substitute-env-vars

substitute-env-vars is a compiled Lisp function in `env.el'.

(substitute-env-vars STRING &optional WHEN-UNDEFINED)

Substitute environment variables referred to in STRING.
`$FOO' where FOO is an environment variable name means to substitute
the value of that variable. The variable name should be terminated
with a character not a letter, digit or underscore; otherwise, enclose
the entire variable name in braces. For instance, in `ab$cd-x',
`$cd' is treated as an environment variable.

If WHEN-DEFINED is nil, references to undefined environment variables
are replaced by the empty string; if it is a function, the function is called
with the variable name as argument and should return the text with which
to replace it or nil to leave it unchanged.
If it is non-nil and not a function, references to undefined variables are
left unchanged.

Use `$$' to insert a single dollar sign.