What's the exactly differences of /etc/init.d/something and /etc/rc.local [closed] - linux

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When I need to create a autostartup task in Ubuntu. I always create a new file and write a bunch of commands into it. Next, I place this file in the /etc/init.d/ directory. Then, I set chmod 755 for this file. Finally, I execute the command line "update-rc.d file_name defaults" to activate it. And it works like a charm.
Recently, I found that there was another way to make it work the same to this above example. That is appending a new command line into /etc/rc.local (place it above the "exit 0" line).
So could you tell the difference between them ? Thank you very much !

Answer 1

To understand this problem, the first thing you should know is run level in *nix. There are total 6 run level in *nix. I won't show details of each run level, you can read more about it here.
Each run level have separate locations under /etc/:
% cuonglm at ~
% ls -l /etc/rc* -d
drwxr-xr-x 2 root root 4096 Feb 20 10:44 /etc/rc0.d
drwxr-xr-x 2 root root 4096 Feb 20 10:44 /etc/rc1.d
drwxr-xr-x 2 root root 4096 Feb 20 10:44 /etc/rc2.d
drwxr-xr-x 2 root root 4096 Feb 20 10:44 /etc/rc3.d
drwxr-xr-x 2 root root 4096 Feb 20 10:44 /etc/rc4.d
drwxr-xr-x 2 root root 4096 Feb 20 10:44 /etc/rc5.d
drwxr-xr-x 2 root root 4096 Feb 20 10:44 /etc/rc6.d
-rwxr-xr-x 1 root root 306 Feb 4 18:58 /etc/rc.local
drwxr-xr-x 2 root root 4096 Feb 4 19:01 /etc/rcS.d
Everytime your system boots, some scripts (which start with S) under the coressponding run level folder is executed. I.E if you boot in to run level 2, some scripts under /etc/rc2.d/ will executed. If you show content of these folder, you'll see that scripts is a symlink of scripts under /etc/init.d/.
% ls -l /etc/rc2.d/
total 4
-rw-r--r-- 1 root root 677 Jul 27 2012 README
lrwxrwxrwx 1 root root 20 Feb 19 11:26 S20kerneloops -> ../init.d/kerneloops
lrwxrwxrwx 1 root root 27 Feb 19 11:26 S20speech-dispatcher -> ../init.d/speech-dispatcher
lrwxrwxrwx 1 root root 20 Feb 19 11:26 S50pulseaudio -> ../init.d/pulseaudio
lrwxrwxrwx 1 root root 15 Feb 19 11:26 S50rsync -> ../init.d/rsync
lrwxrwxrwx 1 root root 15 Feb 19 11:26 S50saned -> ../init.d/saned
lrwxrwxrwx 1 root root 19 Feb 19 11:26 S70dns-clean -> ../init.d/dns-clean
lrwxrwxrwx 1 root root 18 Feb 19 11:26 S70pppd-dns -> ../init.d/pppd-dns
lrwxrwxrwx 1 root root 14 Feb 19 11:26 S75sudo -> ../init.d/sudo
lrwxrwxrwx 1 root root 17 Feb 20 10:44 S91apache2 -> ../init.d/apache2
lrwxrwxrwx 1 root root 22 Feb 19 11:26 S99acpi-support -> ../init.d/acpi-support
lrwxrwxrwx 1 root root 21 Feb 19 11:26 S99grub-common -> ../init.d/grub-common
lrwxrwxrwx 1 root root 18 Feb 19 11:26 S99ondemand -> ../init.d/ondemand
lrwxrwxrwx 1 root root 18 Feb 19 11:26 S99rc.local -> ../init.d/rc.local
This give you an ability to control your service to run under which runlevel. You can make your service run in only run level 2 and stop in others run level. But remember, Only one "runlevel" is executed on bootup, i.e. either runlevel 2 OR 3 OR 4 is executed, not 2 then 3 then 4.
So it leads you to the difference here. In each run level you boot in, after scripts of this run level is executed, the script /etc/rc.local is executed. It means that /etc/rc.local will run at the end of boot process, regardless of run level you boot in.