JOB CONTROL AND ITS COMMANDS ( 7.16 -7.16.1 )

Job control :

Job control is nothing but the ability to stop / suspend the execution of processes ( commands ) and continue/resume their execution as per your requirements . This is done using your operating system and shell such as bash/ksh or POSIX shell .

Job control refers to the ability to selectively stop ( suspend ) the execution of processes and continue ( resume ) their execution at a later point . It keeps a table of currently executing jobs, which may be listed with the jobs command .

To facilitate the implementation of the user interface to job control , the system maintains the notion of a current terminal process group ID . Members of this process group receive keyboard-generated signals such as SIGINT .

If the operating system on which Bash is running supports job control , Bash allows you to use it . You may then manipulate the state of this job , using the bg command to continue it in the background , the fg command to continue it in the foreground , or the kill command to kill it .

The character `%' introduces a job name . Job number n may be referred to as `%n'.

The symbols `%%' and `%+' refer to the shell's notion of the current job , which is the last job stopped while it was in the foreground . Simply naming a job can be used to bring it into the foreground: `%1' is a synonym for `fg %1' bringing job 1 from the background into the foreground .

Similarly, `%1 &' resumes job 1 in the background, equivalent to `bg %1' . You may then use the jobs command to inspect their status .



JOB CONTROL COMMANDS :

Creating your first Linux/Unix job

I am going to run a command called xeyes that displays two googly eyes on screen, enter:



$ xeyes &




Sample outputs:

Fig.01: Running the xeyes command in the background

I started a job in the background with an ampersand (&). The shell prints a line that looks like the following:

[1] 6891

In this example, two numbers are output as follows

• [1] : The xeyes job, which was started in the background, was job number 1.
• 6891 : A process ID of job number 1.
  

I am going to start a few more jobs:

## Start a text editor, system load average display for X, and sleep command ##
gedit /tmp/hello.c &
xload &
sleep 100000 &
 

#2: List the current jobs

To see the status of active jobs in the current shell, type:



$ jobs
$ jobs -l



Sample outputs:

[1]   9379 Running                 xeyes &
[2]   9380 Running                 gedit /tmp/hello.c &
[3]-  9420 Running                 xload &
[4]+  9421 Running                 sleep 100000 &

fg, bg

The fg command switches a job running in the background into the foreground . The bg command restarts a suspended job , and runs it in the background .
If no job number is specified, then the fg or bg command acts upon the currently running job .



EXAMPLE FOR fg & bg COMMAND :

Resume suspended/stopped job in the foreground

Let us resume or bring stopped ping job to the foreground and make it the current job with the help of fg command. The syntax is as follows:

## Job id number 5 for ping command ## fg %5

I can also state any job whose command line begins with the string "ping":

## %String ## fg %ping

Sample outputs:

64 bytes from www.cyberciti.biz (75.126.153.206): icmp_req=3 ttl=53 time=265 ms
64 bytes from www.cyberciti.biz (75.126.153.206): icmp_req=4 ttl=53 time=249 ms
64 bytes from www.cyberciti.biz (75.126.153.206): icmp_req=5 ttl=53 time=267 ms
^C

#5: Resume suspended/stopped job in the background

In this example , I am going to update all installed packages on Red Hat or CentOS Linux production server using yum command background job:

# yum -y update &>/root/patch.log &

However, due to some reason (say load issue) I decided to stop this job for 20 minutes:

# kill -s stop %yum

Sample outputs:

[7]+  Stopped                 yum -y update &>/root/patch.log &

Restart a stopped background yum process with bg

Now, I am going to resume stopped the yum -y update &>/root/patch.log & job, type:

# bg %7

OR

# bg %yum

Sample outputs:

[7]+ yum -y update &>/root/patch.log &