如果等于运算符没有被空格包围，为什么它不起作用? [英] Why equal to operator does not work if it is not surrounded by space?

问题描述

我尝试了以下脚本

#!/bin/bash
var1="Test 1" 
var2="Test 2"
if [ "$var1"="$var2" ] 
  then 
    echo "Equal" 
  else 
    echo "Not equal"
fi

它给了我Equal.虽然它应该打印Not equal

It gave me Equal. Although it should have printed Not equal

仅当我在 = 周围插入空格时，它才能按预期工作

Only when I inserted space around = it worked as intended

if [ "$var1" = "$var2" ] 

并打印不等于

为什么会这样?为什么 "$var1"="$var2" 与 "$var1" = "$var2" 不同?

Why is it so? Why "$var1"="$var2" is not same as "$var1" = "$var2"?

此外，当我写if [ "$var1"="$var2" ]时，它给了

Moreover, when I wrote if [ "$var1"= "$var2" ], it gave

line 4: [: Test 1=: unary operator expected

这是什么意思?它期待的一元运算符是怎么来的?

What does it it mean? How come its expecting unary operator?

推荐答案

test (or [ expr ]) 是一个内置函数.像 bash 中的所有函数一样，你将它的参数作为空格分隔的单词传递.

test (or [ expr ]) is a builtin function. Like all functions in bash, you pass it's arguments as whitespace separated words.

正如 bash 内置程序的手册页所述:每个运算符和操作数必须是一个单独的参数."

As the man page for bash builtins states: "Each operator and operand must be a separate argument."

这就是 bash 和大多数其他 Unix shell 的工作方式.

It's just the way bash and most other Unix shells work.

变量赋值不同.

在 bash 中，变量赋值的语法为:name=[value].您不能在 = 周围放置不带引号的空格，因为 bash 不会将其解释为您想要的分配.bash 将大多数单词列表视为带参数的命令.

In bash a variable assignment has the syntax: name=[value]. You cannot put unquoted spaces around the = because bash would not interpret this as the assignment you intend. bash treats most lists of words as a command with parameters.

例如

# call the command or function 'abc' with '=def' as argument
abc =def

# call 'def' with the variable 'abc' set to the empty string
abc= def

# call 'ghi' with 'abc' set to 'def'
abc=def ghi

# set 'abc' to 'def ghi'
abc="def ghi"

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