[$ var]是检查bash中变量设置/空的可接受方法吗？

Question

I wrote some bash before reading this popular question . I noticed the way I was doing things doesn't show up in any of the answers so I was curious if there is a danger/bug to my logic that I am not seeing.

if [ ! $1 ] || [ $2 ]; then
    echo "Usage: only one var." && exit 1
fi

As you can see, I am testing to see if there is one and only one parameter given on the command line. As far as I have tested, this works. Can someone pass along some more knowledge to a new bash user?

Answer 1

./script one '' oops

[ $2 ] will be false when $2 is the empty string.

And as that other guy points out in a comment, bash will split both strings, opening up a range of issues, including potential security holes.

Answer 2

This is clearer:

if [ $# != 1 ] ; then
    echo "Usage: only one var."; exit 1
fi

Answer 3

Things that will break your test:

• The first parameter is empty ( "" )
• The second parameter is empty ( "" )
• The first or second parameter has more words ( "bla bla" )
• The parameters contain something that will be interpreted by test (eg: -z )

It is simply less clearer than using $# . Also the mere fact that I have to think about all the corner-cases which could potentially break the code makes it inferior.