Controlling interactive process in Unix

In Unix systems, interactive processes refer to processes which are created from terminal sessions. This blog post aims to explain the basics of controlling interactive process in Unix systems.

Foreground and background process

There are two types of interactive process in Unix systems, namely foreground and background process. Foreground process is a process that runs continuously and only returns control to the shell after it finishes. Background process, on the other hand, refers to a process that runs in the background while the user has the control of the shell.

Executing a process in foreground and background mode

Since there are two kinds of interactive process, you can control a process’ mode when you execute it. If you want to run a process as a foreground process, just run it from your terminal. If you want to run in background mode, you have to append & to your command.

$ sleep 60 &

When you start a background process, the control of shell is returned to you and you can continue typing other commands.

Managing background processes

To list all background processes, use command jobs.

$ sleep 60 &
[1] 5483
$ sleep 60 &
[2] 5488
$ jobs
[1]  - running    sleep 60
[2]  + running    sleep 60

Each background process is assigned a number (in square brackets). If you want to kill a background process, use kill %n where n is the assigned number.

$ kill %1
[1]  - 5483 terminated  sleep 60

Switching between the modes

From foreground to background

If you want to switch a process to background in the middle of execution, you should first suspend it with the SIGTSTP signal by typing ^Z (Ctrl + Z). Then type bg to resume execution in background mode. If you have multiple stopped processes, you can specify which process to resume with bg %n.

$ sleep 60
^Z
[1]  + 5513 suspended  sleep 60
$ bg %1
[1]  - 5513 continued  sleep 60

From background to foreground

It’s also possible to bring a process back to foreground mode from background mode. First, you need to find out the assigned number of that process from the jobs command. Then, use the fg %n command to bring a process back to foreground mode.

$ sleep 60 &
[1] 5561
$ jobs
[1]  + running    sleep 60
$ fg %1
[1]  + 5561 running    sleep 60

Conclusion

This post discussed the basics of interactive process management in Unix systems. With these commands, you can easily manage interactive process (e.g. to change its execution mode). Hope you find these useful in your daily work on Unix systems.