The Linux Programming Interface

Process Groups, Sessions, and Job Control 717

We can then see the output of the job by bringing it into the foreground:

$ fg

date

Tue Dec 28 16:20:51 CEST 2010

The various states of a job under job control, as well as the shell commands and ter-

minal characters (and the accompanying signals) used to move a job between these

states, are summarized in Figure 34-2. This figure also includes a notional

terminated state for a job. This state can be reached by sending various signals to the

job, including SIGINT and SIGQUIT, which can be generated from the keyboard.

Figure 34-2: Job-control states

34.7.2 Implementing Job Control

In this section, we examine various aspects of the implementation of job control,

and conclude with an example program that makes the operation of job control more

transparent.

Although optional in the original POSIX.1 standard, later standards, including

SUSv3, require that an implementation support job control. This support requires

the following:

z The implementation must provide certain job-control signals: SIGTSTP, SIGSTOP,

SIGCONT, SIGTTOU, and SIGTTIN. In addition, the SIGCHLD signal (Section 26.3) is also

necessary, since it allows the shell (the parent of all jobs) to find out when one

of its children terminates or is stopped.

z The terminal driver must support generation of the job-control signals, so that

when certain characters are typed, or terminal I/O and certain other terminal

operations (described below) are performed from a background job, an appro-

priate signal (as shown in Figure 34-2) is sent to the relevant process group. In

Control-Z

(SIGTSTP)

fg

(SIGCONT)

fg^

command

kill

kill

bg (SIGCONT)

1. kill –STOP (SIGSTOP)


2. Terminal read (SIGTTIN)


3. Terminal write (+TOSTOP) (SIGTTOU)
   
   
   1. Control-C (SIGINT)
      2. Control-\^ (SIGQUIT)
   
   
   
   

Stopped in

background

Terminated

command &

Running in

background

Running in

foreground