A process is simply a program that is currently running on your system. Whenever you execute a command in Linux, the operating system creates a process to run that command.

• Whenever a command is issued in Unix/Linux, it creates/starts a new process. For example, pwd when issued which is used to list the current directory location the user is in, a process starts.
• Through a 5 digit ID number Unix/Linux keeps an account of the processes, this number is called process ID or PID. Each process in the system has a unique PID.
• Used up pid’s can be used in again for a newer process since all the possible combinations are used.
• At any point of time, no two processes with the same pid exist in the system because it is the pid that Unix uses to track each process.

概念

程序是一系列指令的集合，通常存为可执行文件。进程是程序的执行过程。

进程属性

使用ps可以查看系统正在运行的进程。 ps —help显示该命令的用法。

ps -e -o USER,PID,PPID,GID,NAME
USER           PID  PPID      GID NAME                        CMD            
root             1     0        0 init                        init
root             2     0        0 [kthreadd]                  kthreadd
root             3     2        0 [ksoftirqd/0]               ksoftirqd/0
root             5     2        0 [kworker/0:0H]              kworker/0:0H
root             6     2        0 [kworker/u16:0]             kworker/u16:0
root             7     2        0 [rcu_preempt]               rcu_preempt
root             8     2        0 [rcu_sched]                 rcu_sched
root             9     2        0 [rcu_bh]                    rcu_bh

USER: 进程所属用户的ID PID:进程ID PPID:父进程的ID GID:进程组ID NAME:进程名

孤儿进程

使用fork创建的子进程，父进程应该调用wait函数，等待子进程的退出信息，如果父进程先结束，子进程将变成孤儿进程，过继给init进程， init进程成为它的父进程。

进程组

进程组包含多个进程，进程组会有一个领导进程(process group leader)，该领导进程的PID将作为进程组的PGID。