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Parameter Expansion
     The format for parameter expansion is as follows:

	   ${expression}

     where expression consists of all characters until the matching `}'.  Any
     `}' escaped by a backslash or within a quoted string, and characters in
     embedded arithmetic expansions, command substitutions, and variable
     expansions, are not examined in determining the matching `}'.

     The simplest form for parameter expansion is:

	   ${parameter}

     The value, if any, of parameter is substituted.

     The parameter name or symbol can be enclosed in braces, which are
     optional except for positional parameters with more than one digit or
     when parameter is followed by a character that could be interpreted as
     part of the name.	If a parameter expansion occurs inside double-quotes:

     1.   Pathname expansion is not performed on the results of the expansion.

     2.   Field splitting is not performed on the results of the expansion,
	  with the exception of the special parameter @.

     In addition, a parameter expansion can be modified by using one of the
     following formats.

     ${parameter:-word}
	     Use Default Values.  If parameter is unset or null, the expansion
	     of word is substituted; otherwise, the value of parameter is sub-
	     stituted.

     ${parameter:=word}
	     Assign Default Values.  If parameter is unset or null, the expan-
	     sion of word is assigned to parameter.  In all cases, the final
	     value of parameter is substituted.  Only variables, not posi-
	     tional parameters or special parameters, can be assigned in this
	     way.

     ${parameter:?[word]}
	     Indicate Error if Null or Unset.  If parameter is unset or null,
	     the expansion of word (or a message indicating it is unset if
	     word is omitted) is written to standard error and the shell exits
	     with a nonzero exit status.  Otherwise, the value of parameter is
	     substituted.  An interactive shell need not exit.

     ${parameter:+word}
	     Use Alternate Value.  If parameter is unset or null, null is sub-
	     stituted; otherwise, the expansion of word is substituted.

     In the parameter expansions shown previously, use of the colon in the
     format results in a test for a parameter that is unset or null; omission
     of the colon results in a test for a parameter that is only unset.

     ${#parameter}
	     String Length.  The length in characters of the value of
	     parameter.

     The following four varieties of parameter expansion provide for substring
     processing.  In each case, pattern matching notation (see Shell
     Patterns), rather than regular expression notation, is used to evaluate
     the patterns.  If parameter is one of the special parameters * or @, the
     result of the expansion is unspecified.  Enclosing the full parameter
     expansion string in double-quotes does not cause the following four vari-
     eties of pattern characters to be quoted, whereas quoting characters
     within the braces has this effect.

     ${parameter%word}
	     Remove Smallest Suffix Pattern.  The word is expanded to produce
	     a pattern.  The parameter expansion then results in parameter,
	     with the smallest portion of the suffix matched by the pattern
	     deleted.

     ${parameter%%word}
	     Remove Largest Suffix Pattern.  The word is expanded to produce a
	     pattern.  The parameter expansion then results in parameter, with
	     the largest portion of the suffix matched by the pattern deleted.

     ${parameter#word}
	     Remove Smallest Prefix Pattern.  The word is expanded to produce
	     a pattern.  The parameter expansion then results in parameter,
	     with the smallest portion of the prefix matched by the pattern
	     deleted.

     ${parameter##word}
	     Remove Largest Prefix Pattern.  The word is expanded to produce a
	     pattern.  The parameter expansion then results in parameter, with
	     the largest portion of the prefix matched by the pattern deleted.