kill

Send signals to processes.

Source: src/execution/builtins.f90:1139-1283

Synopsis

kill [-s signal | -signal] pid|jobspec ...
kill -l [signal]

Description

The kill builtin sends signals to processes or jobs. Despite its name, it can send any signal, not just termination signals.

Options

Option	Description
`-s signal`	Specify signal by name
`-signal`	Specify signal (name or number)
`-l`	List signal names
`-l signal`	Convert signal number to name

Usage

Send SIGTERM (default)

kill 1234
kill %1        # Job 1

Send Specific Signal

kill -9 1234           # SIGKILL
kill -KILL 1234        # Same
kill -s KILL 1234      # Same

kill -HUP 1234         # SIGHUP
kill -TERM 1234        # SIGTERM
kill -STOP 1234        # Suspend
kill -CONT 1234        # Resume

List Signals

kill -l
#  1) SIGHUP    2) SIGINT    3) SIGQUIT
#  9) SIGKILL  15) SIGTERM  ...

kill -l 9
# KILL

kill -l TERM
# 15

Kill Job

sleep 100 &
# [1] 12345

kill %1        # Kill job 1
kill %%        # Kill current job
kill %+        # Same as %%

Common Signals

Signal	Number	Action	Use
HUP	1	Terminate	Reload config
INT	2	Terminate	Interrupt (Ctrl+C)
QUIT	3	Core dump	Quit
KILL	9	Terminate	Force kill (can't catch)
TERM	15	Terminate	Graceful shutdown
STOP	19	Stop	Suspend (can't catch)
CONT	18	Continue	Resume
USR1	10	User defined	Custom
USR2	12	User defined	Custom

Examples

Graceful Then Force

pid=1234

# Try graceful first
kill $pid
sleep 2

# Check if still running
if kill -0 $pid 2>/dev/null; then
    kill -9 $pid
fi

Kill Process Group

# Negative PID kills entire group
kill -- -1234

Reload Configuration

# Send SIGHUP to daemon
kill -HUP $(cat /var/run/daemon.pid)

Check If Process Exists

if kill -0 $pid 2>/dev/null; then
    echo "Process $pid is running"
fi

Kill All Background Jobs

kill $(jobs -p)

Signal Script

# In script: handle USR1
trap 'reload_config' USR1

# From outside: send USR1
kill -USR1 $(cat /var/run/script.pid)

Process Groups

# Start process group
(
    trap '' INT   # Ignore in group
    ./process1 &
    ./process2 &
    wait
) &
pgid=$!

# Kill entire group
kill -- -$pgid

Implementation

Source: builtins.f90:1139-1283

Key features:

Signal name/number conversion
Job specification parsing (%n)
Process group handling (negative PID)
Uses c_kill() system call

Exit Status

Status	Condition
0	Signal sent successfully
1	Invalid signal, PID, or permission denied

Notes

kill -0 checks if process exists (no signal sent)
Root can kill any process
Non-root can only kill own processes
-9 should be last resort

Differences from /bin/kill

Feature	builtin	/bin/kill
Job specs	Yes	No
Shell signals	Yes	Limited
Exit status	Shell-aware	Standard