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Bonus: Background Processes

Tools: GCC, Make, ps

Goal

Make the shell from 01-shell-chaining run a command in the background when the line ends with &: start it, do not wait, and give the prompt back at once. Afterwards you will be able to say what a zombie process is, why one exists, and how a parent collects a child that finished while the parent was busy.

Background

sleep 5 & starts sleep and returns to the prompt immediately. The shell still has to call waitpid() for that child at some point: a process that has ended stays in the process table, as a zombie, until its parent asks for its status, and a shell that never asks fills the table with its dead children.

The way to ask without blocking is waitpid() with the WNOHANG flag and -1 as the PID, meaning "any child that has finished, if there is one". Called in a loop before every prompt, it collects whatever finished since the last prompt and returns at once when nothing did. The other way is to have the kernel tell the shell when a child ends, with the SIGCHLD signal; it is harder to get right and not needed here.

The shipped shell.c is the skeleton of 01-shell-chaining with three TODOs added: & as an operator, a background flag for run_command(), and a reap_background() function that main() already calls before every prompt. Start from your finished shell.c and bring the new TODOs over, or paste your finished pieces into the shipped file.

Your Task

  1. TODO 3 — make & an operator. It ends the command before it, like ;, and the command after it always runs.
  2. TODO 4 — in run_command(), when background is set, print the child's PID on stderr and return 0 without waiting. In run_line(), a command runs in the background when the operator after it is &.
  3. TODO 5 — reap_background(): collect every finished child with waitpid(-1, ..., WNOHANG) in a loop, printing a line for each.

Build & Run

make
./shell

minish> sleep 5 &
[started 12345]
minish> echo meanwhile
meanwhile
minish>
[12345 finished]
minish>

Check Your Work

  • sleep 5 & gives the prompt back at once; the next prompt after the five seconds reports the child finished.
  • sleep 2 & echo chained prints the echo immediately; sleep 1 ; echo waited does not.
  • The zombie, on one line so that no prompt comes in between: sleep 1 & sleep 2 ; ps -C sleep -o pid,ppid,stat,cmd. Find the line with state Z and <defunct>, and say what its parent is. Then take the WNOHANG loop out, run several sleep 1 &, wait, and look again from another terminal.
  • Start three background sleeps of different lengths and watch the order the finish lines arrive in.
  • Run under strace -f -e trace=clone,wait4 and type sleep 1 &, then press Enter a few times. Find the wait4(-1, ..., WNOHANG, ...) calls, and what they return before and after the child ends.