Background Processes


Unix has the ability to run your program in the background. This means that instead of waiting for the program to finish execution, the UNIX shell prompts you again and you can run other commands at the same time that the background process is running. To run the program myprog in background, type:

myprog &

The shell will respond with a number, its process identification number (or PID) and then return to the prompt.

It is also possible to leave a big job running in the background even after you logout. To keep the program myprog running in background, type:

nohup myprog &

For example, to run your R job in the background from tcsh with a nice level of 19:

nohup nice +19 R --slave < infile.R &

Or from bash:

nohup nice -19 R --slave < in.R > R.out 2>&1 &

Or MATLAB from tcsh, while running it with a nice level of 19:

nohup nice +19 matlab < infile.m > outfile &


You may alternatively choose to use GNU screen to run your jobs so that you can leave them running when you logout. Just invoke screen from your shell, type a space or carriage return to dismiss the startup message, then start your job. When you want to detach, type Control-a followed by Control-d. You can then exit from your terminal session. When you want to access your job again, connect to the same machine your job is running on and type screen -r.