bash if with or and negation

why does:

#!/bin/bash
wtf=false
if [ $wtf ] || [ ! -f filethatexists.whatever ]
then
 echo "WTF1"
fi
if [ ! -f filethate开发者_如何学运维xists.whatever ]
then
 echo "WTF2"
fi

print:

WTF1

instead of nothing? It is especially perplexing that the second form works as expected and the first not.

The basic test

[ $wtf ]

tests whether the string in the middle is empty or not.

Since $wtf contains the string 'false', the test returns true, or exit status 0 for success, because 'false' is not the same as the empty string '' — and hence you get WTF1 as the response.

Try with:

wtf=''

As pointed out by Gordon Davisson (and Dennis Williamson), it is a good idea to be careful with strings that you are testing. Indeed, I should have stated that I would always use [ -n "$wtf" ] or [ -z "$wtf" ] to test whether a variable is set, because that was necessary when I was learning shell, once upon a quarter century ago. I've had counter stories from Bash afficionados that you don't have to worry about it in Bash - however, I think the code here provides a counter-example that in fact you do still have to worry about it.

So, some best practices:

• Enclose tested variables in double quotes, or
• (In Bash), use [[ $wtf ]] which does know how to handle the variable expansion.
• Use the -n or -z tests to test for non-empty or empty values.

There can be exceptions to the rules - but you will not go far wrong following them.

Consider the code:

wtf="1 -eq 0"
 [  $wtf  ]  && echo "WTF0"
[[  $wtf  ]] && echo "WTF1"
wtf="false"
 [  $wtf  ]  && echo "WTF2"
[[  $wtf  ]] && echo "WTF3"
wtf=""
 [  $wtf  ]  && echo "WTF4"
[[  $wtf  ]] && echo "WTF5"
wtf="false"
 [ "$wtf" ]  && echo "WTF6"
[[ "$wtf" ]] && echo "WTF7"
wtf=""
 [ "$wtf" ]  && echo "WTF8"
[[ "$wtf" ]] && echo "WTF9"

That produces:

WTF1
WTF2
WTF3
WTF6
WTF7

with both bash and ksh (as found on MacOS X 10.6.4, when run with 'bash testcode.sh' or 'ksh testcode.sh'). A real Bourne shell (if you can still find such a thing) would object to the double-bracket operations - it would not be able to find the command '[[' on $PATH.

You can extend the testing to cover more cases ad nauseam.

Here's a handy little trick:

wtf=false
if $wtf || [ ! -f filethatexists.whatever ]

In this form, the contents of the variable are executed and the return value determines whether the test passes or fails. It happens that true and false are Bash builtins that return the appropriate value.

if [ $wtf = true ] || [ ! -f . .