First a note about a programming language that is different from the C
programming language, the GNU shell. When you enter commands in the GNU
shell, they are executed immediately. Moreover, the shell is a
programming language, in that the commands you type are a program,
because you can also create a text file containing many shell commands.
When you run this file, the commands will be executed in sequence.
On the other hand, consider C. While a shell command file can be
executed directly, a C program must be created in two stages:
First, the program is written in the form of text files with a text
editor such as GNU Emacs. This form of the program is called the
source code. A computer cannot execute source code directly.
Second, the completed source code is processed with a compiler -- a
program that generates a new file containing a machine-language
translation of the source code. This file is called an executable
file, or executable. The executable file is said to have been
compiled from the source code.
To run the compiled program, you must usually type the name of the
executable file preceded by a period and a slash, as in this example:
./myprogram
The "dot-slash" prefix tells the GNU shell to look in the current
directory for the executable. You usually do not need to type ./
in front of commands for programs that came with your GNU system, such
as emacs, because the computer already knows where to
look for the executables of those programs, which were placed in special
directories when your GNU system was installed.
A C program is made up of, among other components, variables and functions.
A variable is a way to hold some data which may vary, hence the name.
For example, a variable might hold the number 17, and later the number 41.
Another variable might hold the word "Sue".
A function is a segment of text in the source code of a program
that tells the computer what to do. Programming consists, in large
part, of writing functions.
