Kill built-in

The kill built-in sends a signal to processes.

Synopsis

kill [-s SIGNAL|-n SIGNAL|-SIGNAL] target…

kill -l|-v [SIGNAL|exit_status]…

Description

Without the -l or -v option, the built-in sends a signal to processes.

With the -l or -v option, the built-in lists signal names or descriptions.

The -s or -n option specifies the signal to send. The signal name is case-insensitive, but must be specified without the SIG prefix. The default signal is SIGTERM. (Specifying a signal name with the SIG prefix may be allowed in the future.)

The signal may be specified as a number instead of a name. If the number is zero, the built-in does not send a signal, but instead checks whether the shell can send the signal to the target processes.

The obsolete syntax allows the signal name or number to be specified directly after the hyphen like -TERM and -15 instead of -s TERM and -n 15.

The -l option lists signal names. The names are printed one per line, without the SIG prefix.

The -v option implies and extends the -l option by displaying the signal number before each name. The output format for -v may be changed in the future to include signal descriptions as well.

Operands

Without the -l or -v option, the built-in takes one or more operands that specify the target processes. Each operand is one of the following:

A positive decimal integer, which should be a process ID
A negative decimal integer, which should be a negated process group ID
0, which means the current process group
-1, which means all processes
A job ID, which means the process group of the job

With the -l or -v option, the built-in may take operands that limit the output to the specified signals. Each operand is one of the following:

The exit status of a process that was terminated by a signal
A signal number
A signal name without the SIG prefix

Without operands, the -l and -v options list all signals.

Errors

It is an error if:

The -l or -v option is not specified and no target processes are specified.
A specified signal is not supported by the shell.
A specified target process does not exist.
The target job specified by a job ID operand is not job-controlled, that is, job control was off when the job was started.
The signal cannot be sent to any of the target processes specified by an operand.
An operand specified with the -l or -v option does not identify a supported signal.

Exit status

The exit status is zero unless an error occurs. The exit status is zero if the signal is sent to at least one process for each operand, even if the signal cannot be sent to some of the processes.

Usage notes

When a target is specified as a job ID, the built-in cannot tell whether the job process group still exists. If the job process group has been terminated and another process group has been created with the same process group ID, the built-in will send the signal to the new process group.

Examples

Sending SIGTERM to the last started asynchronous command and showing the signal name represented by the exit status:

$ sleep 10&
$ kill $!
$ wait $!
$ echo "Exit status $? corresponds to SIG$(kill -l $?)"
Exit status 399 corresponds to SIGTERM

Specifying a signal name and job ID:

$ set -m
$ sleep 10&
$ kill -s INT %1
$ wait %1
$ echo "Exit status $? corresponds to SIG$(kill -l $?)"
Exit status 386 corresponds to SIGINT

Specifying a signal number:

$ sleep 10&
$ kill -n 1 $!
$ wait $!
$ echo "Exit status $? corresponds to SIG$(kill -l $?)"
Exit status 385 corresponds to SIGHUP

The -- separator is needed if the first operand starts with a hyphen (a negated process group ID):

$ set -m
$ sleep 10&
$ kill -n 15 -- -$!
$ wait $!
$ echo "Exit status $? corresponds to SIG$(kill -l $?)"
Exit status 399 corresponds to SIGTERM

Compatibility

The kill built-in is specified by POSIX.1-2024.

Specifying a signal number other than 0 to the -s option is a non-standard extension.

Specifying a signal number to the -n option is a ksh extension. This implementation also supports the -n option with a signal name.

The kill -SIGNAL target… form may not be parsed as expected by other implementations when the signal name starts with an s. For example, kill -stop 123 may try to send the SIGTOP signal instead of the SIGSTOP signal.

POSIX defines the following signal numbers:

0 (a dummy signal that can be used to check whether the shell can send a signal to a process)
1 (SIGHUP)
2 (SIGINT)
3 (SIGQUIT)
6 (SIGABRT)
9 (SIGKILL)
14 (SIGALRM)
15 (SIGTERM)

Other signal numbers are implementation-defined.

Using the -l option with more than one operand is a non-standard extension. Specifying a signal name operand to the -l option is a non-standard extension.

The -v option is a non-standard extension.

Some implementations print 0 or EXIT for kill -l 0 or kill -l EXIT while this implementation regards them as invalid operands.

On some systems, a signal may have more than one name. There seems to be no consensus whether kill -l should print all names or just one name for each signal. This implementation currently prints all names, but this behavior may change in the future.

Yash-rs User Manual