What’s a UNIX process?

  • To understand a shell, let’s first understand what processes are.

  • All programs are a process: process is a program in action.

  • Processes have:

    • Process ID or PID (integer)

    • Name (command being run)

    • Command line arguments/options

    • input and output: stdin (input, like from keyboard, another program, another device), stdout (output, like to screen, another program, file, device), stderr (like stdout but for errors)

    • Return code (integer) when complete

    • Working directory

    • environment variables: key-values which get inherited across processes.

  • These concepts bind together all UNIX programs, even graphical ones.

Process listing commands (feel free to try, but we play more with them later):

ps auxw
top              # (q to quit)
htop             # (q to quit)
pstree $USER
pstree -pau $USER

Another way to find out what SHELL you are running:

ps -p $$

Working with processes

All processes are related, a command executed in shell is a child process of the shell. When child process is terminated it is reported back to parent process. When you log out all shell child processes terminated along with the shell. You can see the whole family tree with ps af. One can kill a process or make it “nicer”.

pgrep -af <name>
kill <PID>
pkill <name>
renice #priority <PID>

Making process “nicer”, renice 19 <PID>, means it will run only when nothing else in the system wants to. User can increase nice value from 0 (the base priority) up to 19. It is useful when you backup your data in background or alike.

Foreground and background processes

The shell has a concept of foreground and background processes: a foreground process is directly connected to your screen and keyboard. A background process doesn’t have input connected. There can only be one foreground at a time (obviously).

If you add & right after the command will send the process to background. Example: firefox --no-remote &, and same can be done with any terminal command/function, like man pstree &. In the big picture, the & serves the same role as ; to separate commands, but backgrounds the first and goes straight to the next.

If you have already running process, you can background with Ctrl-z and then bg. Drawback: there is no easy way to redirect the running task output, so if it generates output it covers your screen.

List the jobs running in the background with jobs -l (show real PID as well), get a job back online with fg or fg <job_number>. There can be multiple background jobs.

Kill a foreground job: Ctrl-c

Hint: For running X Window apps while you logged in from other Linux / MacOS make sure you use ssh -X ... to log in. For Windows users, you need to install VcXsrv Windows X Server [1] on your workstation.

Hint: For immediate job-state change notifications, use set notify. To automatically stop background processes if they try writing to the screen stty tostop

Exit the shell and ‘screen’ utility

logout or Ctrl-d (if you don’t want Ctrl-d to quit, set export IGNOREEOF=1 in .bashrc).

Of course, quitting your shell is annoying, since you have to start over. Luckily there are programs so that you don’t have to do this. In order to keep your sessions running while you logged out, you should learn about the screen program.

  • screen to start a session

  • Ctrl-a d to detach the session while you are connected

  • screen -ls to list currently running sessions

  • screen -rx <session_id> to attach the session, one can use TAB for the autocompletion or skip the <session_id> if there is only one session running

  • tmux is a newer program with the same style. It has some extra features and some missing features still.

Some people have their screen open forever, which just keeps running and never gets closed. Wherever they are, they ssh in, connect, and resume right where they left off.

Example: irssi on kosh / lyta

Exercises 1.1


  • Find out which shell you are running, your user name, hostname, system name. For Aalto users: set your SHELL to BASH if you have not yet done so: chsh -s /bin/bash on kosh

  • Find out with man how to use top / ps to list all the running processes that belong to you Tip: top has both command line options and hot keys.

  • Find your shell session’s PID, list the processes tree of all child processes that belong to your current session, with the command line, PID, user

  • With pgrep list all bash processes, if you have SSH access, try both locally, and on a remote Linux server

  • Run nano filename, send it to the background, and return back to the foreground``. Tip: quite nano` with ``Ctrl-x.

  • Run man htop, send it to the backround, and then kill it with kill. Tip: one can do it by background job number or by PID.

  • (*) Run screen session. Detach, close the seesion, open again and attach ‘screen’ back. Exit ‘screen’.

  • (*) Find out how to list a processes tree with ps, both all processes and only your own (but all your processes, associated with all terminals)

  • (*) Try a use case: your current ssh session to a remote host “got stuck” and does not response. Open another ssh session to the same remote host and kill the first one. Tip: echo $$ gives you current bash PID.

  • (*) Get any X Window application (firefox, xterm, etc) to run on a remote Linux machine