On Thu, 2005-04-07 at 19:56 -0500, Dan McGhee wrote:
> I also cannot mentally expand the expression
> $PATHVARIABLE="${!PATHVARIABLE:+:${!PATHVARIABLE}}"
Can't blame you - that's pretty ugly. I've seen (and written) worse
though, so let's break it down:
The exclamation marks are like a pointer - when you see the notation
!PATHVARIABLE, replace it with the value of $PATHVARIABLE. So if
$PATHVARIABLE was equal to the string "PATH", it would be as if the
following command was used instead (note the exclamation marks are
gone):
$PATHVARIABLE="${PATH:+:${PATH}}"
Of course, the left side of the assignment has a dollar sign next to it,
so that also gets expanded. So now, you have:
PATH="${PATH:+:${PATH}}"
That then becomes a simple case of the alternate value syntax, although
the presence of the second colon makes it look a bit more complicated.
Basically, if the variable named before the first colon is set, then it
replaces it with the value after the first colon, which would be the
string ":$PATH".
In short, it takes the variable named in $PATHVARIABLE, and if it's set
to something, prepends a colon.
Simon.
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