In UNIX, signal 0 can be used with kill command to check if a process exists and accepts signals from the current user. However, no signal is sent to the process when kill -0 is run. This command could be used as an alternative to parsing ps output for checking the process existence. Since it’s not shown in most of the help pages or signal list via kill -l, it may not be known to some users.
kill -l
HUP INT QUIT ILL TRAP ABRT EMT FPE KILL BUS SEGV SYS PIPE ALRM TERM URG STOP TSTP CONT CHLD TTIN TTOU IO XCPU XFSZ VTALRM PROF WINCH INFO USR1 USR2 Examples: You can use the following code to check if a process exists.
# Check if a process with given PID exists.
kill -0 $pid; echo "Exit status: $?”The exit status of this command will be ‘0’ if the process with given pid exists, ‘1’ otherwise. errno will be set appropriately if the command fails.
If you are unsure if a process is still running but you want to kill it if it runs, then you can use the below command.
# Kill if the process with given PID exists.
kill -0 $pid && kill $pidFor more info, check kill(2)