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Editorial Team
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Asked: 2026-05-17T00:13:59+00:00

why does: #!/bin/bash wtf=false if [ $wtf ] || [ ! -f filethatexists.whatever ]

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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.

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1 Answer

  1. Editorial Team

    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.

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