Shell Variables

The shell's environment is all the information that the shell will use as it runs. This includes such things as your command search path, your logname (the name you logged in under), and the terminal type you are using. Collectively, they are referred to as your environment variables and individually, as the "so-and-so" environment variable, such as the TERM environment variable, which contains the type of terminal you are using.

When you log in, most of these are set for you in one way or another. (The mechanism that sets all environment variables is shell-dependent, so we will talk about it when we get to the individual shells.) Each environment variable can be viewed by simply typing echo $VARIABLE. For example, if I type

echo $LOGNAME

I get:

Typing

echo $TERM

I get:

In general, variables that are pre-defined by the system (e.g. PATH, LOGNAME, HOME) are written in capital letters. Note that this is not a requirement as there are exceptions.

Note that shell variables are only accessible from the current shell. In order for them to be accessible to child processes (i.e. sub-processes) they must be made available using the export command. In the system-wide shell configuration file or "profile" (etc/profile) many variables, such as PATH are exported. More information on processes can be found in the section on processes in the chapter "Introduction to Operating Systems".

It is very common that users' shell prompt is defined by the systems. For example, you might have something that looks like this:

What this does is to set the first level prompt variable PS1 to include the username, hostname and the current working directory. This ends up looking something like this:

Adding the \A to display the time, we end up with something that looks like this:

One way of using the escape character in your prompt is to send a terminal control sequence. The can be used, for example, to change the prompt so that the time is shown in red:

Which then looks like this:

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