如果没有空间,为什么等于运算符不起作用? [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
.虽然应该已经打印了Not equal
It gave me Equal
. Although it should have printed Not equal
仅当我在=
周围插入空格时,它才按预期工作
Only when I inserted space around =
it worked as intended
if [ "$var1" = "$var2" ]
并打印Not equal
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
是什么意思?它期望的一元运算符怎么来了?
What does it it mean? How come its expecting unary operator?
推荐答案
test
(或[ expr ]
)是内置函数.像bash中的所有函数一样,您将其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.
bash内置手册页指出: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."
这只是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.
变量分配是不同的.
在bash中,变量分配具有以下语法:name=[value]
.您不能在=
周围放置未加引号的空格,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.
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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