在 bash 中 & > 做什么？

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&>word (and >&word redirects both stdout and stderr to the result of the expansion of word. In the cases above that is the file 1.

2>&1 redirects stderr (fd 2) to the current value of stdout (fd 1). (Doing this before redirecting stdout later in the line does not do what you might expect and will split the outputs instead of keeping them combined and is a very common shell scripting error. Contrast this to >word 2>&1 which combines the two fds into one sending to the same location.)

$ { echo stdout; echo stderr >&2; }
stdout
stderr
$ { echo stdout; echo stderr >&2; } >/dev/null
stderr
$ { echo stdout; echo stderr >&2; } >/dev/null 2>&1
$
{ echo stdout; echo stderr >&2; } 2>&1 >/dev/null
stderr

Not that those are, while similar looking, not the same thing.

git status 2&>1 > /dev/null is, in fact, actually running git status 2 with a redirection of &>1 (stdout and stderr to file 1). Almost certainly not what was intended. Your correction almost certainly is what was intended.

$ git init repro
Initialized empty Git repository in /tmp/repro/.git/
$ cd repro/
$ git status
# On branch master
#
# Initial commit
#
nothing to commit
$ ls
$ git status 2>&1
# On branch master
#
# Initial commit
#
nothing to commit
$ ls
$ git status 2&>1
$ ls
1
$ cat 1
# On branch master
#
# Initial commit
#
nothing to commit

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最佳答案

The operators we are using here are:

> Syntax: file_descriptor_opt > file_name
>& Syntax: file_descriptor_opt >& file_descriptor
&> Syntax: &> file_name

If the file descriptor is omitted, the default is 0 (stdin) for input, or 1 (stdout) for output. 2 means stderr.

So we have:

>name means 1>name -- redirect stdout to the file name
&>name is like 1>name 2>name -- redirect stdout and stderr to the file name (however name is only opened once; if you actually wrote 1>name 2>name it'd try to open name twice and perhaps malfunction).

So when you write git status 2&>1, it is therefore like git status 2 1>1 2>1 , i.e.

the first 2 actually gets passed as an argument to git status.
stdout is redirected to the file named 1 (not the file descriptor 1)
stderr is redirected to the file named 1

This command should actually create a file called 1 with the contents being the result of git status 2 -- i.e. the status of the file called 2 which is probably "Your branch is upto-date, nothing to commit, working directory clean", presuming you do not actually track a file called 2.

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All about redirection in bash

What does &> do in bash?

Regarding the erroneous git status 2&>1 > /dev/null...

Can someone clarify the man page and explain exactly what is happening with this syntax?

Yes, I can. I'm also trying to understand a variety of behaviors and bits of bash syntax too, so let's go through the statement in question, and some examples in order to gain a better understanding together.

Quick Summary

If you just want to understand the full meaning of the odd/erroneous syntax in question: git status 2&>1 > /dev/null, jump straight to section 1 below.
If you're here just based on the title of the question itself: "What does &> do in bash?", jump straight down to section 2 below.
If you want to have a quick-reference summary of proper redirection usage in bash, such as redirecting stderr to stdout via 2>&1, or redirecting stdout to a file via >file or 1>file, or redirecting stderr to a file via 2>file, or redirecting both stdout and stderr to a file via &>file or >file 2>&1, look just at section 3 below: "3. Summary of proper redirection in bash".
If you just want to test your bash knowledge on weird, redundant, overriding, or otherwise non-standard usages of redirection, look at section 4 below.

1. Breakdown of the statement `git status 2&>1 > /dev/null`

In the statement

git status 2&>1 > /dev/null

there are 3 separate parts that break out like this (even though this is not intuitive nor obvious, especially considering that there is no space between the 2 and &>1 in 2&>1):

git status 2    # statement 1
&>1             # statement 2
> /dev/null     # statement 3

If you had the following, however, they are all very different than the above. We'll talk about these later in this answer:

# separates into **two separate statements**: `2` and `&>1`
2&>1  # `2` is NOT part of `&>1`


# these are all **single statements**, but mean very different things (more on
# these below)
2>&1
2>1

Statement 1 (git status 2) runs git status 2. Here, 2 is a parameter passed to git status. Since it's not a valid option, git status assumes it to be a path, which can be a file or directory. So, git returns the status of all files named 2, or of all files within a directory named 2. As man git status shows, a better way to specify paths is to separate options from paths with --, like this:
```
git status -- 2  # 2 is a file or folder here
```
Obviously, that's not what you're trying to do. :)
Statement 2 (&>1) redirects stdout and stderr to a file named 1. This is also obviously not what you are trying to do. :) The &>file syntax redirects all stdout and stderr to file. You can read about this in the bash manual online here, or by running man bash and and searching for the section named "Redirecting Standard Output and Standard Error". We'll go over this section of the manual more later.

