Using parameter and variable expansion

A $ followed by a number stands for a positional parameter passed to the script or function. A positional parameter is represented with either a single digit (except 0) or two or more digits in braces; for example, 7 and {15} are both valid representations of positional parameters. For example, if the command:

echo $1

appeared in a shell script, it would echo the first positional parameter.

Similarly, a $ followed by the name of a shell variable (such as $HOME) stands for the value of the variable.

These constructs are called parameter expansions. In this sense, the term parameter can mean either a positional parameter or a shell variable.

The tcsh shell also supports more complicated forms of parameter expansions, letting you obtain only part of a parameter value or a modified form of the value.

Modifier	Description
r	Root of value
e	Extension of value
h	Head of value
t	Tail of value

For example, to extract only part of a file name, you can add one of these modifiers as follows:

File name	r	e	h	t
/usr/bin/vi.txt	/usr/bin/vi	txt	/usr/bin	vi.txt
/u/bobby/mail	/u/bobby/mail	empty	/u/bobby	mail
storybook.pdf	storybook	pdf	empty	storybook.pdf
INSTALL	INSTALL	empty	empty	INSTALL