Processes

Now we know about files, commands, and shells. What more could UNIX possibly have to offer?

UNIX was designed at a time when computers were used largely for crunching huge amounts of numbers. One of the nicest features of UNIX is its ability to multitask - to do more than one thing at a time. A single task is called a process.

The ps Command

Whenever you are logged in, you are executing at least one process. Let's get a list of all the processes I am currently running:

   PID TTY      TIME CMD

 16931 pts/38   0:00 bash

I'm running one process, bash. bash is my shell, so naturally it is running. The output from ps includes the following information:

• PID - the Process ID of the process. Every program that is running has a PID, and you use this number to identify them.
• TTY - the Terminal that the process is running on. This information generally is not very helpful.
• TIME - the time that the process has been running.
• CMD - The name of the command being executed.

ps has a great many options. Unfortunately, every flavor of UNIX uses a different set of options to ps. The following options are fairly standard:

• a - print all running processes, not just yours
• f - generate a "full" listing, including username and command line

Background Processes

UNIX makes a very strong distinction between processes that are running in the foreground and those that are running in the background. A foreground process can talk to your terminal, and you can talk back using the keyboard. A background process is running, but it has no way to communicate with you directly.

This is most useful when you have a large task to complete (say, calculating Pi to a million digits). We'll use the sleep command as an example. Let's try this:

The sleep command takes one argument, the number of seconds to sleep for. sleep then does nothing for that long.

It's pretty boring to sit and watch as sleep just sits there. We will now run sleep as a background process, which will allow us to go about our business.

   PID TTY      TIME CMD

 16931 pts/38   0:00 bash

 22098 pts/38   0:00 sleep

First of all, we use another shell meta-character, & to indicate that the command should be run in the background. You can use the & with any command.

Next, notice that the first command returns a number to us: the Process ID (PID) of the backgrounded process.

Then we use the ps command to confirm that there are now two processes running: bash and sleep.

This is the shell reporting that the command completed successfully.

Killing a Process

Not all processes complete successfuly. You may find yourself stuck with a process that seems to have stopped working, or perhaps you have changed your mind about running the process.

For example, we'll start a new sleep process and kill it.

   PID TTY      TIME CMD

 23360 pts/38   0:00 sleep

 16931 pts/38   0:00 bash

   PID TTY      TIME CMD

 16931 pts/38   0:00 bash

[1]+  Terminated              sleep 300

First, we start a new sleep process. Then we check that it is running with ps. We note the PID, then issue a kill command, passing the PID as an argument. Then we check again using ps. At the same time, the shell prints out a line indicating that our sleep process has been Terminated.