带前置字符的bash if语句[重复]

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,如果不将$variable用“$quotes”括起来,并且变量恰好为空,则不会将其视为空参数,而是将其视为没有争论(没有)

#!/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