方括号中的壳间距 [英] Shell spacing in square brackets

问题描述

作为初学者，我没有在任何地方找到答案，关于间距(语法)和解析的规则.

As a beginner, I did not find answered anywhere, the rules about spacing (grammar) and parsing.

例如.

我可以吗

if [$# -eq 2] ; 
then 
    llll 
fi

或者我必须在对象之间总是有一两个空白，如

or must I always have a blank or two between the objects, as

if [ $# -eq 2 ] ;
then
   llll
fi

和第二个相关问题是关于

and the second related question is about the difference between

if [[ $# -eq 2 ]] ;
then
wafwaf
fi

我担心的是[, ] 之前/之后的间距.

The concern I have is about spacing before/after [, ].

没有搜索为我提供了一套规则.

No searching has provided me with a set of rules.

推荐答案

[ 之后和 ] 之前需要空格.

Spaces are required after [ and before ].

1. [ 实际上是一个命令的名称，test 的别名.这不是一个特殊的符号，它只是一个名字不寻常的命令.

1. [ is actually the name of a command, an alias for test. It's not a special symbol, it's just a command with an unusual name.

$ help '['
[: [ arg... ]
    Evaluate conditional expression.

    This is a synonym for the "test" builtin, but the last argument must
    be a literal `]', to match the opening `['.

因为是普通命令名，不是特殊字符，所以[后面需要一个空格.如果省略空格并写入 [foo，shell 将在 $PATH 中搜索名为 [foo 的命令.

Because it's an ordinary command name and not a special character, a space is required after the [. If you omit the space and write [foo the shell will search the $PATH for a command named [foo.

$ [ foo = foo ] && echo true
true
$ [foo = foo] && echo true
[foo: command not found

为了可读性，[ 期望它的最后一个参数正好是 ].作为一个普通的命令行参数，] 前必须有一个空格.如果没有空格，那么括号将成为前一个参数的最后一个字符，并且 [ 会抱怨它的最后一个参数不是 ].

For readability's sake, [ expects its last argument to be exactly ]. Being an ordinary command-line argument, ] must have a space before it. If there's no space then the bracket will become the last character of the previous argument, and [ will complain about its last argument not being ].

$ [ foo = foo]
bash: [: missing `]'
$ [ foo = 'foo]'
bash: [: missing `]'

[[ 是一个 bash 增强功能，比 [ 有更多的特性，即对不带引号的变量名的处理更加合理.它需要两端有一个空格，与 [ 相同.然而 [[ 实际上是特殊的 shell 语法并且解析方式不同.它不是 [ 那样的普通命令".

[[ is a bash enhancement with more features than [, namely saner handling of unquoted variable names. It requires the a space on both ends, same as [. However [[ is in fact special shell syntax and is parsed differently. It's not an "ordinary command" the way [ is.

[ 和 [[ 区别的详细说明见:

For a detailed explanation of the difference between [ and [[, see:

• [ 和 [[ 之间有什么区别在 Bash 中?

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