[brownaryan]

What is the difference between fork and exec command in Unix?

[eggi]

Hey There,

Simply put, fork duplicates your shell (file descriptors, etc) in a subshell. Usually a forked process will then exec in that subshell.

exec exec's the command in your current shell, basically co-opting it.

In pure versions of exec, if you do, just at the shell prompt:

exec khs <-- instead of exec ksh

your shell will be replaced by khs, which doesn't exist, and you'll lose your shell (get logged out).

Redhat, OpenSolaris, and most Linux brands do a "fake exec" (really a fork and then exec) now, where, even when you call exec and flub it, you don't lose your shell.

Not a very technical explanation, but hopefully helpful

, Mike

[brownaryan]

Thanks Mike. It was indeed helpful. But I am somehow still not able to know the precise difference between both as both the commands spwans a new process at the end.

Regards

Brownaryan
www.unixhelpline.blogspot.com

[eggi]

Hey, no problem,

You're correct. They both do spawn a new process. It's just the manner in which the do it. Try this experiment (dont' use RedHat bash, or any Linux shell that doesn't implement exec like it's supposed to (actually doing a fork and exec to save you from losing your session. I'm pretty sure Solaris and HP-UX, at least up to 9 do exec the "old fashioned way", or the "real way")

login to your shell

type:

blah

(this is doing the standard fork and exec - spawning a subshell and exec'ing the command there) you should get back something like

blah: command not found

and be returned to your command prompt. then type

exec blah

and you should be kicked out of your shell.

Taken with the above explanation, that kind of drives it home. exec spawns a new process by replacing the process space, and fork exec's a new process in a subshell.

Hope that helps

,Mike

[brownaryan]

'exec spawns a new process by replacing the process space, and fork exec's a new process in a subshell.'

I got your above mentioned point Mike. Thanx

Brownaryan

[eggi]

Cool,

I seem to have over-explained again

Glad you're good to go!

, Mike