>> Running UNIX in the background

Running Unix Command / Job in the background :

When you are running a job in unix , it occupies your terminal until the job completes execution. So you have to wait until it completes to run another job. To avoid such situation, you can run the job in the background and use the terminal for other  jobs/process . 

Suppose you run below command for example ;

sleep 600 --- you will have to wait 600 seconds to use the terminal for another command/job.

Use  "&" ( ampersand ) to run job in the background

once you start , until 600 seconds (10 minutes) , your window will be busy and you can not use the terminal until it complets(600 secs).  To avoid such situation, You can run the command in the background by adding "&"( ampersand ) at the end of the code.

sleep 600 &   click ENTER

it displays below output for the instance as soon as you click ENTER :

[JobNumber]       PID Number

for example:

[1]                          12345

Run in the background

To run the job in the background, use "&" at the end of the job or command:

sleep 100 &    click ENTER

You will see below information as soon as you hit ENTER:

[2]     1234

Where "[2]" is the job number and "1234" is the ProcessID-PID.

//OR//

A job running in the foreground ie in the terminal can be run in the background.

Let's run "sleep 1000" , first in the foreground and then stop/pause it and resume it in the back ground.

sleep 1000  click ENTER --- This is running job in the foreground.

To stop or pause , click CTRL+z , the job will be stopped/paused and shows below information:

[1]+  Stopped                 sleep 1000

where [1] is job number , "Stopped" is action/status , "sleep 1000 "  the job/command for which the action is taken.

To  run the above job in the background:

Syntax :  bg   %JobNumber .

For the above paused job:

bg   %1  click ENTER --- it displays below information :

[1]+  sleep  1000 &

and the job is running in the background.

See job status

click "jobs" to see the status as below:

Stopped ----for the stopped /paused job

Running ---- for the job running in the background.

Done ---- for the completed job that was running in the background.

[1]+  Stopped                sleep 100 --- stopped/paused

[3]   Running                 sleep 50 & --- running now

[4]-  Running                 sleep 45 & --- in queue

Let's have an UNIX JOB like below :

Job Name : unixJob

Code :

#!/bin/bash

for i in 1 2

do

   echo "Employee ID# $i  "

sleep 10

done

F --- this is for producing the error

Redirect output

Run a job in the background , redirect the result to a file , redirect errors, if any, to the same or different output file.

Run job or command, for example I am running the job : unixJob

sh unixJob > file1 & --- To run in the background and redirect the output to file1.

sh unixJob  1>file1  & --- To run in the background and redirect the output to file1.

sh unixJob  1>file1  2>&1  & --- To run in the background and redirect the output and error, if any, both to file1.

sh unixJob  1>file1  2>file2  & --- To run in the background and redirect the output  to file1 and error, if any, to file2.

Bring the backGround job to foreGround and vice versa

Let's run sleep  1000  in the background and bring it to foreground. First of all you need to know the job number.

sleep  1000&    click ENTER  --- it displays below information:

[1]    11973   where [1]  is the job number and 11973  is the process ID.

//or //
to know the job number of a running job in the background , ENTER "jobs" :

jobs    click ENTER   --- it displays below information:

[1]   Running                 sleep 1000 &

now to bring this job to foreground.

Syntax:

fg    %JobNumber

fg   %1     click ENTER

Now to bring the foreground job to back ground

Run a job "sleep 1000 " in the foreground.
click CTRL+z  to stop/pause the job, while doing this the job number is displayed like below:

[1]+  Stopped                 sleep 1000

now to run the job in the background ,

bg  %1    click ENTER  --- it displays below information:

[1]+    sleep 1000 &;

Let's check status with "jobs":

jobs    click ENTER   --- it displays below information:

[1]+  Running                 sleep 1000 &   --- the job is running in the background

Kill Process/Job:

you can kill a running process/job  by using "kill" command alongwith its  PID :

Let's  run   sleep   1000&  and then kill it :

sleep  1000&    click ENTER , after you click ENTER you get below information :

[1]    9019     where [1]  is the job number and 9019 is the process ID - PID

You can also get the PID using :  ps    or     ps  -e    or    ps  -ef

such as ,

if you click "ps" , you get below informaiton  :

  PID   TTY          TIME CMD

 4443 pts/1      00:00:00 bash

 9019 pts/1      00:00:00 sleep   ---- this the job/process which we need to kill.

 9048 pts/1      00:00:00 ps

" kill "   or  " kill  -9 "

Syntax :

kill  PID   click ENTER

kill  9019    click ENTER

now check the status of the job  sleep   1000&  by  using :  jobs    or     ps    or     ps  -e    or    ps  -ef

if you check the status with the command : "jobs',

it will show  " Terminated  " :

[1]+  Terminated              sleep 1000

if you check with the command "ps",

it will not show the job/process  like below :

  PID TTY          TIME CMD

 4443 pts/1    00:00:00 bash

 9218 pts/1    00:00:00 ps

In many situations , the job will not be terminated/killed  with only "kill" command. In such situation use "-9"  with "kill".

kill  -9   PID    click ENTER  --- this is killing the job/process forcefully.

kill  -9   9019   click ENTER

Now, if you check the status with the command job, it shows  "killed" :

[1]+  Killed                  sleep 1000

if you check with the command "ps",

it will not show the job/process  like  below :

  PID TTY          TIME CMD

 4443 pts/1    00:00:00 bash

 9218 pts/1    00:00:00 ps