The kill built-in sends a signal to processes.

Syntax

  • kill [-signal|-s signal|-n signal] process

  • kill -l [-v] [signal…]

The kill built-in requires that all options precede operands. Any command line arguments after the first operand are all treated as operands.

Description

When executed without the -l option, the built-in sends a signal to processes. The signal sent can be specified by option. The SIGTERM signal is sent if no signal is specified.

When executed with the -l option, the built-in prints information of signals to the standard output. If no signal is specified, information of all signals is printed.

Options

Signal-specifying options

-signal
-s signal
-n signal

A signal-specifying option specifies a signal to be sent to processes. signal can be specified by name or number. If number 0 is specified, the built-in checks if a signal could be sent to the processes but no signal is actually sent. Signal names are case-insensitive.

You can specify at most one signal-specifying option at a time.

Other options

-l

Print signal information instead of sending a signal.

-v

Print more signal information.

Without this option, the built-in prints the signal name only. This option adds the signal number and a short description.

When the -v option is specified, the -l option can be omitted.

Operands

processes

Specify processes to which a signal is sent.

Processes can be specified by the process ID, the process group ID, or the job ID. The process group ID must be prefixed with a hyphen (-) so that it is not treated as a process ID.

When 0 is specified as process, the signal is sent to the process group to which the shell process belongs. When -1 is specified, the signal is sent to all processes on the system.

signal

Specify a signal of which information is printed.

The signal can be specified by the name, the number, or the exit status of a command that was killed by the signal.

Exit status

The exit status of the kill built-in is zero unless there is any error. If the signal was sent to at least one process, the exit status is zero even if the signal was not sent to all of the specified processes.

Notes

The kill built-in is a semi-special built-in.

Command arguments that start with a hyphen should be used with care. The command kill -1 -2, for example, sends signal 1 to process group 2 since -1 is treated as a signal-specifying option and -2 as an operand that specifies a process group. The commands kill -- -1 -2 and kill -TERM -1 -2, on the other hand, treats both -1 and -2 as operands.

The POSIX standard does not define the -n or -v options, so they cannot be used in the POSIXly-correct mode. The standard does not allow specifying a signal number as the argument of the -s option or a signal name as the signal operand.

The standard requires signal names to be specified without the SIG prefix, like INT and QUIT. If the shell is not in the POSIXly-correct mode, the built-in accepts SIG-prefixed signal names as well.