What does “plus colon” (“+:”) mean in shell script expressions?

Question

What does this mean?

if ${ac_cv_lib_lept_pixCreate+:} false; then :
  $as_echo_n \"(cached) \" >&6
else
  ac_check_lib_save_LIBS=$LIBS

Answer 1

To illustrate what has already been said:

Unset variable (note the blank lines as a result of some echo commands):

$ unset foo
$ echo ${foo}

$ echo ${foo:+:}

$ echo ${foo+:}

Null variable:

$ foo=""
$ echo ${foo}

$ echo ${foo:+:}

$ echo ${foo+:}
:

Non-null variable:

$ foo="bar"
$ echo ${foo}
bar
$ echo ${foo:+:}
:
$ echo ${foo+:}
: