字符串判空有空格报错：binary operator expected

使用-z或-n对一个变量判空时，需要注意若直接使用[ -n ${ARG} ]这种形式，若${ARG}中有空格将会报错，

#!/bin/bash
ARG="sd dd"
if [ -n ${ARG} ]; then
    echo 'ARG:' ${ARG}
else
    echo 'ARG is empty.'
fi

输出：

line 27: [: sd: binary operator expected
ARG is empty.

解决方法，使用[[ -n ${ARG} ]]或[ -n "${ARG}" ]