kill – send signals to processes

kill sends signals to processes, most commonly to terminate them. Despite the name, it can send any signal.

Synopsis

kill [OPTIONS] PID...
kill -SIGNAL PID...

Common Signals

Examples

Terminate process gracefully

$ kill 1234
$ kill -15 1234
$ kill -TERM 1234

Force kill (when TERM doesn’t work)

$ kill -9 1234
$ kill -KILL 1234

Reload configuration

$ kill -HUP $(pgrep nginx)
$ sudo kill -HUP 1    # Reload init

Kill by name

$ pkill firefox
$ killall firefox

Kill all processes of a user

$ pkill -u username

List available signals

$ kill -l
 1) SIGHUP       2) SIGINT       3) SIGQUIT      4) SIGILL
 5) SIGTRAP      6) SIGABRT      7) SIGBUS       8) SIGFPE
 9) SIGKILL     10) SIGUSR1     11) SIGSEGV     12) SIGUSR2
...

Pause and resume

$ kill -STOP 1234    # Pause
$ kill -CONT 1234    # Resume

Kill vs Kill -9

Always try graceful termination first:

# Step 1: Ask nicely
$ kill 1234

# Step 2: Wait a few seconds
$ sleep 5

# Step 3: Force if still running
$ kill -9 1234

Why? SIGTERM allows processes to:

• Save data
• Clean up temp files
• Close connections properly

SIGKILL (-9) terminates immediately without cleanup.

Common Patterns

Kill process on port

$ kill $(lsof -t -i:8080)
$ fuser -k 8080/tcp    # Alternative

Kill all matching processes

$ pkill -f "python script.py"

Kill process tree

$ pkill -P 1234    # Kill children of PID 1234

Tips

• Try TERM first: Always give processes a chance to clean up
• Check before killing: ps -p PID to verify it’s the right process
• Use pkill/killall: Easier than finding PIDs manually
• Zombie processes: Can’t be killed (parent must handle)
• Permission: Need to own process or be root

See Also

Related Commands

• ps — Find process IDs
• pkill — Kill by name
• killall — Kill all matching processes
• top — Interactive process killer

Tutorials

• Basic Unix Commands