chandra) is some what simpler, by simply going into crontab -e and creating a @reboot /some_directory/to_your/script/your_script.txt and then your_script.txt shall be executed every time the system is rebooted. An in depth explanation of @reboot is here

Alternatively by embedding /etc/init.d/your_script.txt into the second line of your script ie:

#!/bin/bash
# /etc/init.d/your_script.txt

You can run chmod +x /etc/init.d/your_script.txt and that should also result for your_script.txt to run every time the system is booted.

Q1: What are the key differences between the two?
Q2: Which is more robust?
Q3: Is there a better one out of the two?
Q4: Is this the correct way of embedding a script to run during booting?

I will be incorporating a bash .sh file to run during startup.

Best Answer

init.d, also known as SysV script, is meant to start and stop services during system initialization and shutdown. (/etc/init.d/ scripts are also run on systemd enabled systems for compatibility).

The script is executed during the boot and shutdown (by default).
The script should be an init.d script, not just a script . It should support start and stop and more (see Debian policy)
The script can be executed during the system boot (you can define when).

crontab (and therefore @reboot).

cron will execute any regular command or script, nothing special here.
any user can add a @reboot script (not just root)
on a Debian system with systemd: cron's @reboot is executed during multi-user.target.
on a Debian system with SysV (not systemd), crontab(5) mention: Please note that startup, as far as @reboot is concerned, is the time when the cron(8) daemon startup. In particular, it may be before some system daemons, or other facilities, were startup. This is due to the boot order sequence of the machine.
it's easy to schedule the same script at boot and periodically.

/etc/rc.local is often considered to be ugly or deprecated (at least by redhat), still it had some nice features:

rc.local will execute any regular command or script, nothing special here.
on a Debian system with SysV (not systemd): rc.local was (almost) the last service to start.
but on a Debian system with systemd: rc.local is executed after network.target by default (not network-online.target !)

Regarding systemd's network.target and network-online.target, read Running Services After the Network is up.

Related Solutions

Solaris 10 – Is it possible to reboot a system from a startup script

I have tested moving this script from /etc/rc0.d to /etc/rcS.d, /etc/rc1.d, /etc/rc2.d and /etc/rc3.d with the following results:

/etc/rcS.d - same behaviour as /etc/rc0.d - /etc/DR_Network_Configured is created, but no reboot occurs.
/etc/rc1.d - /etc/DR_Network_Configured is not created, and no reboot happens.
/etc/rc2.d - /etc/DR_Network_Configured is created and the system is rebooted.
/etc/rc3.d - /etc/DR_Network_Configured is created and the system is rebooted.

To summarise, when the system is booted to its default (milestone/multi-user-server:default, similar to run-level 3), it executes startup scripts located in /etc/rc0.d, /etc/rcS.d, /etc/rc2.d and /etc/rc3.d, but not /etc/rc1.d.

The reboot and init commands do not work when run from a startup script in /etc/rc0.d, /etc/rcS.d (and possibly /etc/rc1.d although I can't confirm this as the startup script in this directory never ran). They do work when run from startup scripts in /etc/rc2.d and /etc/rc3.d.

I imagine that this is designed to prevent a system from constantly rebooting. Should an erroneous startup script in /etc/rc2.d or /etc/rc3.d put the system into an infinite reboot loop, then the system can be fairly easily be rebooted to the single-user milestone and the offending startup script disabled rather than having to locate alternate boot media to boot off, mount the root volume/disk and disable the offending script.

Based on the above, I have modified my network reconfiguration script as follows:

Kept my script in /etc/rc0.d to change the network settings.
Added a function to it so that if the system needs to be rebooted after reconfiguring the network, a new script /etc/rc2.d/S99reboot is created which will reboot the system.
If the /etc/DR_Network_Configured file exists, and /etc/rc2.d/S99reboot exists, then remove the latter to avoid the system constantly rebooting.

My relevant code is:

#!/sbin/sh
reboot_script="/etc/rc2.d/S99reboot"

Create_Reboot_File ()
{
   echo "#!/sbin/sh" > $reboot_script
   echo "case \"\$1\" in" >> $reboot_script
   echo "start)" >> $reboot_script
   echo "  init 6" >> $reboot_script
   echo "  exit 0" >> $reboot_script
   echo "  ;;" >> $reboot_script
   echo "esac" >> $reboot_script
   echo "exit 0" >> $reboot_script
   chmod 740 $reboot_script
   chown root:root $reboot_file
}

case "$1" in
start)
   if [ -f /etc/DR_Network_Configured ]; then
      [ -f $reboot_script ] && rm $reboot_script
      exit 0
   else
      # My reconfigure network functions are here
      # ...
      touch /etc/DR_Network_Configured
      Create_Reboot_File
   fi
   exit 0
   ;;
*)
   echo "Usage: $0 { start }"
   exit 1
   ;;
esac
exit 0

Best Answer

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Solaris 10 – Is it possible to reboot a system from a startup script

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