what does this shell script mean ${display:-:0}

[sean04]

authdisplay=${display:-:0}


what is the ":-:0" mean in the above line?

[Cabhan]

So this is actually pretty freaky, because this looks like one construct, when it's actually another. So first, what does it look like?

alex@danu ~ $ a=hooha; echo ${a:0:2}
ho
alex@danu ~ $ a=hooha; echo ${a:2:3}
oha

Which is to say, ${string:x:y} produces a substring of $string, beginning at index x (indexes are 0-based) of length y.

But this is particularly freaky. Because here, our starting index is '-', obviously not a number. This is because there is another construct:

alex@danu ~ $ a=hooha; echo ${a:-goodbye}
hooha
alex@danu ~ $ a=; echo ${a:-goodbye}
goodbye

A more formal definition: ${string:-word} produces $string if $string is set and non-empty, and word otherwise.

The second construct is being used here. So ${display:-:0} means the value of $display, if it is available, and ":0" otherwise. In this case, ":0" is the first X display, while the current X display is usually stored in $DISPLAY:

alex@danu ~ $ echo $DISPLAY
:0.0

So this is an attempt to ensure that $authdisplay always has a value.

Does this make sense?

[sean04]

Yes. Thank you. Excellent explanation.