UNIX for Dummies Questions & Answers

I am writing a script for the installation of a new version of my executable. The process is started from inittab. The script does the following
1.Comment the process entry from inittab.
2."init q"
3.replace the old executable with the new one
4.Uncomment the entry in initttab
5.init q

Now the process is stopped and the new version is copied. But it is not getting restarted. But if i do these steps manually from the shell the process is getting spawned.

Another interesting observation i had was that if i put a sleep between step 3 & 4 , the process will start.

My doubts are
1) will "init q" check the timestamp of inittab before reading it? (Here if i do it from script it will take less than a second)

2) Is there any other condition,where "init q" wont read inittab?

First off, you don't need to do step 1, 2, or 4. All you have to do is replace the old executable with the new, possibly add a sleep to allow any write cache to clear, and do the init q command.

When making changes like this, the init process isn't re-reading/reloading anything until you do the init q. So you could replace any executable that is run by init and it will not update until the init q (or a reboot).

I don't believe the file timestamp is checked since you are running the init q command.

From the man page for init:

New entries can be added to /etc/inittab at any time; how-
ever, init still waits for one of the above three conditions
to occur before re-examining /etc/inittab. To get around
this, init Q or init q command wakes init to re-examine
/etc/inittab immediately.

It actually would not surprise me if init stat's the file and passes on the reread is the timestamp hasn't changed. That is a common performance hack.

There is a slight problem with replacing a running program. Unix will not allow a unlink() of a file if it is a running process. You will get a "text busy" error message. So you just rename the old one and install a new one. Then perhaps kill the running process to trigger a respawn. Yes this does leave a slight window when init may freak if it can't find a file for a respawn. But init will catch it and sleep with the "respawning too rapidly" error message.