I am extending the functionality of a custom unix shell which I wrote as part of my lab assignment. It currently supports all commands through execvp calls, in-built commands like pwd, cd, history, echo and export, and also redirection and pipes. Now I wanted to add support for running a command in background e.g. $ls -la& I also want to implement bg and fg job control commands.
I know this can be achieved if I execute the command by forking a new child process and not waiting for it in the parent process. But how do I again bring this command to foreground using fg? I have the idea of entering each background command in a list assigning each of them a serial number. But I don't know how do I make the processes execute in the background, then bring them back to foreground. I guess wait() and waitpid() system calls would come handy but I am not that comfortable with them. I tried reading the man pages but still am in the dark. Can someone please explain in a layman's language how to achieve this in UNIX system programming? And does it have something to do with SIGCONT and SIGSTP signals?
Best Answer
Suspending a command (CTRL-Z) works by sending a
SIGTSTPto the child process. The shell is then free to interact with the user via stdin/stdout.Resuming is accomplished by sending
SIGCONTto the child process and then continuing to wait for it to finish.Backgrounding a process is accomplished by sending
SIGCONTbut not waiting for it to finish. In this case, both the shell and the child process can interact with stdout/stderr, but stdin is typically intercepted by the shell.Starting a process in the background is accomplished by doing a fork/exec, but not blocking until the child process exists.
Typically (as in always) a shell will also handle
SIGCHLD, which is the way the OS notifies the parent process that a child processes has finished so that the parent can collect the exit code. Otherwise you end up with "zombie" processes; ones which aren't running but which haven't yet been collected by a parent.