Find the pid of a java process under Linux

Question

I have a Java program running on a Linux computer, and want to find the process ID (pid) of its process. I know the ps command can provide this information, but its output is confusing because it has so much extraneous information. How can I get just the pid?

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I am using MPJ library in java program for Pagerank algorithm. I compile it by

javac -cp .:$MPJ_HOME/lib/mpj.jar MpiPageRank.java 

and run by

mpjrun.sh -np 2 MpiPageRank 

where -np is number of process

Now i have to find its pid

ps -ef|grep java 

like

mpjrun.sh -np 2 MpiPageRank & sleep 2 ps -ef | grep java 

I get

pnewaska 27866 27837 99 21:28 pts/45   00:00:09 java -cp /u/pnewaska/mpj-v0_38/lib/smpdev.jar:/u/pnewaska/mpj-v0_38/lib/xdev.jar:/u/pnewaska/mpj-v0_38/lib/mpjbuf.jar:/u/pnewaska/mpj-v0_38/lib/loader2.jar:/u/pnewaska/mpj-v0_38/lib/starter.jar:/u/pnewaska/mpj-v0_38/lib/mpiExp.jar runtime.starter.MulticoreStarter /nfs/nfs1/home/pnewaska/DistributedSystems/Project3 10 smpdev useLocalLoader EMPTY MpiPageRank -i input.500k0 -n 10 -o 

Now I want to extract MpiPageRank from only 1 linux comman to get its pid ie 27866. how do i do that ?

Answer 1

using ps

ps allows the user to define his own formatting for its output with the -o switch, and -C to select entries by the given command. I would go with:

ps -C java -o pid 

from the man page:

   -C cmdlist      Select by command name                    This selects the processes whose executable name is given in cmdlist.     -o format       user-defined format.                    format is a single argument in the form of a blank-separated or comma-separated list, which offers a way to specify                    individual output columns. The recognized keywords are described in the STANDARD FORMAT SPECIFIERS section below. Headers                    may be renamed (ps -o pid,ruser=RealUser -o comm=Command) as desired. If all column headers are empty                    (ps -o pid= -o comm=) then the header line will not be output. Column width will increase as needed for wide headers; this                    may be used to widen up columns such as WCHAN (ps -o pid,wchan=WIDE-WCHAN-COLUMN -o comm). Explicit width control                    (ps opid,wchan:42,cmd) is offered too. The behavior of ps -o pid=X,comm=Y varies with personality; output may be one                    column named "X,comm=Y" or two columns named "X" and "Y". Use multiple -o options when in doubt. Use the PS_FORMAT                    environment variable to specify a default as desired; DefSysV and DefBSD are macros that may be used to choose the default                    UNIX or BSD columns. 

One can get more accurate results by specifying more restrictions (ie the user under which the process runs, etc). Look at the man page for more information and other switches.

example:

$ sleep 10 & [1] 12654 $ ps -C sleep -o pid 12654 

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using the shell

I don't know why you use an .sh script to run your code and not call java directly, but if in any case you use the & (background) operator, you can grab the pid through your shell, with the $! variable.

for example:

$ sleep 5 & [1] 12395 $ echo $! 12395 

same goes for the java -jar .. & command, $! will be set to the pid of the last backgrounded job.