Processes and Job Control Foreground and Background (1) Unix is a multi-tasking operating system some of these tasks are being done by other users logged in some are being done by you in the background e.g. watching for incoming mail When you run a task (a Unix command, like ls or vi) it executes in the foreground of your shell it has the control" of your screen and keyboard
Foreground and Background (2) If you still want to use the current shell obelix > a_heavy_task &  13607 obelix > When you put a task in background task keeps running, but you continue to work at the shell in the foreground if any output is done, it appears on your screen immediately (can be confusing)
if input is required, process prints a message and stops when it is done, a message will be printed Foreground and Background (3) Explicit background processes are needed less often with windowing systems Just go to another window and run the command But explicit background processes are used often in unix A command needs a long time, you do not want to close that window by accident
Run a job at the background and logout netscape& will open a new window, but leave the current shell window still available to use A Simple Script We use the following shell script to illustrate job control Edit a file make_noise obelix > cat > make_noise #!/bin/sh while [ 1 ] do date
sleep 1 done obelix > chmod u+x make_noise make_noise then is a shell script repeats to print the time for every second, until you terminate it using Ctrl-c. Job Control Suspending Jobs csh, tcsh, and bash allow you to manage the running of different processes Suspending jobs the Ctrl-z special character stops the job obelix > make_noise Fri May 16 14:14:43 EDT 2003
^Z Suspended obelix > vi readme ^Z Job Control - Monitoring Jobs The "jobs" command shows which of your jobs are running and/or stopped. obelix > jobs  + Suspended  + Suspended
make_noise vi readme Here there are two suspended processes, the make_noise and a vi process. Job Control Resuming Jobs Putting jobs back into the foreground: Use the "fg" command to move a job into the foreground. obelix > fg %2 Puts job number 2 into the foreground. Works with either a background or stopped job.
Putting jobs into the background: obelix > bg %1 Job Control Killing Jobs Jobs can also be killed Use the Unix "kill" command obelix > kill %1 or if it won't die ... obelix > kill 9 %1 Jobs can be stopped and continued
obelix > a_heavy_task & obelix > stop %1 obelix > bg %1 Using ps (1) Jobs are really just a special case of Unix processes ps can list the current processes obelix > ps PID TT S TIME COMMAND
2312 pts/0 T 0:00 vi 2296 pts/0 R 0:00 tcsh 2313 pts/0 R 0:00 ps ps can take many options, depending on which version of ps you are using (/usr/bin/ps vs. /usr/ ucb/ps) Using ps (2) The ps command takes a number of options -l gives you a long listing of what is going on -u loginid tells you about loginid's processes
use man ps to see more options kill pid kills the process pid TERM signal will be sent to the process pid kill -9 or kill -KILL will send the KILL signal Use man kill to find out more signals Another useful command: ulimit The ulimit utility sets or reports the file-size writing limit imposed on files written by the shell and its child processes (files of any size may be read). Only a process with appropriate privileges can increase the limit. -a prints all limits -f maximum file size (in 512-byte blocks) -v maximum size of virtual memory (in kbytes) Let us illustrate the interest of ulimit [[email protected] shell]$ ulimit -u 100 [[email protected] shell]$ more foo echo FOO
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