bash script: if condition does not work (is never true) [duplicate]

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• How to set a variable to the output from a command in Bash? 13 answers

Like I mentioned in the title the if condition does not work because is never verified and I am sure the output of the tail command contains the substring I'm looking for.

tail="tail /file.txt"
eval $tail
output=$(eval $x)
echo $output

var1="public";

if [[ "$output" =~ "$var1" ]] 
    then 
        echo Contains the string.
    else
        echo Does not contain the string.
fi

I also tried like this but still does not work.

if [[ $output == *"public"* ]]