If you have a process id (pid), how can you tell if it is still running?
One obvious way to do this is to parse the output of the ps command. For example:
sharfah@starship:~> ps -ef | grep " 1234 " | grep -v grep
sharfah 1234 1 0 10:36:06 pts/65 1:20 java -server
However, a much neater way to do this is to use the kill command to send signal 0 to the process. So kill -0 will not terminate the process and the return status can be used to determine if the process is running. 0 if it is, non-zero otherwise.
sharfah@starship:~> kill -0 1234
sharfah@starship:~> echo $?
0
sharfah@starship:~> kill -0 1234x
bash: kill: 1234x: no such pid
sharfah@starship:~> echo $?
1
For those interested, the kill -l command lists signal numbers and names, but 0 is not listed.
sharfah@starship:~> kill -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL
5) SIGTRAP 6) SIGABRT 7) SIGEMT 8) SIGFPE
9) SIGKILL 10) SIGBUS 11) SIGSEGV 12) SIGSYS
13) SIGPIPE 14) SIGALRM 15) SIGTERM 16) SIGUSR1
17) SIGUSR2 18) SIGCHLD 19) SIGPWR 20) SIGWINCH
21) SIGURG 22) SIGIO 23) SIGSTOP 24) SIGTSTP
25) SIGCONT 26) SIGTTIN 27) SIGTTOU 28) SIGVTALRM
29) SIGPROF 30) SIGXCPU 31) SIGXFSZ 32) SIGWAITING
33) SIGLWP 34) SIGFREEZE 35) SIGTHAW 36) SIGCANCEL
37) SIGLOST
