Dear All,
I have a shell script that can be run by apache.
I want these scripts execute in FIFO (First IN First Out) manner.
Any ideas?

Not quite sure what you mean.

Commands are executed synchronously unless you tell them not to be.

e.g. If you had a script that said:
ls -l
df -h
cat /etc/fstab

It would not start the "df -h" until the "ls -l" had completed. It would not start the "cat /etc/fstab" until the "df -h" had completed.

You can make it asynchronous by putting ampserand (&) after the commands:
ls -l &
df -h &
cat /etc/fstab &
All 3 commands would start as soon as the prior one had started without waiting for a return status.

Some commands do background themselves. You could put in sleep statements to wait for them to complete:

ls -l
sleep 10
df -h
sleep 5
cat /etc/fstab

It would wait 10 seconds after ls -l to start the df -h. It would wait five seconds after the df -h to start the cat /etc/fstab.

Can you detail exactly what you're trying to do?

Thanks for your reply.
I have a script file myscript.sh.
I can run it like myscript.sh & in the background.
I can run more than one instances of myscript.sh & in the background.
I want these instances be run in FIFO manner.

Instead of running them with the "&" run them without it. That way it won't run the second myscript.sh until the first one has completed. That is the say it is the "&" that is making them go at the same time. Taking it out will make them go one after the other.

You really don't want to run several copies of the same script at the same time anyway because it hammers your CPU due to contention for the same resources.