Linux Process relationship

Author: vamei Source: http://www.cnblogs.com/vamei welcome reprint, please also keep this statement. Thank you!

Linux processes have a certain relationship with each other. For example, in the Linux Process base, we can see that each process has a parent process, and all processes take the INIT process as the root to form a tree structure. Here we will explainProcess GroupAndSessionTo manage processes in a richer way.

 

1. Process Group)

Each process belongs to a process group. Each process group can contain multiple processes. The process group has oneProcess Group Leader Process(Process group leader), the PID of the lead process (PID can be found in the Linux Process basis) becomes the process group ID (Process Group ID,PgidTo identify the process group.

$ PS-o pid, pgid, ppid, comm | cat

PID pgid ppid command17763 17763 17751 bash18534 18534 17763 ps18535 18534 17763 cat
 

PID is the ID of the process itself, pgid is the ID of the process group where the process is located, and ppid is the parent process ID of the process. From the above results, we can infer the following relationship:

The arrows in the figure indicate that the parent process generates child processes through fork and exec mechanisms. PS and CAT are both bash sub-processes. The PID of the Process Group's leading process becomes the process group ID. The lead process can be terminated first. At this time, the process group still exists and holds the same pgid until the last process in the Process Group ends.

 

One important reason we classify some processes into a process group is that we can send signals to a process group. All processes in the process group receive this signal. We will discuss this in depth in the next section.

 

2. Session)

Furthermore, when shell supports job control, multiple process groups can also formSession(Session ). Bash (Bourne-again shell) supports work control, while SH (Bourne shell) does not.

A session is created by a process, which is called a session leader ). The session leader's PID is the session ID ). Each process group in a session is calledWork(Job ). A session can contain a processFront-endWork (foreground), while other process groups areBackgroundWork (background ). Each session can be connected to oneControl Terminal(Control terminal ). When the control terminal has input and output, all are passed to the foreground Process Group of the session. BySignal generated by the terminalFor example, CTRL + Z and CTRL + \ are passed to the foreground process group.

The significance of a session is to include multiple tasks in one terminal and take one of them as the frontend to directly receive the input and output of the terminal and terminal signals. Other tasks are run in the background.

 

You can add&To run it in the background:

$ Ping localhost> log &

The terminal displays:

[1] 10141

1 in brackets indicates the work number, and 10141 indicates the pgid.

We can query more detailed information as follows:

$ PS-o pid, pgid, ppid, Sid, tty, comm

(TTY indicates the control Terminal)

 

The signal can be killed

$ Kill-sigterm-10141

Or

$ Kill-sigterm % 1

To the Working Group. The preceding two commands are sent to the pgidAdd-Is a pgid rather than a PID), and one is sent to work 1 (% 1.

 

A job can pass through$ FGFrom background to foreground:

$ Cat> log &

$ FG % 1

After we run the first command, we cannot input the command because it is working in the background, until we bring the work to the foreground, we can enter the command to Cat. After the input is complete, press Ctrl + D to notify the shell that the input is complete.

 

The process group concept is easy to understand. Sessions are mainly created for a terminal. When we open multiple terminal windows, we actually create multiple terminal sessions. Each session has its own foreground and background work. In this way, we have added management and operation for processes.Level. In the era of no graphical interface, sessions allow users to initiate and manage multi-layer processes through shell. For example, I can initiate multiple background jobs through shell, but the standard input and output are not occupied at this time, and I can continue other jobs. Today, graphical interfaces can help us solve this need, but working groups and session mechanisms are still applied in many parts of Linux.

 

Summary:

Process Group, pgid

Session, Sid, job, forground, Background

FG, kill-pid, &, %