Sams Teach Yourself Shell Programming in 24 Hours, Second Edition

Using Double Quotes

In many cases, you will need to quote some meta-characters but allow others to be evaluated by the shell. For example, the following echo command contains some meta- characters that must be quoted and others that should not:

$ echo '$USER owes <-$1250.**>; [ as of (`date +%m/%d`) ]'

Because the string is single quoted, the output is easy to predict—what you see is what you get:

$USER owes <-$1250.**>; [ as of (`date +%m/%d`) ]

As you can imagine, this is not exactly what you wanted; the single quotes have prevented variable substitution and command substitution from occurring, thus the variable $USER, which contains the username of the current user, was not replaced with the appropriate value and the date command was not ...