batch-file - `set VAR 2> nul` 后面的条件执行失败

Question

set command使用变量名(前缀)设置 exit code如果没有定义匹配的变量，则为非零值；退出码可以通过conditional execution operators查询:

set VAR && echo Yes. || echo No!!

如果定义了 VAR(或 VARiable 等)，上面的代码将返回 Yes.，而 No!! 否则，连同来自 set 命令的错误消息 Environment variable VAR not defined。

但是，当我试图抑制错误消息时，No!! 总是被回显，与是否定义了 VAR 无关:

set VAR 2> nul && echo Yes. || echo No!!

当我在命令提示符或批处理文件中执行命令行时，结果是一样的。

为什么插入重定向部分 (2> nul) 会改变运算符 && 和 || 收到的退出代码？

Answer 1

引用命令行:

set VAR 2> nul && echo Yes. || echo No!!

看来nul之间的SPACE和 &&被视为变量名的一部分，所以 set不检查名称以 VAR 开头的变量, 而不是 VAR + SPACE，显然没有定义。

---

我创建了一个包含大量测试的批处理文件，合并了有用的案例 comment按用户LotPings :

@echo on
@rem /* Execute test cases in a sub-routine twice,
@rem    once with variable `VAR` defined and once not: */
@for %%I in ("Value" "") do @(
    set "VAR=%%~I"
    echo/& < nul set /P ="VARIABLE: "
    if defined VAR (set VAR) else echo VAR=
    call :SUB
)
@goto :EOF

:SUB
@rem // This constitutes a list of test cases:
@echo/& echo UNQUOTED (VAR):
set VAR && echo Yes. || echo No!!
set VAR> nul && echo Yes. || echo No!!
set VAR > nul && echo Yes. || echo No!!
set VAR 2> nul && echo Yes. || echo No!!
(set VAR > nul) && echo Yes. || echo No!!
(set VAR 2> nul) && echo Yes. || echo No!!
set VAR > nul&& echo Yes. || echo No!!
set VAR 2> nul&& echo Yes. || echo No!!
> nul set VAR && echo Yes. || echo No!!
2> nul set VAR && echo Yes. || echo No!!
@echo/& echo QUOTED ("VAR"):
set "VAR" && echo Yes. || echo No!!
set "VAR"> nul && echo Yes. || echo No!!
set "VAR" > nul && echo Yes. || echo No!!
set "VAR" 2> nul && echo Yes. || echo No!!
(set "VAR" > nul) && echo Yes. || echo No!!
(set "VAR" 2> nul) && echo Yes. || echo No!!
set "VAR" > nul&& echo Yes. || echo No!!
set "VAR" 2> nul&& echo Yes. || echo No!!
> nul set "VAR" && echo Yes. || echo No!!
2> nul set "VAR" && echo Yes. || echo No!!
@goto :EOF

这是相关的控制台窗口输出:

>>> test-set.bat

VARIABLE: VAR=Value

UNQUOTED (VAR):

>>> set VAR   && echo Yes.   || echo No!!
VAR=Value
Yes.

>>> set VAR  1>nul  && echo Yes.   || echo No!!
Yes.

>>> set VAR   1>nul  && echo Yes.   || echo No!!
Environment variable VAR   not defined
No!!

>>> set VAR   2>nul  && echo Yes.   || echo No!!
No!!

>>> (set VAR  1>nul )  && echo Yes.   || echo No!!
Yes.

>>> (set VAR  2>nul )  && echo Yes.   || echo No!!
VAR=Value
Yes.

>>> set VAR  1>nul  && echo Yes.   || echo No!!
Yes.

>>> set VAR  2>nul  && echo Yes.   || echo No!!
VAR=Value
Yes.

>>> set VAR  1>nul  && echo Yes.   || echo No!!
Yes.

>>> set VAR  2>nul  && echo Yes.   || echo No!!
VAR=Value
Yes.

QUOTED ("VAR"):

>>> set "VAR"   && echo Yes.   || echo No!!
VAR=Value
Yes.

>>> set "VAR"  1>nul  && echo Yes.   || echo No!!
Yes.

>>> set "VAR"   1>nul  && echo Yes.   || echo No!!
Yes.

>>> set "VAR"   2>nul  && echo Yes.   || echo No!!
VAR=Value
Yes.

>>> (set "VAR"  1>nul )  && echo Yes.   || echo No!!
Yes.

>>> (set "VAR"  2>nul )  && echo Yes.   || echo No!!
VAR=Value
Yes.

