Help required SIGQUIT in Bash

/tmp/trap.sh

#! /bin/bash
echo parent
trap signalCaught HUP INT QUIT TERM

signalCaught() {
    echo "SIGNAL DETECTED I am the parent."
}

SLEEP=10
for i in $(seq $SLEEP -1 0); do
    echo "$i"
    sleep 1
done

/tmp/trap2.sh 2>&1 | tee -ai /tmp/garbage.txt

echo " --- terminating \"$0\" "

/tmp/trap2.sh

#! /bin/bash
echo child
trap 'echo signal caught in child' HUP INT QUIT TERM
read JUNK

SLEEP=10
echo sleeping for $SLEEP seconds
sleep $SLEEP

echo " --- terminating \"$0\" "

When I run /tmp/trap.sh, and allow it to call trap2.sh, SIGQUIT is only caught by the parent process (trap.sh). The "echo signal caught in child" is not echoed. I assume, then, that the child do开发者_开发知识库es not catch SIGQUIT.

Is there a reason the child does not catch QUIT? It does catch INT

tee -ai /tmp/garbage.txt is catching SIGQUIT. So for example, when both trap.sh and trap2.sh are running, you have something like:

% pstree 62655
-+= 62655 nicholas -zsh
 \-+= 62867 nicholas /bin/bash ./trap.sh
   |--- 62889 nicholas /bin/bash /tmp/trap2.sh
   \--- 62890 nicholas tee -ai /tmp/garbage.txt

When I press ^\, it gets delivered to the bottom of the tree (pid 62890):

% sudo dtrace -n 'proc:::signal-send /pid/ { printf("%s -%d %d",execname,args[2],args[1]->pr_pid); }'
dtrace: description 'proc:::signal-send ' matched 1 probe
CPU     ID                    FUNCTION:NAME
  1  19556          sigprocmask:signal-send Terminal -3 62890

If I explicitly kill -QUIT 62889, then it does indeed print signal caught in child.

(Thanks to the commenter for making me challenge my assumptions: my previous answer here was completely wrong.)

As @Random832 mentions, process groups are used for delivering signals. In pstree output, = indicates the leader of a process group. You can also see with ps -j output, trap.sh, trap2.sh and tee -ai ... are in the same group:

% pstree 64261
-+= 64261 nicholas -zsh
 \-+= 64551 nicholas /bin/bash ./trap.sh
   |--- 64554 nicholas /bin/bash /tmp/trap2.sh
   \--- 64555 nicholas tee -ai /tmp/garbage.txt
% ps -jxp 64261,64551,64554,64555
USER       PID  PPID  PGID   SESS JOBC STAT   TT       TIME COMMAND
nicholas 64261 64260 64261 90c3998    1 S    s002    0:00.12 -zsh
nicholas 64551 64261 64551 90c3998    1 S+   s002    0:00.01 /bin/bash ./trap.sh
nicholas 64554 64551 64551 90c3998    1 S+   s002    0:00.00 /bin/bash /tmp/trap2.sh
nicholas 64555 64551 64551 90c3998    1 S+   s002    0:00.00 tee -ai /tmp/garbage.txt