英文:
failed to avoid “[: too many arguments”
问题
I know that MyVar="a b" ; [ $MyVar = "a b" ] will signal an error because the resultant expansion is [ a b = "a b" ].
I tried to avoid that case:
$ MyVar="\"a b\""
$ [ $MyVar = "a b" ]
-bash: [: too many arguments
Why did I fail? Didn't it expand to [ "a b" = "a b" ]?
I know there is a [[ ... ]] form; I know I can write "$MyVar".
I am asking why is my approach ineffective?
英文:
I know that MyVar="a b" ; [ $MyVar = "a b" ] will signal an error because the resultant expansion is [ a b = "a b" ].
I tried to avoid that case:
$ MyVar="\"a b\""
$ [ $MyVar = "a b" ]
-bash: [: too many arguments
Why did I fail? Didn't it expand to [ "a b" = "a b" ]?
I know there is a [[ ... ]] form; I know I can write "$MyVar".
I am asking why is my approach ineffective.
答案1
得分: 1
Sure, here's the translated content:
这是因为您遇到了Shell的单词分割：您忘记了对变量进行"引用"。
学习如何在Shell中正确引用非常重要：
对包含空格/元字符和每次扩展的每个字面量都要使用"双引号"：
"$var"，"$(command "$var")"，"${array[@]}"，"a & b"。对于代码或字面量$'s，请使用'单引号'：'Costs $5 US'，ssh host 'echo "$HOSTNAME"'。请查看以下链接以获取更多信息：http://mywiki.wooledge.org/Quotes，http://mywiki.wooledge.org/Arguments，http://wiki.bash-hackers.org/syntax/words，何时需要双引号
在bash中，最好使用这种形式，无需引用：
MyVar="a b" ; [[ $MyVar == "a b" ]]
[[是一个类似于（但更强大的）[命令的bash关键字。请参阅：http://mywiki.wooledge.org/BashFAQ/031和http://mywiki.wooledge.org/BashGuide/TestsAndConditionals。除非您正在编写POSIX sh代码，否则我建议使用[[。
如果您需要编写POSIX代码，您需要使用引号：
MyVar="a b" ; [ "$MyVar" = "a b" ] && echo ok
请注意，对于POSIX shell，必须使用单个=运算符。
英文:
It's because you encounter shell word splitting: you missed to "quote" the variable.
Learn how to quote properly in shell, it's very important :
> "Double quote" every literal that contains spaces/metacharacters and every expansion: "$var", "$(command "$var")", "${array[@]}", "a & b". Use 'single quotes' for code or literal $'s: 'Costs $5 US', ssh host 'echo "$HOSTNAME"'. See
<http://mywiki.wooledge.org/Quotes>
<http://mywiki.wooledge.org/Arguments>
<http://wiki.bash-hackers.org/syntax/words>
when-is-double-quoting-necessary
Better use this form in bash, no need to quote:
MyVar="a b" ; [[ $MyVar == "a b" ]]
[[ is a bash keyword similar to (but more powerful than) the [ command. See <http://mywiki.wooledge.org/BashFAQ/031> and <http://mywiki.wooledge.org/BashGuide/TestsAndConditionals>. Unless you're writing for POSIX sh, I recommend [[
If you need to write POSIX code, you need quotes:
MyVar="a b" ; [ "$MyVar" = "a b" ] && echo ok
Note the use of single = operator mandatory for POSIX shell.
答案2
得分: 1
- 为什么我的方法无效。
这是因为不加引号的 $MyVar 会分成
"a 和 b"（两个字符串），而 == 运算符只在左侧有一个参数和右侧有一个参数时起作用。
请注意，参数扩展是一次性的。它将变量 $MyVar 扩展为 "a 和 b"，然后不会重新评估结果。扩展发生的顺序列在Shell扩展中：
- 扩展的顺序是：大括号扩展；波浪号扩展，参数和变量扩展，算术扩展和命令替换（从左到右进行）；单词分割；和文件名扩展。
英文:
> why is my approach ineffective.
That's because $MyVar without quotes will be split into
"a and b" (two strings) and the == operator only works with one argument on the left hand side and one argument on the right hand side.
Note that parameter expansion is a one-pass thing. It expands the variable $MyVar into "a and b" and does then not reevaluate the result. The order in which the expansions occur is listed in Shell expansion:
> The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and filename expansion.
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