[Shell] Variable Name Expansion

Variable operator	Action	Description
${varname}	Nonambiguous variable substitution	Simple variable substitution occurs with the value of varname being substituted
${varname:=value}	Assign a default value for the variable if a value does not exist.	If varname does not have a value or is set to null, then varname is set to value. Varname is then substituted in the statement.
${varname:+value}	Utilize value if varname is set	If the variable, varname, contains a value and is not null, then the alternate value, value, is substituted instead of the value of the variable varname. Otherwise nothing is substituted.
${varname:-value}	Assign a temporary default value for a variable if one does not exist	If the variable, varname, contains a value and is not null, then it is substituted; otherwise the value, value, is substituted but is not assigned to varname. (different from = operator)
${varname:?value}	Issue an error message if the value of variable is not set to any value.	If the variable, varname, containsa value and is not null, then it is substituted; otherwise an error message containing the value, value, is printed and the Shell exits.
${#varname} (Korn and Bash only)	Return the length of the value contains in varname or the number of positional parameters.	If varname is set, the length of the value of varname is returned. If the special variable * or @ is used as varname then the number of positional parameters are returned.
${varname#pattern} ${varname##pattern} (Korn and Bash only)	Substitue varname with pattern removed from the beginning of varname .	If the pattern matches the begining of varname, then pattern is removed from varname. If the # form is used, then the shortest match is removed. If the ## form is used, then the longest match is replaced.
${varname%pattern} ${varname%%pattern} (Korn and Bash only)	Substitute varname with pattern removed from the end of varname.	If the pattern matches the end of varname, then pattern is removed from varname. If the % form is used, then the shortest match is removed. If the %% form is used, then the longest match is replaced.
${#arrayname[*]} (Korn only)	Substitute the number of elements in the array.	The number of elements in the array arrayname is substituted.

<From : UNIX Shell Programming, FOURTH EDITION, LOWELL JAY ARTHUR, TED BURNS - WILEY COMPUTER PUBLISHING>