A number of in-memory variables are assigned and loaded by default when
you log in. These variables, known as environment variables, can be used by
various commands to get information about your environment, such as the
type of system you are running, your /home directory, and the shell in use.
Environment variables are used to help tailor the computing environment of
your system and include helpful specifications and setup, such as default
locations of executable files and software libraries. If you begin writing shell
scripts, you might use environment variables in your scripts. Until then, you
need to be aware only of what environment variables are and do.
The following list includes a number of environment variables, along with
descriptions of how the shell uses them:
PWD—Provides the full path to your current working directory, used by
the pwd command, such as /home/matthew/Documents
USER—Declares the user’s name, such as matthew
LANG—Sets the default language, such as English
SHELL—Declares the name and location of the current shell, such as
/bin/bash
PATH—Sets the default locations of executable files, such as /bin,
/usr/bin, and so onTERM—Sets the type of terminal in use, such as vt100, which can be
important when using screen-oriented programs, such as text editorsYou can print the current value of any environment variable by using echo
$VARIABLENAME, like this:
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matthew@seymour:~$ echo $USER
matthew
matthew@seymour:~$
NOTE
Each shell can have its own feature set and language syntax, as well as a
unique set of default environment variables.You can use the env or printenv command to display all environment
variables, as follows:
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matthew@seymour:~$ env
ORBIT_SOCKETDIR=/tmp/orbit-matthew
SSH_AGENT_PID=1729