bash 如果等于运算符没有被空格包围，为什么它不起作用？
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Why equal to operator does not work if it is not surrounded by space?
提问by Andrew-Dufresne
I tried the following script
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
It gave me Equal. Although it should have printed Not equal
它给了我Equal。虽然它应该打印Not equal
Only when I inserted space around =it worked as intended
只有当我在=它周围插入空间时才按预期工作
if [ "$var1" = "$var2" ]
and printed Not equal
并打印 Not equal
Why is it so? Why "$var1"="$var2"is not same as "$var1" = "$var2"?
为什么会这样？为什么"$var1"="$var2"不一样"$var1" = "$var2"？
Moreover, when I wrote if [ "$var1"= "$var2" ], it gave
此外，当我写的时候if [ "$var1"= "$var2" ]，它给了
line 4: [: Test 1=: unary operator expected
What does it it mean? How come its expecting unary operator?
这是什么意思？它期待的一元运算符是怎么来的？
回答by CB Bailey
test(or [ expr ]) is a builtin function. Like all functions in bash, you pass it's arguments as whitespace separated words.
test(或[ expr ]) 是一个内置函数。像 bash 中的所有函数一样，您将它的参数作为空格分隔的单词传递。
As the man page for bash builtins states: "Each operator and operand must be a separate argument."
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。”
It's just the way bash and most other Unix shells work.
这就是 bash 和大多数其他 Unix shell 的工作方式。
Variable assignment is different.
变量赋值不同。
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.
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有参数的命令。
E.g.
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"
回答by William Pursell
When the shell reads
当外壳读取
if [ "$var1" = "$var2" ]
it invokes the command [ with 4 arguments. Whether [ is a builtin or an external command is irrelevant, but it may help to understand that it may be the external command /bin/[. The second argument is the literal '=' and the fourth is ']'. However, when the shell reads
它调用带有 4 个参数的命令 [。[ 是内置命令还是外部命令无关紧要，但理解它可能是外部命令 /bin/[ 可能会有所帮助。第二个参数是文字“=”，第四个参数是“]”。但是，当 shell 读取
if [ "$var1"= "$var2" ]
[ only gets 3 arguments: the expansion of $var1 with '=' appended, the expansion of $var2, and ']'. When it gets only 3 arguments, it expects the last argument to be ']' and the first argument to be a unary operator.
[ 只得到 3 个参数：附加了 '=' 的 $var1 的扩展、$var2 的扩展和 ']'。当它只得到 3 个参数时，它期望最后一个参数是 ']'，第一个参数是一元运算符。
回答by Jahid
To add to the existing explanation, "$var1"="$var2"is just a single non-empty string, and thus always evaluates as true in a conditional.
添加到现有解释中，"$var1"="$var2"只是一个非空字符串，因此在条件中始终评估为真。
[ "$var1"="$var2" ] && echo true
The above command will always print out true(even if var1and var2be empty).
上面的命令将始终打印出来true（即使var1并且var2为空）。
回答by knesenko
In bash the best is to use [[ ]]:
在 bash 中最好使用 [[ ]]：
x="test"
y="test"
if [[ "${x}" = "${y}" ]]; then
echo "Equals"
else
echo "No equals"
fi
