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Linux Tutorial - Shells and Utilities - Shell Variables
  Directory Paths ---- Permissions  


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

I get:

jimmo

Typing

I get:

ansi

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:

PS1='\u@\h:\w> '

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:

jimmo@linux:/tmp>

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

10:09 jimmo@linux:/tmp>
Variable Meaning
\u Username
\h Hostname
\H The fully-qualified hostname
\w Current working directory
\d date
\t the current time in 24-hour HH:MM:SS format
\T the current time in 12-hour HH:MM:SS format
\@ the current time in 12-hour am/pm format
\A the current time in 24-hour HH:MM format
\l the basename of the shell's terminal device
\e Escape character
\n newline
\r carriage return

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:

PS1='\e[31m\A\e[0m \u@\h:\w> '

Which then looks like this:

10:09 jimmo@linux:/tmp>

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Copyright 2002-2009 by James Mohr. Licensed under modified GNU Free Documentation License (Portions of this material originally published by Prentice Hall, Pearson Education, Inc). See here for details. All rights reserved.
  
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