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This article introduces the knowledge of "how to understand the meaning and difference of shell 1 > & 22 > & 1 & > filename redirection". In the operation of actual cases, many people will encounter such a dilemma. Then let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

When I saw "> & 1" and "> & 2" in shell, I never understood what it meant. The puzzle was solved by searching on the Internet. In fact, these are two kinds of output.

In shell programs, there are about three most commonly used FD (file descriptor), which are:

0 is a file descriptor that represents standard input (stdin)

1 is a file descriptor that represents standard output (stdout)

2 is a file descriptor that indicates standard error (stderr)

In a standard case, these FD are associated with the following devices:

Stdin (0): keyboard keyboard input and return at the front end

Stdout (1): the correct return value of monitor is output to the front end

Stderr (2): monitor error return value output to the front end

Give an example:

There is only one file a.txt in the current directory.

[root@redhat box] # ls

A.txt

[root@redhat box] # ls a.txt b.txt

Ls: b.txt: No such file or directory returns an error value because there is no b.txt file, which is called 2 output

A.txt, and this is the so-called 1 output

Let's move on to:

[root@redhat box] # ls a.txt b.txt 1 > file.out 2 > file.err

After execution, there is no return value. The reason is that the return values are redirected to the corresponding file instead of being displayed at the front end.

[root@redhat box] # cat file.out

A.txt

[root@redhat box] # cat file.err

Ls: b.txt: No such file or directory

Generally speaking, "1 >" can usually be omitted as ">".

You can write the above command as: ls a.txt b.txt > file.out 2 > file.err

Only with these understandings can we understand "1 > & 2" and "2 > & 1".

1 > & 2 the correct return value is passed to the 2 output channel & 2 represents the 2 output channel

If it is mistakenly written as 1 > 2, it means that the output of 1 is redirected to file 2.

The 2 > & 1 error return value is passed to the 1 output channel, and the & 1 represents the 1 output channel.

For instance。

[root@redhat box] # ls a.txt b.txt 1 > file.out 2 > & 1

[root@redhat box] # cat file.out

Ls: b.txt: No such file or directory

A.txt

Now, both the correct output and the wrong output are directed to the file.out file instead of being displayed at the front end.

In addition, there are not only 1 and 2 outputs, but also other types, which are only the most commonly used and basic.

> is a redirector, which redirects the previous output to the location specified later, such as (example 1):

Echo "some content" > filename.txt

The above example writes "something" to the filename.txt file.

> you can add a number to indicate what is redirected to the file, and the standard output is redirected to the file by default, so the following example is the same as the one above (example 2):

Echo "some content" 1 > filename.txt

If it is an error message, it will not be output to filename.txt (example 3):

$ls nodir 1 > filename.txt$ ls: nodir: No such file or directory

Nodir does not exist in the above example, so when querying through the ls command, the error message will be output to 2 (stderr), but what we specify is to redirect 1 to filename.txt, so after the above command is executed, there is no content in filename.txt. But executing the following command will write the error message to filename.txt (example 4):

$ls nodir 2 > filename.txt$ cat filename.txt$ ls: nodir: No such file or directory

& is a descriptor, and if you don't add & before 1 or 2, it will be treated as a normal file.

1 > & 2 means to redirect standard output to standard error.

2 > & 1 means to redirect standard error output to standard output.

& > filename means to redirect both standard output and standard error output to the file filename

Let's look at another example (listing 5):

$ls nodir 1 > filename.txt 2 > & 1$ cat filename.txt$ ls: nodir: No such file or directory

The above example redirects the standard output to the file filename.txt, and then redirects the standard error to the standard output, so the final error information is also written into the file through the standard output.

The following is what Baidu knows, you can refer to it:

Question: how to understand Linux redirection > & 2?

Question to add: echo "aaaaaaaaaaaaaaaa" > & 2 how to understand?

A:

> & 2, that is, 1 > & 2, that is, output the result to the same as standard error; if a standard error was redirected to a log file before, the standard output will also be redirected to this log file

For example: ls 2 > A1 > & 2 (equivalent to ls > A1 > & 1)

Redirect both standard output and standard error to A1, and no information can be seen on the terminal.

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