kill Command Cheat Sheet

The kill command is a built-in shell utility (and a standalone binary /bin/kill) used to send signals to running processes. Despite its name, it can send any signal, not just termination signals.

Synopsis

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

Basic Termination

Terminate a Process (Graceful)

Sends SIGTERM (15) by default. This asks the process to stop nicely (save data, close files, etc.).

kill 1234

Force Kill (Immediate)

Sends SIGKILL (9). The kernel removes the process immediately. The process cannot catch or ignore this signal. Data loss may occur.

kill -9 1234

Signals

List All Available Signals

kill -l

Common Signals

ID	Name	Effect	Use Case
1	SIGHUP	Hangup	Reload configuration (e.g., `kill -1 <pid>` for nginx/apache)
2	SIGINT	Interrupt	Equivalent to `Ctrl+C`
3	SIGQUIT	Quit	Quit and dump core (`Ctrl+\`)
9	SIGKILL	Kill	Force kill (Cannot be caught)
15	SIGTERM	Terminate	Polite kill (Default)
18	SIGCONT	Continue	Resume a stopped process
19	SIGSTOP	Stop	Pause process (Cannot be caught)

Sending Signals by Name or Number

All equivalent:

kill -1 1234
kill -HUP 1234
kill -SIGHUP 1234

Advanced Usage

Kill Multiple Processes

kill 1234 1235 1236

Kill All Processes You Can

⚠️ Dangerous: kill -9 -1 kills all processes you are allowed to kill (except the killing process itself). If run as root, this can crash the system.

Checking if a Process Exists

Sending signal 0 checks permissions and existence but doesn't affect the process.

if kill -0 1234; then
    echo "Process 1234 is running."
else
    echo "Process 1234 is NOT running."
fi

Process Groups

You can kill an entire process group by specifying a negative PID.

# Kill process group 1234
kill -TERM -1234

Exit Status

Code	Meaning
0	Success
1	Failure (Permissions, No process found)

Notes

Bash Builtin: type kill usually returns kill is a shell builtin. This allows you to kill jobs using %1 syntax.
Alternatives:
- pkill: Kill by name pattern.
- killall: Kill by exact name.
- xkill: Kill graphical window by clicking on it.