A Reprise

Recall that every command in UNIX is a program. Now let us take a little look under the hood. When you log in, shell is launched. The shell accepts commands you enter at the prompt and sends them to the kernel, or operating system, which runs the program. This can cause output to be put to the screen, as in ls, or happen without comment, as in rm.

Programs that are running in UNIX are called processes. Every process has an owner and an integer associated with it called a process ID (PID). The user who spawns a process will generally be its owner. You are the owner of all processes you spawn. Many, such as ls, last such a short time you never notice them beyond the output they produce; they terminate in a fraction of a second after you enter them. When you log into your host, you actually are launching a program; this is your shell. When the shell terminates, your terminal session will be gone. At the command line, enter ps and you will see something like this.

unix> ps
PID TTY
10355 pts/1
10356 pts/1
unix>
TIME CMD
00:00:00 bash
00:00:00 ps

The ps command shows all processes currently running spawned by your shell. On this machine, the shell’s (bash) process ID is 10355. By entering ps aux at the command line, you can see all processes running on your UNIX server, along with their process IDs and an abundance of other information. Try this at several different times. If you are using a server, you will see processes spawned by other users. You will also see other processes being run by the system to support your machine’s operation.

An example of a program that does not finish its work immediately is the program bc. We show a sample bc session here; this application is a simple arbitrary-precision calculator.

unix> bc
bc 1.06.94
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006
Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type ‘warranty’.
3+4
7
4*5
20
2^20
1048576
2^100
1267650600228229401496703205376
quit

When you type bc at the command prompt, the shell runs the bc program. This program continues to run until you stop it by typing quit. To see bc’s process ID, start bc and then type Control-Z to put it to sleep. This interrupts the bc process, puts it in the background, and returns you to your shell. Then enter ps at the command prompt to see the process ID for your bc session.

unix> bc
bc 1.06.94
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 
Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type ‘warranty’.
[1]+
unix> ps
PID
14110
14253
14254
unix>
Stopped
TTY
pts/4
pts/4
pts/4
bc
TIME
00:00:00
00:00:00
00:00:00
CMD
bash
bc
ps

Try typing exit to log out; you will see something like this.

unix> exit 
exit
There are stopped jobs.
unix>

Now type jobs at the command prompt. You will see this.

unix> jobs
[1]+ Stopped
unix>
bc

You can end the job bc labeled [1] by doing the following

unix> kill %1
unix> jobs
[1]+ Terminated
unix> jobs
unix>
bc

If several jobs are stopped, each will be listed with a number. You can end any you wish to by entering a kill command for each job. When you type jobs at the command line the first time, it will tell you what jobs it has suspended. After that, you will see a (possibly empty, like here) list of jobs still in the background. Do not dismiss a shell with running jobs; end them to preserve system resources.

You can bring your stopped job into the foreground by entering fg at the command prompt.

Programming Exercises

Start up a session of bc and put it into the background using control-Z. Do this for several sessions. Type in some calculations into some of the sessions and see if they reappear when you bring the bc session containing that calculation into the foreground.
The bc calculator has variables which allow you to store numbers under a name. These play the role of the symbols described in Chapter 0, but they are limited to storing numbers. Here we show some variables being created and some expressions being evaluated.
```
unix> bc
bc 1.06.94
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 
Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type ‘warranty’.
cow = 5
pig = 2
horse = 7
horse + cow
12
horse/pig
3
pig/horse
0
cow^horse
78125
```
Replicate this session. Put it into the background and bring it into the foreground. Were your variables saved? Notice that this calculator does integer arithmetic. The = sign you see is actually assignment, and not test of equality.
Look at one of the algorithms for converting a binary number into a decimal number. Can you step through the process using bc and make it work?