Re: QA needed for insecure LD_LIBRARY_PATH in many

网上看到老外回答的变量设置方式的区别，挺好，收藏，感谢作者！

> is unset. (Actually, some scripts use "${LD_LIBRARY_PATH+:
> $LD_LIBRARY_PATH}", which seems to work, too. But this is not 
> documented in the bash man page, at least I can't find it.)

The difference between ${PARAMETER:+WORD} and ${PARAMETER+WORD} is
subtle, and you're right, it's not documented in the bash man page.
It is part of the POSIX shell standard, though.  ${PARAMETER:+WORD}
substitutes WORD if PARAMETER is set and non-empty.  ${PARAMETER+WORD}
substitutes WORD if PARAMETER is set, empty or not.  For example:

vineet@sprocket:~$ FOO=
vineet@sprocket:~$ echo ${FOO+BAR}
BAR
vineet@sprocket:~$ echo ${FOO:+BAR}

vineet@sprocket:~$ unset FOO
vineet@sprocket:~$ echo ${FOO+BAR}

vineet@sprocket:~$ echo ${FOO:+BAR}

vineet@sprocket:~$ 

In many cases they'll be equivalent, but in the LD_LIBRARY_PATH case,
I'd recommend using the colon-form.  If someone has set an empty
LD_LIBRARY_PATH, the correct behavior is just to add the directory you
want; you don't want to stick an extra empty pathname component in
there.

good times,
Vineet