Managing UNIX processes

The ‘ps’ command

The output of ps will indicate the name of the program, its process I.D., and the amount of CPU time it has consumed so far.

  1. To display information about a specific user’s processes:

where USERNAME is your username

  1. To display information about a specific running program:
ps -ef | grep PROGRAM

where PROGRAM is the program’s name

See man ps for additional information.

Killing Processes

To kill a running program, type:

kill PID

where PID is the process I.D. obtained by running ps.

If the first form does not kill your process, try

kill -9 PID

To kill all your background processes, execute:

kill 0

Input and Output

Most programs initially inherit the current terminal or pseudo-terminal for doing input and output. This means that these programs have a “controlling terminal”. The “controlling terminal”, and therefore all output, is lost if you logout before your background job completes. To avoid this loss, you should either code your program to read all input and write all output to specific files, or your should redirect input and output for the program by using the shell operators: <, >, >>, and >&.

It is recommended that you redirect standard output and standard error to a log file for all programs run in the background. For example:

myprog >& logfile &

will run myprog in the background, redirecting all output to the file ‘logfile’ in the current directory. See man bash for details on shell redirection operators.

Running Intensive Processes

A “big job” is any CPU-bound process which requires over one minute of CPU time.


In order to run big jobs without slowing down the entire system and inconveniencing all users, everyone must follow certain procedures.

  1. Class accounts are restricted to running their jobs on the public workstations (Use the command sitehosts public for a list of public workstations). Class accounts are not allowed to run a job on a faculty workstation without the prior consent of the faculty member.

  2. Run the program in background. Do not run the program as “a.out”. Please rename the “a.out” file before executing the program and do not remove the program “binary” before the program has completed executing.

  3. Run only one background job at a time per machine. If you need to run multiple jobs on one machine, run them in sequence, not in parallel, by putting all the commands in a shell script. Select another computer if a large job is already running on the computer that you have selected. Use the command idle to select a host and top to examine the most CPU intensive jobs are running on that host.

  4. Please do not submit jobs to more than 8 computers at any one time. If you have special requirements, you can request an increase in the number of computers to use concurrently by notifying the EML staff. This will enable the EML staff to evaluate load requirements and to monitor the system resources, which will help to avoid conflicts with other users.

  5. Big jobs should run at low priority, or high “nice value”, of 18 or 19. By default, all programs run at nice value 0. But on the EML systems, the following minumum nice values are required for big jobs:

    Table 1. Appropriate nice values

    CPU time

    Minimum nice value

    1 - 5 minutes


    over 5 minutes


For example, suppose one wanted to run Matlab in the background with nice value of 19:

% nice +19 matlab &

For more information on these commands, see “man nice” (but note that where the manual says “nice -10” the C shell requires “nice +10”. Each user is responsible for ensuring that his/her big jobs are running at appropriate priorities. The superuser is free to renice appropriately any process which is slowing the system down.

If you need to nice a process while it is running, look up the PID as described above and run:

% renice +19 PID

Memory Usage

Computer memory is necessary for the computer to operate on your data. As the size of the data you’re working with gets bigger and bigger, the computer tends to need more memory. Thus, when you’re working with lots of data and your program prints an error message, especially one which reports a number of bytes or kilobytes of memory requested, the problem is most likely computer memory. The first step is to lift the default restriction on the use of memory which is imposed by the shell, by typing the command:

limit datasize unlimited

at the UNIX command prompt, before you run the program which is failing to obtain the required memory. If the problem persists, in general your only alternative is to run the job on a computer which has more memory. Please send mail to consult to get information about the memory resources of the different EML machines.

Hints to Improve Efficiency

Some people run 2-hour CPU-time jobs only to discover afterwords that the program didn’t even do what they wanted. Avoid this. Debug your program using small test cases until you’re sure you’ve got it right. Only then should you run the big monster.

If it’s a very long computation and you can wait for the results, use the “batch” and “at” commands to run it when the system is unloaded.

Timing Jobs

Two common questions when running big jobs are “How do I find out the running time?” and “How do I capture the program output which would normally go to the screen?”. Here is one simple way to do both (as:

% nice +18 /usr/bin/time program-name >& ouput-filename

Where program-name is the name of your program and ouput-filename is the name of the file in which you want to capture output. The running time will be the last line of the output file, formatted like this:

60.0 real        10.0 user         0.5 sys

In this example, the cpu time used was 10.5 seconds (10.0 user + 0.5 sys) and the elapsed (wall-clock) time was 60.0 seconds. By division, your program used 10.5/60 or just over 1/6th of the available cpu time while it ran.

Running sequential jobs

Users should run only one background job at a time per machine. If you must run several jobs in background, run them sequentially, not simultaneously. If your programs are ‘prog1’, ‘prog2’ and ‘prog3’, run them in background via the shell command:

(prog1 ; prog2 ; prog3) >& log &

Another way is to use a semicolon:

run1 >& run1.log ; run2 >& run2.log

where run1 and run2 are the programs you wish to run and run1.log and run2.log are the logfiles.

Yet annother way is to set up a shell script file, for example ‘run_all’, containing:

#!/bin/sh run1 >& run1.log run2 >& run2.log

By specifying the > sign, you save the output from run1 into file run1.log. By also including the & sign, it also saves any error message output into run1.log.

Then from the unix prompt:

% chmod +x run_all

to allow the script to be executable, and then type:


to run the script. You could also type:

./run_all &

to have it run in the background.

Scheduling Jobs

The at and batch commands allow the system to queue up big jobs and run them at a later time. at allows you to specify when the commands should be executed, while jobs queued with batch will execute as soon as the system load level permits. These commands provide a mechanism for big jobs to run without slowing down interactive response and interfering with other people trying to use the computer.

Queue Execution

To use at or batch, create a script file which contains the unix commands you want to run. Suppose your script file is called ‘filename’. To run it in batch, type the command:

batch filename

To run the script at a specific time, use:

at time date filename

where time is in the form 0815, 0815am, 8:15am, now, and 5 pm; and date is in the form Jan 24, Friday, tomorrow, and today.

If you leave out the date field, the date will default to today.

The computer will respond:

job N at <full date>

where ‘N’ is the job number it creates. When the job finishes, it will mail you the output of the script, unless output was redirected. (see below)

Shell Features

By default, /bin/sh is used as the shell interpreter for the commands in your script. If your script-file is a /bin/csh script, use the ‘-c’ flag, as in ‘at -c 1 pm script’.

If the commands in your script file need any input, create separate input files which contain the necessary input and use the ‘<’ shell feature in the script file. To redirect the output of a particular command in your script, use the ‘>’ shell feature. For example, your script file might contain the line:

proga < inputa > outputa

This would cause the program ‘proga’ to take its input from the file ‘inputa’ and send output to ‘outputa’.

Controlling jobs

To find out the status of your jobs, type the command:

at -l

This will report both ‘batch’ and ‘at’ jobs. If ‘N’ is the job number reported by ‘at -l’ then the command:

at -r N

will remove that job from the queue (whether or not it is already running) and interrupt it (if it is already running).