What's the best way to send a signal to all members of a process group?

You don't say if the tree you want to kill is a single process group. (This is often the case if the tree is the result of forking from a server start or a shell command line.) You can discover process groups using GNU ps as follows:

 ps x -o  "%p %r %y %x %c "

If it is a process group you want to kill, just use the kill(1) command but instead of giving it a process number, give it the negation of the group number. For example to kill every process in group 5112, use kill -TERM -- -5112.

Kill all the processes belonging to the same process tree using the Process Group ID (PGID)

• kill -- -$PGID     Use default signal (TERM = 15)
• kill -9 -$PGID     Use the signal KILL (9)

You can retrieve the PGID from any Process-ID (PID) of the same process tree

• kill -- -$(ps -o pgid= $PID | grep -o '[0-9]*')   (signal TERM)
• kill -9 -$(ps -o pgid= $PID | grep -o '[0-9]*')   (signal KILL)

^{Special thanks to tanager and Speakus for contributions on $PID remaining spaces and OSX compatibility.}

Explanation

• kill -9 -"$PGID" => Send signal 9 (KILL) to all child and grandchild...
• PGID=$(ps opgid= "$PID") => Retrieve the Process-Group-ID from any Process-ID of the tree, not only the Process-Parent-ID. A variation of ps opgid= $PID is ps -o pgid --no-headers $PID where pgid can be replaced by pgrp.
  But:
  • ps inserts leading spaces when PID is less than five digits and right aligned as noticed by tanager. You can use:
    PGID=$(ps opgid= "$PID" | tr -d ' ')
  • ps from OSX always print the header, therefore Speakus proposes:
    PGID="$( ps -o pgid "$PID" | grep [0-9] | tr -d ' ' )"
• grep -o [0-9]* prints successive digits only (does not print spaces or alphabetical headers).

Further command lines

PGID=$(ps -o pgid= $PID | grep -o [0-9]*)kill -TERM -"$PGID"  # kill -15kill -INT  -"$PGID"  # correspond to [CRTL+C] from keyboardkill -QUIT -"$PGID"  # correspond to [CRTL+\] from keyboardkill -CONT -"$PGID"  # restart a stopped process (above signals do not kill it)sleep 2              # wait terminate process (more time if required)kill -KILL -"$PGID"  # kill -9 if it does not intercept signals (or buggy)

Limitation

• As noticed by davide and Hubert Kario, when kill is invoked by a process belonging to the same tree, kill risks to kill itself before terminating the whole tree killing.
• Therefore, be sure to run the command using a process having a different Process-Group-ID.

Long story

> cat run-many-processes.sh#!/bin/shecho "ProcessID=$$ begins ($0)"./child.sh background &./child.sh foregroundecho "ProcessID=$$ ends ($0)"> cat child.sh#!/bin/shecho "ProcessID=$$ begins ($0)"./grandchild.sh background &./grandchild.sh foregroundecho "ProcessID=$$ ends ($0)"> cat grandchild.sh#!/bin/shecho "ProcessID=$$ begins ($0)"sleep 9999echo "ProcessID=$$ ends ($0)"

Run the process tree in background using '&'

