Sending SIGINT from keyboard to piped commands in bash

Question

If in bash I run a | b | c | d on the command line and then press ^C, which process gets the signal?

Answer 1

In short, they all do.

When setting up a pipeline, the shell creates a process group. ^C is interpreted by the kernel's line discipline as the user's request to interrupt the process group currently running in the foreground. Sending a signal such as SIGINT to a process group automatically delivers the signal to all processes in the group.

Answer 2

I like experimentation better:

#!/bin/bash
# FILE /tmp/bla.sh
# trap ctrl-c and call ctrl_c()
trap ctrl_c INT

MY_ID=$1 # Identifier for messages

function ctrl_c() {
    echo >&2 "GOODBYE $MY_ID"
    exit
}

# This will continue until interrupted, e.g. if the input/output get closed
cat
# If we somehow got to the end
echo >&2 "grace $MY_ID"

Chaining them, running and breaking them

nitz@mars:~$ /tmp/bla.sh 1 | /tmp/bla.sh 2
^CGOODBYE 2
GOODBYE 1
0

As you can see, both executions got the interrupt signal, meaning they all get killed. Furthermore, the order in which they output that they were killed is random, e.g.:

nitz@mars:~$ /tmp/bla.sh 1 | /tmp/bla.sh 2 | /tmp/bla.sh 3 | /tmp/bla.sh 4
^CGOODBYE 2
GOODBYE 4
GOODBYE 1
GOODBYE 3