击：变量赋值命令替换的退出code [英] Bash: exit code of variable assignment to command substitution

问题描述

我感到困惑的是什么明明白白，用命令替换执行变量赋值时错误code中的命令将返回：

I am confused about what error code the command will return when executing a variable assignment plainly and with command substitution:

a=$(false); echo $?

它输出 1 ，这让我觉得变量赋值不扫或在最后一个产生新的错误code。但是，当我尝试这样做：

It outputs 1, which let me think that variable assignment doesn't sweep or produce new error code upon the last one. But when I tried this:

false; a=""; echo $?

它输出 0 ，显然这是什么 A =回报，它会覆盖 1 由归国假。

It outputs 0, obviously this is what a="" returns and it override 1 returned by false.

我想知道为什么会这样，有没有变量赋值，从其它正常的命令不同的特殊性？或只是事业 A = $（假）被认为是一个命令，只有命令替换一部分有意义吗？

I want to know why this happens, is there any particularity in variable assignment that differs from other normal commands? Or just be cause a=$(false) is considered to be a single command and only command substitution part make sense?

- 更新 -

谢谢大家，从答案和意见，我拿到了一点当你使用命令替换为变量赋值，退出状态是命令的状态。 （由@Barmar），这个解释是出色清晰和易于理解，但说话并不precise足够的程序员，我希望看到当局如TLDP或GNU手册页该点的参考，请大家帮忙我再次找到它，谢谢！

Thanks everyone, from the answers and comments I got the point "When you assign a variable using command substitution, the exit status is the status of the command." (by @Barmar), this explanation is excellently clear and easy to understand, but speak doesn't precise enough for programmers, I want to see the reference of this point from authorities such as TLDP or GNU man page, please help me find it out, thanks again!

推荐答案

当执行命令为 $（命令）允许的output更换自己。

当你说：

a=$(false)             # false fails; the output of false is stored in the variable a

由命令假生成的输出存储在变量 A 。此外，出口code是由命令产生的相同。 帮助虚假会告诉：

the output produced by the command false is stored in the variable a. Moreover, the exit code is the same as produced by the command. help false would tell:

false: false
    Return an unsuccessful result.

    Exit Status:
    Always fails.

在另一方面，他说：

$ false                # Exit code: 1
$ a=""                 # Exit code: 0
$ echo $?              # Prints 0

将导致出口code对分配给 A 要返回这是 0 。

编辑：

从手动：

如果在扩张中的一个包含一个命令替换，退出
  该命令的状态是最后一个命令的退出状态
  替代执行。

If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed.

BASHFAQ / 002 ：

我怎么能存储在一个返回值和/或命令的输出
  变量？

...

输出= $（命令）

状态= $？

分配到输出对命令的退出状态，不影响其
  仍然是 $？。

The assignment to output has no effect on command's exit status, which is still in $?.

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