>>> set "VAR"  1>nul  && echo Yes.   || echo No!!
Yes.

>>> set "VAR"  2>nul  && echo Yes.   || echo No!!
VAR=Value
Yes.

>>> set "VAR"  1>nul  && echo Yes.   || echo No!!
Yes.

>>> set "VAR"  2>nul  && echo Yes.   || echo No!!
VAR=Value
Yes.

VARIABLE: VAR=

UNQUOTED (VAR):

>>> set VAR   && echo Yes.   || echo No!!
Environment variable VAR  not defined
No!!

>>> set VAR  1>nul  && echo Yes.   || echo No!!
Environment variable VAR  not defined
No!!

>>> set VAR   1>nul  && echo Yes.   || echo No!!
Environment variable VAR   not defined
No!!

>>> set VAR   2>nul  && echo Yes.   || echo No!!
No!!

>>> (set VAR  1>nul )  && echo Yes.   || echo No!!
Environment variable VAR  not defined
No!!

>>> (set VAR  2>nul )  && echo Yes.   || echo No!!
No!!

>>> set VAR  1>nul  && echo Yes.   || echo No!!
Environment variable VAR  not defined
No!!

>>> set VAR  2>nul  && echo Yes.   || echo No!!
No!!

>>> set VAR  1>nul  && echo Yes.   || echo No!!
Environment variable VAR  not defined
No!!

>>> set VAR  2>nul  && echo Yes.   || echo No!!
No!!

QUOTED ("VAR"):

>>> set "VAR"   && echo Yes.   || echo No!!
Environment variable VAR not defined
No!!

>>> set "VAR"  1>nul  && echo Yes.   || echo No!!
Environment variable VAR not defined
No!!

>>> set "VAR"   1>nul  && echo Yes.   || echo No!!
Environment variable VAR not defined
No!!

>>> set "VAR"   2>nul  && echo Yes.   || echo No!!
No!!

>>> (set "VAR"  1>nul )  && echo Yes.   || echo No!!
Environment variable VAR not defined
No!!

>>> (set "VAR"  2>nul )  && echo Yes.   || echo No!!
No!!

>>> set "VAR"  1>nul  && echo Yes.   || echo No!!
Environment variable VAR not defined
No!!

>>> set "VAR"  2>nul  && echo Yes.   || echo No!!
No!!

>>> set "VAR"  1>nul  && echo Yes.   || echo No!!
Environment variable VAR not defined
No!!

>>> set "VAR"  2>nul  && echo Yes.   || echo No!!
No!!

唯一失败的情况是:

set VAR > nul && echo Yes. || echo No!!
set VAR 2> nul && echo Yes. || echo No!!

当命令解析器识别并暂时删除重定向部分(参见 How does the Windows Command Interpreter (CMD.EXE) parse scripts? 中第 2 阶段的描述)时，> 之前的潜在 SPACE和/或在 nul 之后被遗留下来，这似乎被 set 视为变量名称的一部分 | , 除非给定的变量名被引用；不过，总共只有一个 SPACE 似乎是可以容忍的。

另请查看错误消息 Environment variable VAR not defined变量名后面有不同数量的SPACE。

---

set命令似乎以特定方式处理其参数:

@set "VAR=Value"
rem // No trailing spaces -- returns `VAR=Value`:
set VAR
rem // One trailing space -- returns `VAR=Value`:
set VAR 
rem // Two or more trailing spaces -- returns an ERROR!
set VAR  
rem // No trailing spaces -- returns `VAR=Value`:
set "VAR"
rem // One trailing space -- returns `VAR=Value`!?
set "VAR "
rem // Two or more trailing spaces -- returns an ERROR!
set "VAR  "

不带引号的语法似乎不遵循标记化的标准规则，其中两个或多个连续的标记分隔符(如 SPACE)合并为一个。

令我大吃一惊的是，即使是引用的语法容差也是一个SPACE，尽管它忽略了结束" 之后的所有SPACE。至少。

更令人惊讶的是，在 SPACE 后面的一些其他文本(如 X)仍然没有返回任何错误:

@set "VAR=Value"
rem // No spaces behind `X`, one in front -- returns `VAR=Value`!?
set VAR X
rem // No spaces behind `X`, two in front -- returns an ERROR!
set VAR  X
rem // No spaces behind `X`, one in front -- returns `VAR=Value`!?
set "VAR X"
rem // No spaces behind `X`, two in front -- returns an ERROR!
set "VAR  X"