killall

Kill processes by name.

Syntax
     killall [-delmsvz] [-help] [-u user] [-t tty] [-c procname] [-SIGNAL] [procname ...]

Key
   -d | -v     Be more verbose about what will be done.  For a single
               -d option, a list of the processes that will be sent
               the signal will be printed, or a message indicating
               that no matching processes have been found.

   -e          Use the effective user ID instead of the (default) real
               user ID for matching processes specified with the -u option.

   -help       Give a help on the command usage and exit.

   -l          List the names of the available signals and exit, like in kill.

   -m          Match the argument procname as a (case sensitive) regular expression
               against the names of processes found.
               CAUTION!  This is dangerous, a single dot will match any process running
               under the real UID of the caller.

   -s          Show only what would be done, but do not send any signal.

   -SIGNAL     Send a different signal instead of the default TERM.
               The signal can be specified either as a name (with or without
               a leading SIG), or numerically.

   -u user     Limit potentially matching processes to those belonging
               to the specified user.

   -t tty      Limit potentially matching processes to those running
               on the specified tty.

   -c procname
               When used with the -u or -t flags, limit potentially matching
               processes to those matching the specified procname.

   -z          Do not skip zombies.  This should not have any effect except to print a 
               few error messages if there are zombie processes that match the specified pattern.

All Processes

Sending a signal to all processes with uid XYZ is already supported by kill. So use kill for this job (e.g. $ kill -TERM -1 or as root $ echo kill -TERM -1 | su -m user)

Common Kill Signals

Signal name	Signal value	Effect
SIGHUP	1	Hangup
SIGINT	2	Interrupt from keyboard
SIGQUIT	3	Quit
SIGABRT	6	Abort
SIGKILL	9	Kill signal
SIGTERM	15	Termination signal - allow an orderly shutdown
SIGSTOP	17,19,23	Stop the process

Exit Status

0  Success
1  No matching process has been found or not all processes have been signalled successfully.
2  Command error.

Examples

Check if the process ss64 is running:
$ $ ps -aef | grep ss64

If so, kill it:
$ killall ss64

Kill two processes at once:
$ killall process1 p

Flush the DNS cache, the mDNSResponder will automatically restart.

# MacOS 10.7, 10.8, and 10.9, 10.10.4 and above:
$ sudo killall -HUP mDNSResponder

# for older (pre 2009) versions of macOS see dscacheutil

# Tip: You can use an alias to save  remembering long commands:
$ alias flushdns='sudo killall -HUP mDNSResponder'

"Love never dies a natural death. It dies because we don't know how to replenish it's source. It dies of blindness and errors and betrayals.
It dies of illness and wounds; it dies of weariness, of witherings, of tarnishings" ~ Anais Nin

Related macOS commands:

ctrl+z / ctrl+c - Suspend/Interrupt a program
ps - List running processes (returns PID)
pkill - Kill processes by a full or partial name.
kill - Kill a process by specifying its PID.
sigaction(2)
lsof - List open files