带有前置字符的 Bash if 语句 [重复]

Question

我正在尝试理解一个script，它会在以 root 身份执行时停止：

#!/usr/bin/env bash

if [ x"$(whoami)" = x"root" ]; then
    echo "Error: don't run this script as root"
    exit 1
fi

我已经对此进行了测试，即使我删除了 if 语句中的 x，它也能按预期工作。我的问题是为什么需要x"$(whoami)" 和x"root" 中的x？

Answer 1

基本上 [ 是指向名为 test 的外部程序的软链接，因此条件作为程序参数传递给它，并且如果您不使用 "$quotes" 包围 $variable 并且变量恰好是为空它不会被视为空参数，它将被视为无参数（无）

#!/bin/bash -eu

var=bla

if [[ $var == bla ]];then
  echo first test ok
fi

var=""

if [[ $var == "" ]];then
  echo second test ok
fi

if [ "$var" == "" ];then
  echo third test ok
fi

if [ x$var == "x" ];then
  echo fourth test ok
fi

echo this will fail:

if [ $var == "" ];then
  echo fifth test ok
fi

echo because it is the same as writing:

if [ == "" ];then
  echo sixth test is obviously eroneous
fi

echo but also you should quote your variables because this will work:

var="a b"

if [ "$var" == "a b" ];then
  echo seventh test ok
fi

echo ... but this one won\'t as test now has four arguments:
if [ $var == "a b" ];then
  echo eighth test ok
fi