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The shell can also help you in running multiple programs at a time. Suppose you want to run a word count on a large file but you don't want to wait for it to finish. Then you can say:
wc hugefile > wc.output &
The ampersand & at the end of a command line says to the shell to take this command and start executing it in the background and get ready for further commands on the command line. If you don't redirect the output to a file it will show up on your terminal some time later when the wc program is done! The command jobs lists all background jobs that you have started from the current shell.
If you start a bunch of processes with the ampersand you can use the jobs command to list them. Then you can selectively bring one into the foreground by the command fg %n, where n is the job number as listed by the jobs command. You can also suspend a running program with Ctrl-z and put it in the background with the command bg.
Each running program is knon as a process. The number printed by the shell for each running program is a unique process-id that the operating system assigns to the process when it is created. To check for running processes use the ps command (ps is for process status). A sample session is shown below.
[alice@localhost]: date Mon Oct 25 14:40:52 MDT 2012 [alice@localhost]: wordfreq Encyclopedia.txt > output & [1] 19027 [alice@localhost]: jobs [1]+ Running wordfreq Encyclopedia.txt >output & [alice@localhost]: ps PID TTY TIME CMD 19018 ttyp1 00:00:00 bash 19027 ttyp1 00:00:00 wordfreq 19033 ttyp1 00:00:02 sort 19034 ttyp1 00:00:00 uniq 19035 ttyp1 00:00:00 sort 19036 ttyp1 00:00:00 wc 19037 ttyp1 00:00:00 ps [alice@localhost]: [1]+ Done wordfreq Encyclopedia.txt >output [alice@localhost]: date Mon Oct 25 14:41:20 MDT 2012 [alice@localhost]:
To see all processes on the system, use the command ps augxw. To search for processes owned by a user bcatherm, use grep as shown below:
ps augxw | grep bcatherm
Here is a sample output
[alice@localhost alice]$ ps augxw | grep bcatherm bcatherm 14322 0.0 0.0 6760 2020 ? S Dec01 0:02 /usr/sbin/sshd bcatherm 14328 0.0 0.0 4396 1480 pts/14 S Dec01 0:00 -bash bcatherm 16659 0.0 0.0 6792 2032 ? S 00:10 0:00 /usr/sbin/sshd bcatherm 16667 0.0 0.0 4392 1472 pts/0 S 00:10 0:00 -bash bcatherm 20128 0.0 0.0 4152 1072 pts/0 S 00:53 0:00 /bin/sh /usr/local/bin/pbsget -4 bcatherm 20134 0.0 0.0 1584 656 pts/0 S 00:53 0:00 qsub -v DISPLAY -q interactive -I /tmp/cluster.pbs.611 bcatherm 20140 0.0 0.0 4392 1472 ttyp0 S 00:53 0:00 -bash bcatherm 20141 0.0 0.0 1456 312 ttyp0 S 00:53 0:00 pbs_demux bcatherm 20818 0.0 0.0 1688 808 ttyp0 S 01:08 0:00 /usr/share/pvm3/lib/LINUXI386/pvmd3 -nws00 bcatherm 20901 0.4 0.0 8360 2868 pts/14 S 01:09 0:00 vim control/pvm/stage3.c alice 20907 0.0 0.0 3592 628 pts/19 S 01:09 0:00 grep bcatherm [alice@localhost alice]$
If you start a background job with the ampersand & and logout, normally the background job is terminated. However, you can ask the shell to let the background job continue running after you log out by using the prefix nohup. For example:
nohup mylongrunningprogram >& output &
logout
Next time you login, you can use the ps command to check whether the job has finished. Then check for the output file.