A $N construct in a shell script can be enclosed in double or single quotation marks.

• When double quotation marks are used, the parameter is replaced by the appropriate value from the command line. For example, suppose that the file search contains:

  grep "$1" *

  If you enter the command:

  search 'two words'

  the parameter value 'two words' replaces the construct $1 in the grep command:

  grep "two words" *

  If the grep command does not contain the double quotation marks, the parameter replacement would result in:

  grep two words *

  which has an entirely different meaning.
• When you use single quotation marks to enclose a $N construct in a shell script, the $N is not replaced by the corresponding parameter value. For example, if the file search contains:

  grep '$1' *

  grep searches for the string $1. The $1 is not replaced by a value from the command line. In general, single quotation marks are “stronger” than double quotation marks.