Hi all,
Alina Friedrichsen napsal(a):
Hi!
The correct syntax is [ -n "${var}" ] (notice the quotes).
Thanks to Vasilis.
A security notice:
It can become a security issue when any function is wrongly used in a security
context.
I would mark this one only as a general warning.
Don't use more then one expression in one test call.
You can freely use test concatenated by logical operators if you know how the
used shell handles them and prevent the wrong behavior by protecting variables.
It is just not portable.
Use instant two test calls:
if [ -z "$x" ] && [ -z "$y" ]; then
echo x and y are empty
fi
For example:
if [ "x$x" = x -a "x$y" = x ] ; then
or
if [[ -z "$x" && -z "$y" ]] ; then ## in bash
would do the same thing but either way is not portable (-a and [[).
if [ -z "$x" ] || [ -z "$y" ]; then
echo x or y is empty
fi
If more then one expression is done with test the comparison is exploitable.
It's a design error of the UNIX shell and can't be fixed.
I would not dare to say the one "UNIX shell". There are many shells. And every
one of them has its own issues or quirks or exceptions.
Busybox (the original complaint was targeted to it) claims its POSIX compliance
and it should handle -z/-n tests properly but the result is the same like in
bash. The good way to handle it safely is to use portable expressions.
Some of them are well documented differences, you can look to the Autoconf
manual. It has had to handle them to become a really multiplatform tool.
See for example:
http://www.gnu.org/software/autoconf/manual/html_node/Limitations-of-Builtins.html#index-g_t_0040command_007btest_007d-1431
Regards
Alina
Best regards,
Lubos
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