44.3.1 How Shells Run Other Programs

For each command it runs, a shell does a series of steps. First, if the shell is reading commands from a terminal (interactively), it prints a prompt (such as a % or $ ) and waits for you to type something. Next, the shell reads the command line (like cat -v afile bfile > cfile ), interprets it (8.1 , 8.5 ) , and runs that command line. When the command finishes running (unless the command is in the background (1.26 , 1.27 ) ), the shell is ready to read another command line.

44.3.2 Interactive Use vs. Shell Scripts

A shell can read command lines from a terminal or it can read them from a file. When you put command lines into a file, that file is called a shell script (44.2 ) 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 % or $ prompts). 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.

44.3.3 Types of Shells

There are two main kinds of shells in UNIX:

• 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 quirks (47.2 ) 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 ksh and bash (1.8 ) , combine handy interactive C shell-like features with Bourne shell syntax.)

44.3.4 Shell Search Paths

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 .

44.3.5 Bourne Shell Used Here

Most serious shell programmers write their scripts for the Bourne shell. So do we.

Newer Bourne shells have features - like shell functions (10.9 ) , 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 ($ ) in it. If your account isn't running the Bourne shell, start one by typing sh . Your prompt should change to a dollar sign ($ ). You'll be using the Bourne shell until you type CTRL-d at the start of a line:

% sh


$ 
$ ...Enter commands...

$ [CTRL-d]

%