Menu
why does:
#!/bin/bash
wtf=false
if [ $wtf ] || [ ! -f filethatexists.whatever ]
then
echo "WTF1"
fi
if [ ! -f filethatexists.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.
233
How to negate an if condition in a Bash if statement? (if not command or if not equal) To negate any condition, use the ! operator, for example: if ! <test-command>; then <command-on-failure>; fi .
Negation of "If A, then B". To negate a statement of the form "If A, then B" we should replace it with the statement "A and Not B". This might seem confusing at first, so let's take a look at a simple example to help understand why this is the right thing to do.
NegationWhen we use the not operator outside the [[, then it will execute the expression(s) inside [[ and negate the result. If the value of num equals 0, the expression returns true. But it's negated since we have used the not operator outside the double square brackets.
The origin of != is the C family of programming languages, in which the exclamation point generally means "not". In bash, a ! at the start of a command will invert the exit status of the command, turning nonzero values to zero and zeroes to one.
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:
[[ $wtf ]] which does know how to handle the variable expansion.-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.
160
If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
