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There are two types of variables:
- Shell variables: Shell variables are variables that
are used to configure options of the shell as well as allow
the user to specify/modify an item or list of items as
variables.
- Environmental variables: Environmental variables are mainly used
to
configure external programs by setting different values that
they each can view. Only environmental variables are visible
to external programs.
There are many built in shell variables that allow you to
view modify, turn on and off certain features of the shell.
Some variables need not have a value, just the fact that they
are set (i.e. the variable exist but with out a value
associated with it) triggers a feature it controls to exist.
Some of the variables, such as cwd, are there for mostly
internal purposes only-they may be given a different value,
but it has no significance to the shell and may be
overwritten at any time. To view all of the variables and any
value associated with them, type set by itself. If you want
to view a single variables and it's value (if any) type echo
$<variable>, if the variable exists it will
print the variable name, and any value that it may have. You
use the $ character in front of the variable name to indicate
to the shell that the name is actually a variable, however
the $ is not part of the variable name.
To set a variable, you use the shell command set followed by
the variable name. If you wish to give a variable to a value,
follow the variable by an equals sign and the value you wish
to set (i.e. set x=mrvalue). If you need to give a variable
more than one value, group all the values together using
parentheses, or the shell will give the variable the value of
the first item: set x=(1 2 3 4). To take a variable off the
variable list, use the unset command. This will both remove
the value of a variable and remove it from the list of
variables.
Some of the more interesting variables are listed
here, for a more complete list, check out the shell man
page.
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