I tried the code in this SO post - How to evaluate a boolean variable in an if block in bash? and it does not work. It looks like there are no booleans in BASH. Is there a flawless workaround which lets me set and check booleans in BASH ?
My code -
#!/bin/bash
flag=true
if [ $flag ]
echo 'True'
#flag=false
#echo 'Now changed to false'
fi
Even if flag=false in line 2, output is still True. Why ?
Try without square brackets, like this
#!/bin/bash
flag=true
if $flag ; then
echo 'True'
#flag=false
#echo 'Now changed to false'
fi
true command returns true, and the false command returns false. You can also set flag="rm -r /*", and have the condition delete all your files.It evaluates to true because of this part of 'man test' -
[ expression ]
string True if string is not the null string.
You need to use something like -
#!/bin/bash
flag=1
if [ ${flag} -eq 1 ]
then
echo 'True'
#flag=0
#echo 'Now changed to false'
fi
${flag} and $flag are the same. You shouldn't use double quotes and == here, though, because you want to do a numeric comparison, not a string comparison.
Setting
To set a bash variable $subject to boolean true or false, use subject=true or subject=false.
To set a vash variable to null, use subject= or subject=$() or unset subject
Testing
The use of single brackets and no brackets is suspicious and I've found several discrepancies in the expected behavior around how empty strings, null values, and integer 0 and 1 are evaluated.
First, defining the correct behavior of a strict boolean test as follows:
if test for boolean true evaluates to true only when the subject is boolean true. Such a test will evaluate to false for any other value, including 1 and any string with value 'true'if test for false evaluates to true only when the subject is boolean false. Such a test will evaluate to false for any other value, including 0, null, and an empty string ''The correct method to accomplish this is to use double square brackets and explicitly test against true or false as follows:
if [[ ${subject} = true ]]; then { ... } fi # only if boolean true
if [[ ${subject} = false ]]; then { ... } fi # only if boolean false
Note that if you use such a test for boolean true (or false), an else block will not only cover the opposite condition, but also the entire set of possible values that are not boolean false (or true) respectively.
