bash if ステートメントの他の動作

ステートメントの動作についていくつかのテストを行いましたが、bash ifそのステートメントが出力される理由を正確に理解しているかどうかはわかりません。

以下は、それぞれの異なる明細出力で私が考えた理由ですif。すべての理由は正しいですか？私も答えが見つかりません。行動の一つ以下に説明します。バッシュバージョンは4.1.2です。

#!/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