Linux Process – Managing Shell Jobs on CentOS/RHEL 7

In Linux, there are a lot of services that starts when your machine boots up. These services in turn launch lots of processes that the service requires. To view a full list of all these services you do:

$  ps -ef | wc -l
158
$  ps aux | wc -l       # alternative command. Not used that much anymore. 
158

In my case, I piped the output to wc command to do a line count instead. Hence in my case I have 158 processes that are currently running.

However some of the processes in the above list are not started by a service, instead they are triggered by you, when you run various commands in your current bash/putty terminal. These are a special type of processes, and they are referred to as "jobs". These jobs get terminated as soon as you close your bash/putty terminal. To view a list of your jobs, do:

$ ps -u
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root      1367  0.0  2.6 193316 26832 tty1     Ss+  May01   0:11 /usr/bin/Xorg :0 -background none -verbose -auth /run/gdm/auth-for-
root      8886  0.0  0.2 116752  2860 pts/0    Ss   07:01   0:01 -bash
root      9820  0.0  0.2 116248  2336 pts/1    Ss+  07:55   0:00 /bin/bash
root     10578  1.0  0.4 153128  4676 pts/0    T    08:38   0:00 vim
root     10579  0.0  0.1 123372  1380 pts/0    R+   08:38   0:00 ps -u

Sometime a job is keeping your shell busy and stopping you from doing anything else, e.g. when you run the "find" command to search the entire filesystem for a file. In these situations, you can do ctrl+z keyboard shortcut to first pause the process:

$ find / -name hello.txt
^Z
[2]+  Stopped                 find / -name hello.txt
$

Then use the bg command to start the process again, but this time running it in the background:

$ bg
[2]+ find / -name hello.txt &

You can then monitor the job's progress using the jobs command:

$ jobs
[2]-  Done                    find / -name hello.txt

There's a shorthand way to make a command run in the background as soon as it is triggered, and that is by ending the command with an ampersand:

$ jobs
$ sleep 200 &
[1] 3837
$ jobs
[1]+  Running                 sleep 200 &
$

Here's another example:

$ sleep 600
^Z
[1]+  Stopped                 sleep 600
$ jobs
[1]+  Stopped                 sleep 600
$ bg
[1]+ sleep 600 &
$ jobs
[1]+  Running                 sleep 600 &

Note: bg by default always gets applied to the last shell job.

If you want to bring a background job back to the foreground, the you use the fg command followed by the job id:

$ sleep 200          # create a process
^Z                                           # send the job the background. 
[1]+  Stopped                 sleep 200
$ sleep 600
^Z
[2]+  Stopped                 sleep 600
$ sleep 300
^Z
[3]+  Stopped                 sleep 300
$ jobs
[1]   Stopped                 sleep 200
[2]-  Stopped                 sleep 600
[3]+  Stopped                 sleep 300
$ bg
[3]+ sleep 300 &
$ jobs
[1]-  Stopped                 sleep 200
[2]+  Stopped                 sleep 600
[3]   Running                 sleep 300 &
$ jobs 2                 # this displays a particular job by it's id. 
[2]+  Stopped                 sleep 600
$ jobs
[1]-  Stopped                 sleep 200
[2]+  Stopped                 sleep 600
[3]   Running                 sleep 300 &
$ bg 2
[2]+ sleep 600 &
$ jobs
[1]+  Stopped                 sleep 200
[2]   Running                 sleep 600 &
[3]-  Running                 sleep 300 &
$ bg 1
[1]+ sleep 200 &
$ jobs
[1]   Running                 sleep 200 &
[2]-  Running                 sleep 600 &
[3]+  Running                 sleep 300 &
$ fg 2                                     # This brings job with id "2" to the foreground
sleep 600
^C                                         # Here we did ctrl+c to terminate the job
[root@localhost ~]# jobs
[1]-  Running                 sleep 200 &
[2]+  Running                 sleep 300 &

You can also kill a background job like this:

$ kill -9 %{job id}

Here we used the "%" to indicate that this is a job id rather than a process id.

Tip: while using vim, You can use ctrl+z to put vim in the background so that you can look up something. Then use fg to return back to your vim session.

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