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

Question

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?

For OP: get an idea from [**this code**](http://www.ladweb.net/src/ladsh4.c.html) how shell actually implements `|` operator. search this line in code: `if (!(newJob.progs[i].pid = fork()))` Then you came to know ls, grep, wc are child processes of shell...The code is not bash code but Bash do similarly.. — Grijesh Chauhan, Mar 03 '13 at 21:41

Oliver Charlesworth · Accepted Answer · 2013-03-03T21:47:22.003

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.

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

1 Answers1