UNIX for Advanced & Expert Users

I have read that exec "replaces the current process with a new one".

So I did

$ exec ls

and after this executed, my shell disappeared. I am assuming that my shell had PID xyz, and when I did exec ls, this ls got pid xyz, and when it terminated, there was no more shell process running, and so it terminated. Is this correct??

Second question, say I am running a Java program, and I do a system call, of exec ls. Does this mean that the Java process will be replaced by ls?

Thanks in advance.

for exec ls,

the child process ls - will replace the address space of the parent process which is the interpretor, after exec effectively there is only one address space in execution previously it was the parent ( shell ) and now its the child ( ls )

After execution of ls, like any other binary - ls also terminates. Once it terminates the session also closes as there is no effective program running

As the adverts says: "It does exactly what it says on the tin."

oh, I get it. The parent is replaced by the child. Yes, makse sense.

Thanks.

Almost, there is no child process created.

In UNIX a process is a running entity managed by the operating system and a program is a file on disk.

In MS-DOS, Windows, OS/2, classic MacOS the only way to create a process was to run a program and the OS would create a new process to run the program.

In UNIX they are two separate activities (a) create a new process, ie, fork and (b) replace the current running program with a new program, eg exec.

So in Windows what would be "CreateProcess" is in UNIX a fork followed by exec. Although 99% of the time we want to fork/exec there are a few occasions where replacing the current program is the thing to do.

Ah. I get it (this time I do!!!)

Thanks!