Re: Testing for a value not sent to a shell script

Pete,

That code doesn't do what you think it does!!

#!/bin/bash
if [ $1 ] ; then
echo 'Yes!'
else
echo 'No!'
fi

What this is doing is trying to execute $1 as an expression for the "test" utility to evaluate. If test evaluates to true, it'll print yes, if not, it'll print no. So this isn't really testing to see if a parameter is passed! It's evaluating the parameter as an expression!

For example, try this:

$ ./myscript '1 -eq 0'
No!

See what I mean? I *did* pass it a parameter, but it says no. Why? Because it's running my parameter as an expression. Since 1 is not equal to 0, it says "No!"

If you just want to test if a parameter is empty, do this instead:

if [ -z "$1" ] ; then
echo 'No!'
else
echo 'Yes!'
fi

I would've given that answer at first if I had known that was what John was asking. But there are so many things he could've meant...