Note that &>1 (written as 2&>1--two separate statements: 2 and &>1, in your question) is not the same syntax nor concept as 2>&1. The latter, 2>&1, redirects file descriptor 2 (stderr) to file descriptor 1 (stdout), where the 3 primary file descriptors are:
1. 0 = stdin
2. 1 = stdout
3. 2 = stderr
Statement 3 (> /dev/null): this is where it starts to get tricky. This can also be written without the space, as >/dev/null. It redirects stdout (file descriptor 1) to the /dev/null Linux pseudofile, which discards any output written to it. Writing >/dev/null is exactly identical to 1>/dev/null, as file descriptor 1 (stdout) is implied as the default option if not specified, as the bash manual states. Read more about it in the "Redirecting Output" section of the bash manual here.

It starts to get tricky here for two reasons:
1. First, because the 1 means two different things: in &>1 the one is a file named "1". This is likely a mistake on the user's part, but that's what it is. In >/dev/null there is an implied 1 as in 1>/dev/null, and that refers to file descriptor 1, which is stdout.
2. Second, it is tricky because you have redirection overrides going on, where the last one sticks. &>1 redirects both stdout and stderr to a file named "1", but 1>/dev/null then redirects stdout to the "/dev/null" file. The latter redirection of stdout overrides the former, so the end result is that stderr is redirected to a file named "1" and stdout is redirected to the "/dev/null" file. This could have also been written as:
```
# redirect stderr to a file named "1", and redirect stdout to the file
# named "/dev/null" to discard it.
2>1 1>/dev/null


# same thing
&>1 > /dev/null
```

The above information is begging for more explanation, so let's explain &> more and then give a detailed summary of how to correctly do redirection in bash.

2. Just addressing the title of this question: "What does `&>` do in bash?"

As @Casey Jones implies in his (unfortunately) deleted answer here, I think the bash manual explains it best: https://www.gnu.org/software/bash/manual/bash.html#Redirecting-Standard-Output-and-Standard-Error

From it we learn that all 4 of these syntaxes are nearly identical, with the 1st being recommended over the 2nd, and the 3rd being the more-universal way to do it outside of bash:

# 4 ways in bash to redirect both stdout and stderr to `file`


# 1. recommended in bash
# Think of the `&` symbol here as meaning "1 AND 2", since it redirects
# both stdout (file descriptor 1) AND stderr (file descriptor 2) to `file`
&>file
# 2. works, but `file` may NOT expand to a number or to `-`.
>&file


# 3. the universal way to do it in or outside of bash: first redirect stdout to
# file, and then redirect stderr to stdout
>file 2>&1
# 4. exact same as 3 above
1>file 2>&1

So, &>file means "redirect both stdout and stderr to file."

Here is how the manual phrases it:

3.6.4 Redirecting Standard Output and Standard Error

This construct allows both the standard output (file descriptor 1) and the standard error output (file descriptor 2) to be redirected to the file whose name is the expansion of word.

There are two formats for redirecting standard output and standard error:
&>word
and
>&word
Of the two forms, the first is preferred. This is semantically equivalent to
>word 2>&1
When using the second form, word may not expand to a number or ‘-’. If it does, other redirection operators apply (see Duplicating File Descriptors below) for compatibility reasons.

