Process basic properties

One . brief introduction

This article mainly introduces the basic properties of a process , Basic attributes include : process ID、 The parent process ID、 Process group ID、 Session and control terminals

Two . Process properties

1. process ID（PID） Each process has a unique process represented by a nonnegative integer ID（PID）. Like our ID cards , Everyone's ID number is unique . Because of the process ID Identifiers are always unique , It is often used as part of other identifiers to ensure its uniqueness . process ID（PID） It cannot be changed at the user level .

stay Linux In the system ,PID by 0 The process of is usually a scheduling process . It is often called an exchange process , It is also the first system process . The first user process is init process . Its PID by 1.

2. The parent process ID（PPID） No matter what process （ except init process ） Are created by another process . This process is called the parent process of the created process . The process that is created is called a child process . The parent process ID Cannot change at the user level . The process of the parent process ID It is the parent process of the child process ID（PPID）.

3. Process group ID（process group ID PGID） stay Linux In the system . Every user is a user ID（UID） User group ID（GUID）. identical , Processes also have their own processes ID（PID） And process groups ID（PGID）. A process group is a collection of one or more processes ; They are related to the same assignment . Each process group has a unique process group ID（PGID）, Process group ID（PGID） Can be changed at the user level . example . Add a process to another process group , Is the use of setpgid() Function changes its process group ID.

A process can only set up process groups for itself or its child processes ID（PGID）, Suppose... Is called in its child process exec() And so on , The process group of the child process can no longer be changed ID（PGID）.

4. conversation （session） A session is a collection of one or more process groups . System call function getsid() Used to get the session of a process ID(SID).

such as , We go through SSH logon server , Will open a control terminal （TTY）, This control terminal corresponds to a session . And the commands we run in the terminal and their child processes , It forms process groups one by one , among , Commands running in the background , Make up a background process group ; Commands running in the foreground , Constitute the foreground process group .