Command not found in Bash's IF-ELSE condition when using [! -d “$DIR”]

Question

I have a code like this

#!/bin/bash 
DIR=\"test_dir/\";
if [! -d \"$DIR\"]; then
    # If it doesn\'t create it
    mkdir $DIR
fi

But why

Answer 1

Add space between [ and !. And before ] as well.

#!/bin/bash 
DIR="test_dir/";
if [ ! -d "$DIR" ]; then
    # If it doesn't create it
    mkdir $DIR
fi

It's also a good idea to quote your variable:

    mkdir "$DIR"