bash if 문의 다른 동작

명령문 동작에 대해 몇 가지 테스트를 수행했지만 bash if해당 명령문이 출력되는 이유를 정확하게 이해하고 있는지 잘 모르겠습니다.

다음은 각각의 다른 명세서 출력에서 ​​제가 생각해낸 이유입니다 if. 모든 이유가 맞습니까? 나도 답을 못찾겠다행동 중 하나아래에 설명된 바와 같습니다. 배쉬 버전은 4.1.2입니다.

#!/bin/bash

set -x

# behavior 1
if [ $anything ]; then
        echo 'true 1'
fi
# result is false cause $anything will be translated by bash to an empty string, and in 'test' command, EXPRESSION omitted will be false

# behavior 2
if [ '' ];then
        echo 'true 2'
fi
# result is false cause in 'test' command, EXPRESSION omitted will be false

# behavior 3
if [ 0 ]; then
        echo 'true 3'
fi
# result is true cause 0 is a STRING and that is same with '-n STRING', since the length of string '0' is definitely nonzero, so it is true

# behavior 4
if [ ]; then
        echo 'true 4'
fi
# result is false cause in 'test' command, EXPRESSION omitted will be false

# behavior 1a
if $anything; then
        echo 'true 1a'
fi
# result is true. But I don't understand why since bash will translate $anything to an empty string, shouldn't this will result in syntax error ?

# behavior 2a
if ''; then
        echo 'true 2a'
fi
# syntax error cause 'list' is empty and command can not be found

# behavior 3a
if 0; then
        echo 'true 3a'
fi
# syntax error cause 'list' is 0 and there is no such command as 0

# behavior 4a
if ; then
        echo 'true 4a'
fi
# syntax error cause list is empty


set +x