这个 bash 命令 1>&2 中的和号表示什么

What does the ampersand indicate in this bash command 1>&2

提问by mikip

Quick one, 2>&1redirects stderr to stdout, but what does the ampersand mean? I know if we had 2 > 1it would output to a file named 1, what does the ampersand do?

快速一个，2>&1将 stderr 重定向到 stdout，但是 & 号是什么意思？我知道如果我们有2 > 1它会输出到一个名为 的文件1，与号有什么作用？

采纳答案by Ignacio Vazquez-Abrams

It copies file descriptor 1 to file descriptor 2. FD2 is stderr and FD1 is stdout, so it makes any output to stderr go to stdout instead.

它将文件描述符 1 复制到文件描述符 2。FD2 是标准错误，而 FD1 是标准输出，因此它使标准错误的任何输出改为标准输出。

回答by paxdiablo

2>&1redirects standard error (file handle 2) to the same file that standard output (file handle 1) is currentlygoing to.

2>&1将标准错误（文件句柄 2）重定向到标准输出（文件句柄 1）当前要去的同一个文件。

It's also a position-dependent thing so:

这也是一个依赖位置的事情，所以：

prog >x 2>&1 >y

will actually send standard error to xand standard output to yas follows:

实际上将标准错误发送到x标准输出，y如下所示：

• Connect standard output to x;
• Thenconnect standard error to same as current standard output, which is x;
• Thenconnect standard output to y;

• 将标准输出连接到x;
• 然后将标准错误连接到与当前标准输出相同的位置，即x;
• 然后将标准输出连接到y;

回答by Rose Perrone

The &in &1duplicates the file descriptor 1. The duplicated descriptor actually does not behave like a copy, but like an alias of the old one. Duplicating 1allows multiple streams to be redirected to 1without overwriting each other.

将&在&1复制的文件描述符1。重复的描述符实际上不像副本，而是旧描述符的别名。复制1允许将多个流重定向到1而不相互覆盖。

Example: (no &)

例子：（没有&）

$ ls existing-file non-existent-file > tmp 2> tmp 
$ cat tmp
existing-file
nt-file: No such file or directory

Note that 1overwrote what 2wrote. But not when we use &:

请注意，1覆盖了2所写的内容。但不是当我们使用&：

$ ls existing-file non-existent-file > tmp 2>&1 
$ cat tmp
ls: non-existent-file: No such file or directory
existing-file

A file descriptor is a handle to a file (or other input/output resource, such as a pipe or network socket). When 1and 2are separately redirected to tmp(as in the first example), they move their tmpfile pointer independently. That's why the file descriptors overwrote each other.

文件描述符是文件（或其他输入/输出资源，例如管道或网络套接字）的句柄。当1和2分别重定向到tmp（如第一个示例中）时，它们tmp独立地移动它们的文件指针。这就是文件描述符相互覆盖的原因。

According to the Linux man page:

根据Linux 手册页：

[Duplicate file descriptors] refer to the same open file description and thus share file offset and file status flags; for example, if the file offset is modified by using lseek(2) on one of the descriptors, the offset is also changed for the other.

[重复文件描述符] 引用相同的打开文件描述，从而共享文件偏移量和文件状态标志；例如，如果在其中一个描述符上使用 lseek(2) 修改了文件偏移量，则另一个描述符的偏移量也会更改。

Note that even though &acts like an alias, 2>&1means redirect 2to the stream that 1in currentlypointing to. When 1is redirected to something else, 2points to the same file it did independently of 1.

需要注意的是，即使&像一个别名行为，2>&1手段重定向2到该流1在目前指向。当1被重定向到其他东西时，2指向它独立于1.

Observe:

观察：

$ ls existing-file non-existent-file > tmp 2>&1 > tmp1
$ cat tmp1
existing-file
$ cat tmp
ls: non-existent-file: No such file or directory

回答by sleske

The ampersand belongs to the "1", so the snippet really has three parts: "2", ">", "&1". They mean, respectively, "take the data from output stream 2 (which is standard error)", "redirect it", and the redirection target, which is output stream 1. So "&" here allows you to redirect to an existing stream, rather than to a file.

＆符号属于“1”，因此该代码段实际上包含三个部分：“2”、“>”、“&1”。它们的意思分别是“从输出流 2（这是标准错误）中获取数据”、“重定向它”和重定向目标，即输出流 1。所以这里的“&”允许您重定向到现有流, 而不是文件。

回答by Douglas Leeder

From info bash:

来自info bash：

3.6.7 Duplicating File Descriptors
----------------------------------

The redirection operator
     [N]<&WORD
   is used to duplicate input file descriptors.  If WORD expands to one
or more digits, the file descriptor denoted by N is made to be a copy
of that file descriptor.  If the digits in WORD do not specify a file
descriptor open for input, a redirection error occurs.  If WORD
evaluates to `-', file descriptor N is closed.  If N is not specified,
the standard input (file descriptor 0) is used.

   The operator
     [N]>&WORD
   is used similarly to duplicate output file descriptors.  If N is not
specified, the standard output (file descriptor 1) is used.  If the
digits in WORD do not specify a file descriptor open for output, a
redirection error occurs.  As a special case, if N is omitted, and WORD
does not expand to one or more digits, the standard output and standard
error are redirected as described previously.

So 2>&1duplicates fd 1 onto fd 2.

所以2>&1将 fd 1 复制到 fd 2 上。

回答by Pete Kirkham

The ampersanddoesn't do anything - it's the character in the 2>&1operator rather than being a thing in its own right.

该符号并不做任何事情-它在字符2>&1操作，而不是在自己的权利的事情。

bash supports several redirection operators, the 2>&1operator or the &>operator tie together the streams of the process before or after redirection.

bash 支持多个重定向操作符，2>&1操作符或&>操作符将重定向前后的流程流联系在一起。