Before things get messy:
for a newbie it might be best to think of every shell command line
to expand into setting finally the integer-variable $? representing
the final value of the command. Therefore, a line like that written
below will expand to
$? = ( xxx == TRUE && yyy == TRUE )
and in a shell, the "&&" works like you used to from C as that it
is short-circuting - if the first execution (xxx == TRUE) returns
false, the second will never get executed. As homework, explain
the following line, and notice that the "test" command will only
receive arguments up to but _not_ including "||".
test -d "/tmp/my" || mkdir "/tmp/my"
and note that shell-assignments survive a "&&" as long as you do
not try to put them into a subshell with the innocent-looking
round paratheses, so that in the follwoing snippets the first
one has a different results on the terminal than the latter two:
prefix=NONE
test "x$prefix" = xNONE && prefix="/usr/local"
echo $prefix
- vs. -
prefix=NONE
(test "x$prefix" = xNONE && prefix="/usr/local")
echo $prefix
- vs. -
prefix=NONE
(test "x$prefix" = xNONE) && (prefix="/usr/local")
echo $prefix
Es schrieb Ionutz Borcoman:
>
> Evrika :-)
>
> Yes, you're right. But I've been for too long a C programmer (and never
> a shell one). I've thought '&&' was for test command. Something like in:
> if ( xxx == TRUE && yyy == TRUE ) {};
>
> So the C++ equivalent of this bash line:
> test xxx="xxx" && xxx="zzz"
> is
> if( xxx == "xxx" ) { xxx = "zzz"; }
>
> Right ?
>
> TIA,
>
> Ionutz
>
> Andreas Schwab wrote:
> >
> > ??? Of course, it is used, in a conditional.
