Although you may not have thought of it as such, the Unix commands that you type into your keyboard consititute a programming language. This language is interpreted by a program called a shell.

The Unix operating system is positively overrun with different shell languages. The C-shell (csh) or TC-shell (tcsh) that you have probably been using as your primary means of interaction is just one example. And, although we have not touched on it yet, that is a full programming language, including control flow instructions for branching and loops.

csh and tcsh constitute one family of commonly used shells. They languages they accept are nearly identical. tcsh adds features that are of especial use when typing commands directly to the shell. csh is preferred when writing scripts, short programs in the shell language, if only because csh is more likely to be installed on any given Unix system.

Another family is the sh family, which contains sh, bsh, and bash. sh is commonly used for writing scripts but lacks some of the conveniences we would want in an interactive shell. bsh is the “Bourne shell” (named for its inventor), and bash, the default interactive shell for Linux and CygWin, is the “Bourne Again shell” .

These are by no means the only shell or scripting languages available for Unix, but they'll do for now.

What distinguishes these shell languages from more general-purpose programming languages is their emphasis on simplicity. Scripting languages are designed to permit immediate translation. If, every time you typed a command, you had to wait while a full-fledged compiler was loaded and executed to process the characters you had typed, the time delays would have you screaming in frustration. Instead, these languages are designed to very quickly determine (usually by examining the first word you typed) if the command is a special command “built in” to the shell. Anything else is a program to be launched.

The command type can be used to determine if a given word represents a built-in shell command, a program (and if so, where that program is located), or something else. Try logging in and giving the following commands:

We'll concentrate on the csh for scripting purposes, with some notes on how sh-like languages, including bash, differ from csh.

In any programming language, we expect to have variable names, and shells are no exception. A typical shell variable name begins with an alphabetic character and continues with zero or more alphanumeric characters. Oddly enough, when we want to get the value of a shell variable, we add a $ to the front of it.

Try this:

csh, tcsh	sh, bash
set A=hello echo $A	A=hello echo $A

and the echo command will produce the string “Hello” .

All shell variables hold strings, although in selected instances we may be able to interpret those strings as numbers.

The scope rules for shell variables are a bit odd. Most variables are local to the process where they are assigned. For example, look at the following command sequence:

csh, tcsh	sh, bash
set A=hello echo $A hello csh echo $A A: undefined variable exit echo $A hello	A=hello echo $A hello sh echo $A A: undefined variable exit echo $A hello

What's going on? The first = and echo are obvious enough. We set A to a value and then printed it. In the next line, we start another copy of the shell running (csh or sh). This runs as a separate process (the old one is temporarily suspended). This new “child” process does not inherit the variable A, so when we try to print it, we are told that A is undefined. The exit command shuts down the child process, returning us to the original (parent) process where we had previously defined A, and so we were able to print it again.

If we prefer, we can make a variable exported, meaning that its value will be seen by child processes. In csh and tcsh this is done by using setenv instead of set. In sh and bash this is done by naming the variable in an export command.

csh, tcsh	sh, bash
setenv B goodbye echo $B goodbye csh echo $B goodbye exit echo $B goodbye	B=goodbye export B echo $B goodbye csh echo $B goodbye exit echo $B goodbye

An annoying inconsistency in the csh family: You must use an = sign when doing set:

Many Unix programs use exported shell variables to control or modify their behavior. Examples that may be familiar include:

Another important exported environment variable is PATH. This determines what programs you can execute without typing in a full path name. If you type a command that is not a built-in shell command, and does not contain a '/', then the shell looks at each directory listed in $PATH to see if the program you have requested can be found there. For example, suppose you have compiled a C++ program and produced a new program, yourProgram, in your current working directory. Some people will be able to execute the program this way:

Adding additional directories to your $PATH is one of the more common customizations that people make to their Unix environments.

To see some of the other environment variables that are already defined for you, try giving the command env.

A final caution: when writing commands that involve environment variables, it helps to keep in mind the different forms of quoting in Unix shells, especially the difference between 'single quotes' and "double quotes".

Sooner or later, you're going to decide that you don't like some of the default behaviors of your favorite Unix programs. When that happens, you will be pleased to learn that there's are great deal you can do to modify your Unix environment.