Bash - String Manipulation Minus Sign (-) (hyphen) -

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• usage of :- (colon dash) in bash 2 answers

while looking @ apache run script /etc/init.d/apache2 realise different in string manipulating substring extraction. example:

if [ "${apache_confdir##/etc/apache2-}" != "${apache_confdir}" ] ; 

or

if [ -n "${pidtmp:-}" ] && kill -0 "${pidtmp:-}" 2> /dev/null; 

the second 1 seems use default values if pidtmp unset or null didn't find logical in one.

what use of minus sign (-) in each case? brief explanation helpful.

note:i interested in minus signs in braces , checked :-word .

it seems me has no real effect. :- supposed mean: "if not set, use following default value", since nothing follows, afaik has no effect:

• if variable set, default value not taken
• if variable unset, expand "", have happened anyway when using "${pidtmp}".

the use case can imagine make sure expansion contains @ least empty string , not "unset variable" (this makes difference when -u option in effect).

in bash man page:

${parameter:-word} use default values. if parameter unset or null, expansion of word substituted. otherwise, value of parameter substituted.

and

-u treat unset variables , parameters other special parameters "@" , "*" error when performing parameter expansion. if expansion attempted on unset variable or parameter, shell prints error message, and, if not interactive, exits non-zero status.