Unix - What is Job Control?

1. What is Job Control?

Job control allows you to manage multiple processes (jobs) started from the same shell. You can:

• Pause (stop) a job

• Resume it in the foreground or background

• List running/stopped jobs

2. Important Commands

a) & (Run in Background)

• Appending & to a command sends it to the background immediately.

• The shell prints the job number and process ID (PID).
  Example:

$ sleep 60 &
[1] 2345

• 1 = job number

• 2345 = PID of the process

b) jobs (List Jobs)

• Shows all jobs started in the current shell, along with their status:

  • Running = process is active

  • Stopped = process paused

Example:

$ jobs
[1]+  Running    sleep 60 &
[2]-  Stopped    vim file.txt

c) fg (Bring Job to Foreground)

• Brings a background or stopped job to the foreground so you can interact with it.
  Syntax:

fg %[job_number]

Example:

$ fg %1

• Job 1 now runs in the foreground.

d) bg (Resume Job in Background)

• Resumes a stopped job in the background.
  Syntax:

bg %[job_number]

Example:

$ bg %2

• Job 2 will now continue running in the background.

3. Pausing and Resuming Jobs

• Ctrl + Z: Pause the current foreground job and move it to the background (stopped).

• Then you can use bg to resume it in the background or fg to resume in foreground.

Example Workflow:

$ vim file.txt   # open vim
# press Ctrl+Z
[1]+  Stopped    vim file.txt
$ bg %1          # resume in background
[1]+ vim file.txt &
$ fg %1          # bring back to foreground

4. Quick Tips

• % = prefix to specify job number (from jobs command)

• Use jobs -l to also see the PID of each job

• You can have multiple jobs running simultaneously in the background

Summary Table

If you want, I can draw a small diagram showing jobs moving between foreground, background, stopped, and running states. This makes it really easy to visualize.