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up vote 3 down vote favorite
2
 
phone_missing=false
echo "missing $phone_missing"

if [ ! $phone_missing ]
then
        echo "Lost phone at $readabletime"
        $phone_missing=true
fi

I just can't wrap my head around this, as stupid it may sound. The line 

echo "missing $phone_missing"

echos missing false, I would very much expected the statement 

if [ ! $phone_missing ]

to be true and enter the if clause, but it doesn't? What am I missing here!?

share|improve this question
    
possible duplicate of Invert boolean variable –  casey Dec 19 '13 at 18:52

4 Answers 4

up vote 9 down vote accepted

$phone_missing is a string that happens to contain "false". And a non-empty string evaluates to true. See also http://www.linuxintro.org/wiki/Babe#empty_strings

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Ah alright. I would have declared that variable as boolean (at least not with the declare word) but the ash console won't let me. I also don't want to compare Strings. How is this done right? –  Ascorbin Dec 19 '13 at 18:57
1  
stackoverflow.com/questions/2953646/…, but take care: this guy EXECUTES a string variable, and as well false as true are unix commands. –  Thorsten Staerk Dec 19 '13 at 19:01
up vote 7 down vote

I often use "true" and "false" since they are also commands that merely return success and failure respectively. Then you can do

if "$phone_missing"; then ...
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@coffeMug, I appreciate the edit, but this wording expresses my thoughts better. –  glenn jackman Dec 19 '13 at 20:35
up vote 3 down vote

Here's one way to do this, while retaining the true/false values.

phone_missing=false
if [ "$phone_missing" != false ]; then
    echo "phone_missing is not 'false' (but may be non-true, too)"
fi
if [ "$phone_missing" == true ]; then
    echo "phone_missing is true."
fi

The double quotes around $phone_missing are to protect against the case where variable phone_missing is not defined at all. Another common idiom to ward against this is [ x$phone_missing != xfalse ], but the quotes seem more natural to me.

The hint is in the bash help page for test:

  STRING      True if string is not empty.
  ...
  ! EXPR      True if expr is false.

So, basically [ $foo ] will be true if $foo is non-empty. Not true or false, just non-empty. [ ! $foo ] is true if $foo is empty or undefined.

You could always change your code to just set phone_missing to a non-empty value, which will denote true. If phone_missing is unset (or empty — phone_missing=""), it will be false. Otherwise, you should be using the string testing operators (= and !=).

The other slight issue is the assignment. You have it as $phone_missing=true, whereas it should be phone_missing=true (no dollar sign).

Sorry if this is a bit dense, it's because I am. It's been a long day. :)

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up vote -1 down vote

The correct syntax is if ! $bool; then [statements]; fi.

Example:

bool=false

if ! $bool; then
    echo "This is correct!"
fi

if [ ! $bool ]; then
    echo "This is wrong!"
fi

Output: This is correct!

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There’s a kernel of truth in your answer, but, lacking any explanation, it causes more confusion than it remedies.  Also, it is little more than a repeat of one of the previous answers.  If you don’t want your answer to be removed, explain why it is right. –  G-Man May 24 at 6:17

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