Unix Lesson 7: Processes
Processes are the running programs that are currently being executed by the operating system. Each process has a unique identifier called a "process ID", or "PID" for short. This is an increasing number that "rolls over" after reaching an upper limit.
Process ID #1 is special. It belongs to a program called init, which is responsible for the supervision of all the other, "child" processes. Processes are arranged in a tree of parent/child relationships.
To view the processes that are the children of your shell:
$ ps
Will show this list. ps has many flags. Most ps flags are different than the switches we've seen before; they are not proceded with a dash (-). The flags are:
a: all processes (the default is just the shell's processes)u: display all processes that this user ownsx: "extended output" (see manual)w: "wide" output (kind of like extended; can be repeated)
These are combined as a single argument, for example:
$ ps auxww
Processes change frequently, so they can be hard to track with single commands run once at a time. To watch a periodically updating list of the most hungry processes, you can use the top command to get this. Press "q" to quit once starting it.
Jobs can also be run in the background. To do this, append an & (ampersand) to the end of the command-line. This will launch the command, and immediately return you to the shell. It still keeps standard I/O tied to the shell and terminal, so you may see output.
Another way of doing this is to press ^Z (control+z) after launching the command. This will immediately suspend the program. After that, you can type bg to force the process to run in the background, similar to the ampersand usage. To get it back, type fg.
To manage and see background processes, type jobs. Each process will have a job control number. You can use these numbers with a % symbol to control processes with tools like kill. kill takes a PID normally, but you can replace it with a job control number, too.
$ kill %1
For example.
Assignment
Start a cat process in the background, list its job control number, and then kill the process. Explain the commands you used to do this, from the launching of cat forward.