Killing Processes

Sending the Default Signal

kill pid without a signal name sends SIGTERM, a polite termination request that a process can trap and respond to before actually exiting, unlike a forceful kill.

Example: Sending the Default Signal

#!/bin/bash
sleep 5 &
pid=$!
kill "$pid"
wait "$pid" 2>/dev/null
echo "background sleep was asked to terminate, wait returned status $?"

Checking Before You Kill

kill -0 pid sends no actual signal but checks whether the process exists and is signalable, returning a normal exit status; this avoids errors from trying to kill an already-finished job.

Example: Checking Before You Kill

#!/bin/bash
sleep 0.2 &
pid=$!
if kill -0 "$pid" 2>/dev/null; then
    echo "process $pid is still running"
fi
wait "$pid"
if ! kill -0 "$pid" 2>/dev/null; then
    echo "process $pid is no longer running"
fi

Forceful Termination with SIGKILL

kill -9 pid (SIGKILL) cannot be caught, blocked, or ignored by the target process, so it always terminates it immediately, but the process has no opportunity to run its own cleanup code.

Note: Reserve SIGKILL for processes that ignore a normal SIGTERM request, since it skips any cleanup the process might have wanted to run.

Example: Forceful Termination with SIGKILL

#!/bin/bash
sleep 5 &
pid=$!
kill -9 "$pid"
wait "$pid" 2>/dev/null
status=$?
echo "exit status after SIGKILL: $status"

Understanding Signal-Based Exit Codes

When a process is terminated by a signal instead of exiting normally, its reported exit status is conventionally 128 plus the signal's number (for example, SIGTERM is 15, so a terminated process often reports 143). Note $BASHPID (the actual subshell's PID) is used here rather than $$, since $$ always refers to the original top-level shell even inside a subshell.

Warning: Sending SIGTERM to $$ instead of $BASHPID from inside a subshell would terminate the whole script, not just the subshell.

Example: Understanding Signal-Based Exit Codes

#!/bin/bash
( kill -TERM $BASHPID ) &
sub_pid=$!
wait "$sub_pid"
echo "the subshell that sent itself SIGTERM reported status: $?"