> ./run-many-processes.sh &    ProcessID=28957 begins (./run-many-processes.sh)ProcessID=28959 begins (./child.sh)ProcessID=28958 begins (./child.sh)ProcessID=28960 begins (./grandchild.sh)ProcessID=28961 begins (./grandchild.sh)ProcessID=28962 begins (./grandchild.sh)ProcessID=28963 begins (./grandchild.sh)> PID=$!                    # get the Parent Process ID> PGID=$(ps opgid= "$PID")  # get the Process Group ID> ps fj PPID   PID  PGID   SID TTY      TPGID STAT   UID   TIME COMMAND28348 28349 28349 28349 pts/3    28969 Ss   33021   0:00 -bash28349 28957 28957 28349 pts/3    28969 S    33021   0:00  \_ /bin/sh ./run-many-processes.sh28957 28958 28957 28349 pts/3    28969 S    33021   0:00  |   \_ /bin/sh ./child.sh background28958 28961 28957 28349 pts/3    28969 S    33021   0:00  |   |   \_ /bin/sh ./grandchild.sh background28961 28965 28957 28349 pts/3    28969 S    33021   0:00  |   |   |   \_ sleep 999928958 28963 28957 28349 pts/3    28969 S    33021   0:00  |   |   \_ /bin/sh ./grandchild.sh foreground28963 28967 28957 28349 pts/3    28969 S    33021   0:00  |   |       \_ sleep 999928957 28959 28957 28349 pts/3    28969 S    33021   0:00  |   \_ /bin/sh ./child.sh foreground28959 28960 28957 28349 pts/3    28969 S    33021   0:00  |       \_ /bin/sh ./grandchild.sh background28960 28964 28957 28349 pts/3    28969 S    33021   0:00  |       |   \_ sleep 999928959 28962 28957 28349 pts/3    28969 S    33021   0:00  |       \_ /bin/sh ./grandchild.sh foreground28962 28966 28957 28349 pts/3    28969 S    33021   0:00  |           \_ sleep 999928349 28969 28969 28349 pts/3    28969 R+   33021   0:00  \_ ps fj

The command pkill -P $PID does not kill the grandchild:

> pkill -P "$PID"./run-many-processes.sh: line 4: 28958 Terminated              ./child.sh background./run-many-processes.sh: line 4: 28959 Terminated              ./child.sh foregroundProcessID=28957 ends (./run-many-processes.sh)[1]+  Done                    ./run-many-processes.sh> ps fj PPID   PID  PGID   SID TTY      TPGID STAT   UID   TIME COMMAND28348 28349 28349 28349 pts/3    28987 Ss   33021   0:00 -bash28349 28987 28987 28349 pts/3    28987 R+   33021   0:00  \_ ps fj    1 28963 28957 28349 pts/3    28987 S    33021   0:00 /bin/sh ./grandchild.sh foreground28963 28967 28957 28349 pts/3    28987 S    33021   0:00  \_ sleep 9999    1 28962 28957 28349 pts/3    28987 S    33021   0:00 /bin/sh ./grandchild.sh foreground28962 28966 28957 28349 pts/3    28987 S    33021   0:00  \_ sleep 9999    1 28961 28957 28349 pts/3    28987 S    33021   0:00 /bin/sh ./grandchild.sh background28961 28965 28957 28349 pts/3    28987 S    33021   0:00  \_ sleep 9999    1 28960 28957 28349 pts/3    28987 S    33021   0:00 /bin/sh ./grandchild.sh background28960 28964 28957 28349 pts/3    28987 S    33021   0:00  \_ sleep 9999

The command kill -- -$PGID kills all processes including the grandchild.

> kill --    -"$PGID"  # default signal is TERM (kill -15)> kill -CONT -"$PGID"  # awake stopped processes> kill -KILL -"$PGID"  # kill -9 to be sure> ps fj PPID   PID  PGID   SID TTY      TPGID STAT   UID   TIME COMMAND28348 28349 28349 28349 pts/3    29039 Ss   33021   0:00 -bash28349 29039 29039 28349 pts/3    29039 R+   33021   0:00  \_ ps fj

Conclusion

I notice in this example PID and PGID are equal (28957).
This is why I originally thought kill -- -$PID was enough. But in the case the process is spawn within a Makefile the Process ID is different from the Group ID.

I think kill -- -$(ps -o pgid= $PID | grep -o [0-9]*) is the best simple trick to kill a whole process tree when called from a different Group ID (another process tree).

pkill -TERM -P 27888

This will kill all processes that have the parent process ID 27888.

Or more robust:

CPIDS=$(pgrep -P 27888); (sleep 33 && kill -KILL $CPIDS &); kill -TERM $CPIDS

which schedule killing 33 second later and politely ask processes to terminate.

See this answer for terminating all descendants.