The Linux kernel: Processes
In this article, we will explain 30 most useful examples of 'ps ps L. The command below allows you to view the PID, PPID, user name and. Differences between parent and child PID PPID Return value from fork Pending from CSC H at University of Toronto. Solaris Admin - PID and PPID What are the differences between PID and PPID. Give example? the PPID of Its a parent-child relationship.
Are you having the same questions on your mind? If so, you are in the right place to get all the details. What Is Process ID? PID stands for process identification number which is generally used by most operating system kernels such as Linux, Unix, macOS and Windows.
It is a unique identification number that is automatically assigned to each process when it is created in an operating system. A process is a running instance of a program. The default maximum value of PIDs is 32, You may ask, why we need such amount of PIDs? Also in order to prevent possible errors.
The PID for the running processes on the system can be found by using the below nine methods such as pidof command, pgrep command, ps command, pstree command, ss command, netstat command, lsof command, fuser command and systemctl command. This can be achieved using the below six methods.
Make sure your need to input your process name instead of us. Using pidof Command pidof used to find the process ID of a running program. To demonstrate this, we are going to find out the Apache2 process id from Debian 9 stretch system.
It could be the first number. Using pgrep Command pgrep looks through the currently running processes and lists the process IDs which match the selection criteria to stdout.
Using pstree Command pstree shows running processes as a tree.
Linux PID, PPID, PGID (Process Group Leader), SID (Session Leader)
The tree is rooted at either pid or init if pid is omitted. A process can determine the foreground process group in its session using tcgetpgrp fdwhere fd refers to its controlling tty.
If there is none, this returns a random value larger than 1 that is not a process group ID. A process can set the foreground process group in its session using tcsetpgrp fd,pgrpwhere fd refers to its controlling tty, and pgrp is a process group in the its session, and this session still is associated to the controlling tty of the calling process. How does one get fd? There is also the function ctermid to get the name of the controlling terminal.
Opening the name of the controlling tty gives a file descriptor fd. Background process groups All process groups in a session that are not foreground process group are background process groups. Since the user at the keyboard is interacting with foreground processes, background processes should stay away from it. Normally, that will stop it, the job control shell notices and tells the user, who can say fg to continue this background process as a foreground process, and then this process can read from the terminal.
Indeed, the idea is to tell the process that reading from the terminal is not allowed right now. If it wouldn't see the signal, then it will see the error return. Orphaned process groups The process group leader is the first member of the process group. It may terminate before the others, and then the process group is without leader. A process group is called orphaned when the parent of every member is either in the process group or outside the session. In particular, the process group of the session leader is always orphaned.
The idea is that perhaps the parent of the process group leader is a job control shell. In the same session but a different process group.
As long as this parent is alive, it can handle the stopping and starting of members in the process group. When it dies, there may be nobody to continue stopped processes. Note that the process group of the session leader is already orphaned, so no signals are sent when the session leader dies. Note also that a process group can become orphaned in two ways by termination of a process: Furthermore, that a process group can become orphaned other than by termination of a process, namely when some member is moved to a different process group.
When the process is created, it becomes a member of the session of its parent. By convention, the session ID of a session equals the process ID of the first member of the session, called the session leader. A process finds the ID of its session using the system call getsid.
Every session may have a controlling tty, that then also is called the controlling tty of each of its member processes. And when that fails, there was no controlling tty.
9 Easiest Ways To Find Out Process ID (PID) In Linux
Given a file descriptor for the controlling tty, one may obtain the SID using tcgetsid fd. A session is often set up by a login process. The terminal on which one is logged in then becomes the controlling tty of the session.
All processes that are descendants of the login process will in general be members of the session. In order to be sure of that we fork first: Moreover, it becomes process group leader of a new process group. Both session and process group contain only the single process pid. Furthermore, this process has no controlling tty. The restriction that the current process must not be a process group leader is needed: Getting a controlling tty How does one get a controlling terminal?
Nobody knows, this is a great mystery.