Shell parameter and Variable Expansion

The "$" character introduces parameter expansion, command substitution, or arithmetic expansion. The parameter name or symbol to be expanded may be enclosed in braces, which are optional but serve to protect the variable to be expanded from characters immediately following it which could be interpreted as part of the name.

When braces are used, the matching ending brace is the first "}" not escaped by a backslash or within a quoted string, and not within an embedded arithmetic expansion, command substitution, or parameter expansion.

The basic form of parameter expansion is "${PARAMETER}". The value of "PARAMETER" is substituted. The braces are required when "PARAMETER" is a positional parameter with more than one digit, or when "PARAMETER" is followed by a character that is not to be interpreted as part of its name.

If the first character of "PARAMETER" is an exclamation point, Bash uses the value of the variable formed from the rest of "PARAMETER" as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of "PARAMETER" itself. This is known as indirect expansion.

You are certainly familiar with straight parameter expansion, since it happens all the time, even in the simplest of cases, such as the one above or the following:

franky ~> echo $SHELL
/bin/bash

The following is an example of indirect expansion:

franky ~> echo ${!N*}
NNTPPORT NNTPSERVER NPX_PLUGIN_PATH

Note that this is not the same as echo $N*.

The following construct allows for creation of the named variable if it does not yet exist:

${VAR:=value}

Example:

franky ~> echo $FRANKY

franky ~> echo ${FRANKY:=Franky}
Franky

Special parameters, among others the positional parameters, may not be assigned this way, however.

We will further discuss the use of the curly braces for treatment of variables in Chapter 10. More information can also be found in the Bash info pages.