Processes

Any program running on a Linux machine is a process. These processes have a parent-child organization that forms a tree-like hierarchy.

New processes are created when you run a command in your bash shell. For example, if you run top, the bash program will run fork() to create a new process. In this case, bash is the parent process and top is the child process. We commonly phrase it such that a parent “forks” a new process.

Process Tree

init is the first process in the tree. Each process has a user id (UID), a process ID (PID), and a parent process ID (PPID). The UID determines what the process is allowed to do. If a process dies, but already forked children, then any children are reparented to init.

When you log into the supercomputer, how many processes are created? The supercomputing login server has an SSH server program called sshd (SSH Daemon), which listens for incoming connections. When you first run ssh, you’re establishing a connection with that program. sshd then forks off a child process that handles authentication and data exchange for your session. Once authenticated, the sshd that handles your connection forks off another child which becomes the shell. If someone else were to connect, the process is the same.

Shell Commands

You should become familiar with ps and htop–run the following examples to see what results:

Command	Purpose	Examples
ps	Shows currently running processes	ps -u $USER ps -aux
htop	Dynamically display running processes	htop htop -u $USER