killall, pidof and finding a process by name

From names to numbers

kill speaks in PIDs, but people think in names: *stop the backup*, *restart the web server*, *how many shells are open?*. Copying numbers out of ps works, yet it is slow and error-prone. Two small tools do the lookup for you. pidof NAME prints the PIDs of every process running the program NAME. killall NAME skips the printing and sends a signal straight to all of them. On full distributions you will also meet pgrep and pkill, which match patterns instead of exact names; this BusyBox does not include them, so here pidof, killall and the ps pipelines from lesson 52 are the whole toolbox.

~% sleep 1000 &
[1] 883
~% sleep 2000 &
[2] 884
~% pidof sleep
884 883
~% pidof -s sleep
884
~% pidof sh
881 859 855
~% pidof nosuch; echo $?
1

pidof prints all the matches on one line, separated by spaces, which is exactly the shape kill wants: kill $(pidof sleep) sends TERM to both sleeps. With -s it prints a single PID, handy when you need to plug one number into a path such as /proc/$(pidof -s sleep)/status. -o PID omits a PID from the answer, and the special form -o %PPID omits the shell that ran pidof, which is how a script asks *is another copy of me already running?*. The exit status is 0 when something matched and 1 when nothing did, so if pidof sleep >/dev/null; then … is a clean *is it running?* test.

What counts as the name

Both tools compare against the program name, the same short name ps -o comm shows, not the full command line. pidof 'sleep 1000' finds nothing, and pidof sleep cannot tell your two sleeps apart. Scripts are a classic surprise: a script started as sh backup.sh is a process named sh, so killall sh would hit every shell on the machine, including your own. When the name is not precise enough, go back to the command line: ps -o pid,args | grep '[s]leep 2000' finds exactly one instance, and awk '{print $1}' extracts its PID for kill.

killall: signal by name

~% killall sleep
[2]+  Terminated                 sleep 2000
[1]+  Terminated                 sleep 1000
~% killall sleep
killall: sleep: no process killed
~% killall -q sleep; echo $?
1
~% yes > /dev/null &
~% killall -STOP yes
~% killall -CONT yes
~% killall -9 yes

killall takes signals exactly like kill (lesson 55): nothing means TERM, and -9, -KILL, -HUP, -STOP or -CONT choose another. -l lists the signal names and -q keeps it quiet when nothing matched, useful in scripts that clean up *if* something is running. Its exit status is 0 if at least one process received the signal. Treat it with the same respect as kill -9: one command can reach dozens of processes, so run pidof NAME first and look at what you are about to hit.