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cforkconceptual

What Does the Process Tree Look Like Using Fork()?

Say, for example, we have the following executed in the shell: 

ls | grep "abc" | wc

I understand how a child process would fork from the shell and how its a child, like this,

Shell (pid=12)
       \
        \
         ls (pid=13)

but I'm not sure how the pipe and the trailing commands fit in. Are they also children of the shell, and not ls?

Solution

  • Each component of the pipeline will be a child process of the shell. If you use pstree -p from another terminal, you'll probably see something like this:

    ...
sshd(11)---bash(12)-+-ls(13)
                    |-grep(14)
                    \-wc(15)
...

    (assuming you can run this whilst your pipeline is still running!)

    However, note that all the components will form a single process group.

    The piping itself is a feature of Linux (or whatever OS you're using), it's not a separate user process. The shell creates some anonymous pipes, and hooks them up to the relevant file descriptors for each pair of processes in the pipeline.