B.4.2 Environment substitutions

Much of the power of environment variables lies in the way that they may be substituted into configuration files. Variable substitutions are described below.

The line is scanned from left to right. In this scan, all characters other than dollar signs ($) are retained unmolested. Dollar signs, however, introduce an environment variable reference. References take three forms:

$var

Replaced by the value of environment variable var, determined as specified in Variable values. var must be one of the following:

• One or more letters.
• Exactly one nonalphabetic character. This may not be a left brace ({).

${var}

Same as above, but var may contain any character (except }).

$$

Replaced by a single dollar sign.

Undefined variables expand to a empty value.