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Processes

A process is just a piece of running code which is executing some task. It is active, in that it has been loaded into the system RAM and is being processed by the CPU.

Every process has...

  • a process ID, pid
  • a parent ID, ppid

With the exception of the init process, all processes are forked from another process.

The init process

When you boot the machine, it does a lot in kernel space. When this is done, it calls one thing in user space: the init process. Once that's done, the init system is responsible for starting all other processes in user space.

The init process is the only one without a parent. Its pid is 1 and ppid is 0.

There are a number of init systems, including systemd, upstart, sysvinit, launchd, bash, and openrc. Ubuntu 18.04 uses bash.

Exec processes

Exec processes replace an existing process image with a new image. The current process dies and is replaced by the same process with the same PID; there is no child process created. The process will begin again starting at the top of the new image.

Fork and exec examples

When you run ls -l in a terminal window, this happens:

  • The parent process (in this case, a bash shell) is copied (fork)
  • The copy (the child process) is replaced by ls
  • The child process runs and then dies on completion (exec)

Forked processes inherit stdin, stdout, and stderr from the parent.

What about exec ls -l?

  • Whatever is running in the current process stops, and the process runs ls -l
  • The terminal window in which this is run will most likely close

Note that exec is not a direct evocation of an exec process, it's a shell command which triggers an exec process somewhere down the line

Process signals

Keyboard shortcuts for processes running in foreground:

  • Ctrl+C = SIGINT (interrupts process)
  • Ctrl+\ = SIGQUIT (quits process)
  • Ctrl+Z = SIGTSTP (suspends process)
  • kill -l gives you a list of signals

Ending processes with kill, in order or severity:

  • kill -15 [pid] sends SIGTERM
    • kill [pid] does same thing
  • kill -2 [pid] sends SIGINT
  • kill -1 [pid] sends SIGHUP
  • kill -9 [pid] sends SIGKILL
    • This gives the process no chance to clean up before shutdown
    • Can never fail

Useful commands in Linux

  • ps -e | wc -l count all processes
  • ps -e | grep Chrome list all processes related to Chrome

Sources