kill / pkill / killall

Use kill to send a signal by specifying a PID.

kill PID
kill -signal_name PID
kill -signal_number PID

Use pkill to search by process name and send a signal.

pkill process_name
pkill -signal process_name
pkill -u username process_name

Use killall to send a signal to all processes with an exactly matching name.

killall process_name
killall -signal process_name

List available signals.

kill -l

Signal	Number	Description
SIGTERM	15	Requests graceful termination (default). The process can clean up before exiting.
SIGKILL	9	Forces immediate termination. The process cannot ignore this signal and exits without cleanup.
SIGINT	2	Interrupt signal, equivalent to pressing Ctrl+C.
SIGHUP	1	Hangup signal. Many daemons use this to reload their configuration files.
SIGSTOP	19	Pauses a process (cannot be ignored).
SIGCONT	18	Resumes a stopped process.
SIGUSR1/2	10/12	User-defined signals for application-specific handling.

The following examples assume that sleep 1000 is running with PID 12345.

ps aux | grep "sleep" | grep -v grep
alice  12345  0.0  0.0  5536  1024 pts/0  S  10:00  0:00 sleep 1000

Request graceful termination with SIGTERM. When no signal is specified, the default SIGTERM (15) is sent.

kill 12345

Use SIGKILL (-9) to force-terminate an unresponsive process. The process stops immediately without any cleanup.

kill -9 12345
kill -SIGKILL 12345   # same effect

Send SIGHUP to nginx to reload its configuration file. Most daemons reload their config when they receive this signal.

kill -HUP $(cat /var/run/nginx.pid)

Use pkill to stop a process by name, saving you the trouble of looking up its PID.

pkill sleep          # send SIGTERM to processes whose name contains "sleep"
pkill -9 myapp       # force-terminate
pkill -u www-data php-fpm   # stop a process owned by a specific user

killall sends a signal to all processes whose name matches exactly.

killall nginx

Use kill -0 to check whether a process exists without sending an actual signal. This is useful for monitoring processes in scripts.

check_process.sh

kill 12345 && echo "Signal sent successfully"
sleep 1
if ! kill -0 12345 2>/dev/null; then
    echo "Process has terminated"
fi

bash check_process.sh
Signal sent successfully
Process has terminated

Example of capturing a background process PID and stopping it.

kill_bg.sh

sleep 100 &
bg_pid=$!
echo "Background PID: $bg_pid"
kill "$bg_pid"
echo "Stop signal sent"

bash kill_bg.sh
Background PID: 12348
Stop signal sent

The recommended practice is to first send SIGTERM (-15) to give the process a chance to exit gracefully, and only use SIGKILL (-9) if the process does not terminate. SIGKILL skips flushing open files and releasing resources, which can result in data corruption.

$ kill -0 PID only checks whether the process exists without sending any signal (exit status 0 = exists, non-zero = does not exist). This is useful for process monitoring in scripts.

To inspect processes, see ps / top / htop. For running processes in the background, see & (background execution).