A.out >outfile 2>&1 redirection problem in "Turn" Linux

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a.out > outfile 2>&1 redirection issues under Linux

The problem stems from the UNIX advanced environment programming

a.out > OutFile 2>&1

A.out 2> &1 >outfile

1. First 1> can be abbreviated to >, followed by 1 for stdout, output for the terminal, 2 for stderr, I also feel on the terminal. in front of 1 plus & is to show that &1 is stdout, not a file named 1 ;

2. The first of the topics in the first a.out (that is, a.out 1>outfile) output directed stdout to OutFile, and 2 (stderr) directed to &1 (stdout), and directed to OutFile, that is, the knot The output stdout and stderr of A.out are directed to OutFile .

3. The second a.out stderr directed to Stdout,stdout and directed to OutFile. (as a result, stderr was directed to the terminal, and StdIn was directed to outfile.) is not the same as the result of the first one!!! ）

That's what you understand.

The following is an explanation of 2>&1 in the Advanced Programming of UNIX environment

a.out > OutFile 2>&1

Set the standard output to outfile first, and then execute the DUP to copy the standard output to descriptor 2 (standard error), with the result that standard output and standard errors are set to the same file, that is, descriptors 1 and 2 point to the same file table entry. And for the command line

A.out 2>&1 >outfile

Because the DUP is executed first, the descriptor 20% is the terminal (assuming the command is executed interactively), and the standard output is redirected to outfile. The result is a descriptor 1 that points to the file table entry for outfile, descriptor 2 to the terminal's File table entry .

The "2>&1" meaning script in the Linux shell is:

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

For & 1 More accurate should be the file descriptor 1, and 1 is generally representative of Stdout_fileno, in fact, this operation is 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 file descriptors 1 and 2 point to the same file table entry , or the wrong output is merged. where 0 means keyboard input 1 means that the screen output 2 indicates an error output. REDIRECT standard error to standard output, and 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 redirects the command output to the Out.file file, which means that the output is not printed to the screen, but is output to the Out.file file. 2>&1 is redirecting standard errors to standard output, where the standard output has been redirected to the Out.file file, and the standard error is output to the Out.file file. The last & 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, Errors and outputs are uploaded to the 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 !!!

A.out >outfile 2>&1 redirection problem in "Turn" Linux