A shell is a program that interprets your command lines and runs other programs. Another name for the shell is "command interpreter." This article covers the two major UNIX shells, including discussion about how shells run, how they search for programs, and how they read shell script files.
For each command it runs, a shell does a series of steps.
if the shell is reading commands from a terminal (interactively),
it prints a prompt (such as a
$) and waits for you
to type something.
Next, the shell reads the command line (like cat -v afile bfile > cfile),
and runs that command line.
When the command finishes running (unless the command is
the shell is ready to read another command line.
A shell can read command lines from a terminal or it can read them from
When you put command lines into a file, that file is called a
or shell program.
The shell handles the shell script just as it handles the commands you type
from a terminal (though it doesn't print the
With this information, you already know how to write simple shell scripts - just put commands in a file and feed them to the shell!
In addition though, there are a number of programming constructs that make it possible to write shell programs that are much more powerful than just a list of commands.
The C shell (csh) is especially good for working on a terminal. csh will read shell scripts and has some useful features for programmers. Unfortunately, it has some that can make shell programming tough.
The Bourne shell (sh) and shells like it are probably used more often for shell programming. (Some newer sh-like shells, including , combine handy interactive C shell-like features with Bourne shell syntax.)
As article 8.7 explains, if the shell is trying to run a command and the command isn't built-in to the shell itself, it looks in a list of directories called a search path. UNIX systems have standard directories with names like /bin and /usr/bin that hold standard UNIX programs. Almost everyone's search path has these directories.
If you do much shell programming, you should make a directory on your account for executable files. Most people name theirs bin and put it under the home directory. See article 4.2.
Most serious shell programmers write their scripts for the Bourne shell. So do we.
Newer Bourne shells have features - like , an unset command for shell variables, and others - that the earlier Version 7 Bourne shell didn't. Most scripts in this book are written to work on all Bourne shells, though - for portability, the scripts don't use these new features.
For the rest of
these introductory articles,
it may be easier if you have a terminal
close by so you can try the examples.
If your account uses the Bourne shell or one of its relatives (ksh,
bash, etc.), your prompt probably has a dollar sign (
If your account isn't running the Bourne shell, start one by typing
should change to a dollar sign (
You'll be using the Bourne shell until you type
at the start of a
sh$ $ ...Enter commands... $ [CTRL-d] %