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| <h3 class="section">27.1 Concepts of Job Control</h3> |
| |
| <p><a name="index-shell-3200"></a>The fundamental purpose of an interactive shell is to read |
| commands from the user's terminal and create processes to execute the |
| programs specified by those commands. It can do this using the |
| <code>fork</code> (see <a href="Creating-a-Process.html#Creating-a-Process">Creating a Process</a>) and <code>exec</code> |
| (see <a href="Executing-a-File.html#Executing-a-File">Executing a File</a>) functions. |
| |
| <p>A single command may run just one process—but often one command uses |
| several processes. If you use the ‘<samp><span class="samp">|</span></samp>’ operator in a shell command, |
| you explicitly request several programs in their own processes. But |
| even if you run just one program, it can use multiple processes |
| internally. For example, a single compilation command such as ‘<samp><span class="samp">cc |
| -c foo.c</span></samp>’ typically uses four processes (though normally only two at any |
| given time). If you run <code>make</code>, its job is to run other programs |
| in separate processes. |
| |
| <p>The processes belonging to a single command are called a <dfn>process |
| group</dfn> or <dfn>job</dfn>. This is so that you can operate on all of them at |
| once. For example, typing <kbd>C-c</kbd> sends the signal <code>SIGINT</code> to |
| terminate all the processes in the foreground process group. |
| |
| <p><a name="index-session-3201"></a>A <dfn>session</dfn> is a larger group of processes. Normally all the |
| processes that stem from a single login belong to the same session. |
| |
| <p>Every process belongs to a process group. When a process is created, it |
| becomes a member of the same process group and session as its parent |
| process. You can put it in another process group using the |
| <code>setpgid</code> function, provided the process group belongs to the same |
| session. |
| |
| <p><a name="index-session-leader-3202"></a>The only way to put a process in a different session is to make it the |
| initial process of a new session, or a <dfn>session leader</dfn>, using the |
| <code>setsid</code> function. This also puts the session leader into a new |
| process group, and you can't move it out of that process group again. |
| |
| <p>Usually, new sessions are created by the system login program, and the |
| session leader is the process running the user's login shell. |
| |
| <p><a name="index-controlling-terminal-3203"></a>A shell that supports job control must arrange to control which job can |
| use the terminal at any time. Otherwise there might be multiple jobs |
| trying to read from the terminal at once, and confusion about which |
| process should receive the input typed by the user. To prevent this, |
| the shell must cooperate with the terminal driver using the protocol |
| described in this chapter. |
| |
| <p><a name="index-foreground-job-3204"></a><a name="index-background-job-3205"></a>The shell can give unlimited access to the controlling terminal to only |
| one process group at a time. This is called the <dfn>foreground job</dfn> on |
| that controlling terminal. Other process groups managed by the shell |
| that are executing without such access to the terminal are called |
| <dfn>background jobs</dfn>. |
| |
| <p><a name="index-stopped-job-3206"></a>If a background job needs to read from its controlling |
| terminal, it is <dfn>stopped</dfn> by the terminal driver; if the |
| <code>TOSTOP</code> mode is set, likewise for writing. The user can stop |
| a foreground job by typing the SUSP character (see <a href="Special-Characters.html#Special-Characters">Special Characters</a>) and a program can stop any job by sending it a |
| <code>SIGSTOP</code> signal. It's the responsibility of the shell to notice |
| when jobs stop, to notify the user about them, and to provide mechanisms |
| for allowing the user to interactively continue stopped jobs and switch |
| jobs between foreground and background. |
| |
| <p>See <a href="Access-to-the-Terminal.html#Access-to-the-Terminal">Access to the Terminal</a>, for more information about I/O to the |
| controlling terminal, |
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