3. Summary of proper redirection in bash

File descriptor reminder:

0 = stdin
1 = stdout
2 = stderr

Good, proper-usage syntax you should use:

# 1. file descriptor redirection


# redirect stderr to stdout
2>&1
# redirect stdout to stderr
1>&2




# 2. redirection to a file


# 2.A. redirect stdout to `file`
>file
1>file  # (same thing; the 1 is implied above)


# 2.B. redirect stderr to `file`
2>file


# 2.C. redirect BOTH stdout and stderr to `file` (4 ways)
# Think of the `&` symbol in this 1st example as meaning "1 AND 2", since it
# redirects both stdout (file descriptor 1) AND stderr (file descriptor 2)
# to `file`
&>file       # recommended in bash      <===
>&file       # not recommended
>file 2>&1   # universal and fine       <===
1>file 2>&1  # exact same as just above <===

Examples:

# print "hey 2 " to stdout.
# - Note: the `"%s "` format specifier gets applied to each input argument
#   thereafter. So, calling `printf "%s " "hello" "world"` is as though you had
#   called `printf "%s %s " "hello" "world"`.
printf "%s " "hey" 2


# redirect stdout to `file`; "file" now contains "hey 2 "
printf "%s " "hey" 2 >file
printf "%s " "hey" 2 1>file  # (same thing)


# redirect stderr to `file`; "file" remains empty since no stderr was printed
printf "%s " "hey" 2 2>file


# redirect BOTH stdout and stderr to `file`
printf "%s " "hey" 2 &>file
printf "%s " "hey" 2&>file        # don't do this! (same thing in this case,
# but looks awkward without the space before
# the `&`)
printf "%s " "hey" 2 >file 2>&1   # (same thing)
printf "%s " "hey" 2 1>file 2>&1  # (same thing)

4. "Weird" examples that we will present as "puzzles" and to teach and test understanding

# print "hey 2 " to stdout, redirecting stdout to a file named "1", then to "2",
# then to "3", then to "4", then to "5". Ultimately, `>5` overrides all
# previous stdout redirections, resulting in 5 files being created
# (named "1", "2", "3", "4", and "5"), with **only file "5"** containing
# the "hey 2 " text, and all other files being empty. Read the contents of all
# files all at once with `grep '' *`.
printf "%s " "hey" 2 >1 >2 >3 >4 >5
# OR, exact same thing:
printf "%s " "hey" 2 1>1 1>2 1>3 1>4 1>5


# read the contents of all files so you can see that only file "5" above has
# "hey 2 " in it
grep '' *


# print "hey " to a file named "5", while also creating empty files
# named "1", "2", "3", and "4". Note: `21>1` is a bit nonsensical in this case.
# It redirects file descriptor 21 to a file named "1". That doesn't really do
# anything. All the redirections thereafter redirect file descriptor 1 (stdout)
# to a file with a number for a name, as specified.
printf "%s " "hey" 21>1 1>2 1>3 1>4 1>5


# print "hey " to a file named "5", while creating empty files
# named "1", "2", "3", and "4". stderr gets redirected to the file named "1",
# but it also ends up empty since no stderr output was produced by this command.
printf "%s " "hey" 2>1 1>2 1>3 1>4 1>5


# Print some error output to a file named "1", since stderr gets redirected to
# it. Stdout gets ultimately redirected to a file named "5", but no stdout
# output is printed since `--invalid_arg` is not a valid argument. The stderr
# error message printed to the file named "1" is:
#       bash: printf: --: invalid option
#       printf: usage: printf [-v var] format [arguments]
printf --invalid_arg "%s " "hey" 2>1 1>2 1>3 1>4 1>5


# print "hey 2 " to a file named "5", while also creating empty files
# named "1", "2", "3", and "4". Initially, both stdout and stderr get
# redirected to the file named "1", via `&>1`, but then the stdout redirection
# gets overridden repeatedly until the last one that "sticks" is the stdout
# redirection to the file named "5" via `1>5`.
printf "%s " "hey" 2 &>1 1>2 1>3 1>4 1>5
printf "%s " "hey" 2&>1 1>2 1>3 1>4 1>5     # (same as above, but even more
# awkward-looking)
printf "%s " "hey" 2&>1 >2 >3 >4 >5         # (same as above)
printf "%s " "hey" 2 &>1 >2 >3 >4 >5        # (same as above)
printf "%s " "hey" 2 >1 2>&1 >2 >3 >4 >5    # (same as above)
printf "%s " "hey" 2 1>1 2>&1 >2 >3 >4 >5   # (same as above; reminder: `1>1`
# redirects stdout (file descriptor 1) to a file named "1", whereas
# `2>&1` redirects stderr (file descriptor 2) to stdout
# (file descriptor 1); make sure you understand that by now)


# (NOT the same as above! See this example & description previously a few
# examples up)
printf "%s " "hey" 2>1 1>2 1>3 1>4 1>5