Bash-Beginners-Guide

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:
Bash Guide for Beginners
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.
3.4.5. Command substitution
Command substitution allows the output of a command to replace the command itself. Command substitution
occurs when a command is enclosed like this:
$(command)
or like this using backticks:
`command`
Bash performs the expansion by executing COMMAND and replacing the command substitution with the
standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but
they may be removed during word splitting.
franky ~> echo `date`
Thu Feb 6 10:06:20 CET 2003
When the old-style backquoted form of substitution is used, backslash retains its literal meaning except when
followed by "$", "`", or "\". The first backticks not preceded by a backslash terminates the command
substitution. When using the "$(COMMAND)" form, all characters between the parentheses make up the
command; none are treated specially.
Command substitutions may be nested. To nest when using the backquoted form, escape the inner backticks
with backslashes.
Chapter 3. The Bash environment 48