The meaning of "2>&1" in Linux

Article excerpt from: http://os.chinaunix.net/a2009/0903/996/000000996941.shtml

The script is:

nohup/mnt/nand3/h2000g >/dev/null 2>&1 &

Right

In & 1 more accurately said file descriptor 1, and 1

The general Representative is Stdout_fileno, which is actually a dup2 (2) call. He standard output to All_result

, and then copy the standard output to the file Descriptor 2 (Stderr_fileno), the consequence is that the file descriptor 1 and 2 point to the same file table entry, or that the output of the error is merged. 0 of them

Indicates that keyboard input 1 indicates screen output

2 indicates an error output. REDIRECT standard error to standard output, then throw it under/dev/null. In layman's words, all standard output and standard errors are thrown into the trash.

Command >out.file 2>&1 &

Command

>out.file is to redirect the command output to the Out.file file, that is, the output is not printed to the screen, but is output to the Out.file file.

2>&1

is to redirect standard error to standard output, where standard output has been redirected to the Out.file file, and the standard error is output to the Out.file file. The last One &

is to have the command execute in the background.

Imagine what 2>1 stands for, the combination of 2 and > represents error redirection, while 1 represents the error redirection to a file 1, not the standard output;

Replacing 2>&1,& with 1 represents the standard output, which becomes an error redirect to the standard output.

You can use

LS 2>1 test, will not report the error of No 2 file, but will output an empty file 1;

ls xxx 2>1 test, no xxx This file error output to 1;

ls xxx 2>&1 test, will not generate 1 of this file, but the error ran to the standard output;

ls xxx >out.txt 2>&1, actually can be replaced with ls xxx 1>out.txt 2>&1; redirect symbol > default is 1, error and output are uploaded to OUT.txt.

why 2>&1 to write in the back?

command > File 2>&1

The first is command > file to redirect the standard output to file, 2>&1 is the standard error copy of the standard output behavior, which is also redirected to file, the end result is that the standard output and errors are redirected to file.

Command 2>&1 >file

The 2>&1 standard error copies the behavior of the standard output, but at this point the standard output is still in the terminal. The output is redirected to file after >file, but the standard error remains at the terminal.

With Strace you can see:

1. Command > File 2>&1

The key system call sequences that implement redirection in this command are:

Open (file) = = 3

Dup2 (3,1)

Dup2 (ON)

2. Command 2>&1 >file

The key system call sequences that implement redirection in this command are:

Dup2 (ON)

Open (file) = = 3

Dup2 (3,1)

Consider what kind of file-sharing structure the different dup2 () call sequences will produce. Please refer to Apue 3.10, 3.12

The meaning of "2>&1" in Linux