Shell input and output redirection __linux under Linux

on the shell:
0 indicates standard input, input data from the terminal, generally refers to the keyboard
1 indicates the standard output, the data displays to the terminal, generally refers to the screen
2 indicates standard error output, Error prompts display to terminal, generally refers to screen
> default to standard output redirection, same as 1>

2>&1 means redirecting the standard error output to the standard output.

2>1 indicates that the standard error is redirected to file 1, using & to differentiate between the file name and the filename descriptor

&>file means redirecting both standard output and standard error output to file files

<,<<,>,>> Difference

"<" indicates the input redirection operator, and ">" represents the output redirection t operator.

"<<" This input tells the shell that the current standard input is from the command line to the start tag and the end tag tag between the contents as input.

> Output, a file is automatically created if the file does not exist, overwriting the previous entry if the second time.

">>" means to append the second output to a file instead of overwriting it.

PS EF > A.txt redirects the execution results of the PS command to the A.txt file, not on the screen.

Wc<a.txt WC originally directly statistics from the terminal input lines of text information (line number, word number, character number), here will be input redirect to A.txt file

Example 1 <<:

pp@pp-satellite-l700~$ cat << EOF
> Hello
> What?
> OK
> Bye
> EOF
Hello
what?
OK
bye
pp@pp-satellite-l700$ cat >test.txt << EOF
> Hello
> What?
> OK
> Bye
> EOF
pp@pp-satellite-l700$ cat test.txt 
Hello
what?
OK
bye

The first command: Cat <<eof, indicating an end to EOF, where the contents are transmitted to the cat as input.
The second command: Cat > Test.txt << EOF, first redirect the output of the cat to the file Test.txt, and then redirect the input to the command line end with EOF as the end tag. Equivalent to Cat 1>test.txt 0<< EOF and Cat 0<<eof 1>test.txt

Example 2:
Note that for ease of understanding, you must set up an environment that enables the grep da* command to have normal output and error output, and then use the following command to generate three files, respectively:
grep da * > Greplog1
grep da * > Greplog2 1>&2
grep da * > Greplog3 2>&1//grep da * 2>greplog4 1>&2 results
#查看greplog1会发现里面只有正常输出内容
#查看greplog2会发现里面什么都没有

#查看greplog3会发现里面既有正常输出内容又有错误输出内容

Example 3:

LS 1>/dev/null 2>/dev/null

/dev/null is a special file device, all input will be discarded. Used to filter out the output you don't want to see.

Reference:

Http://blog.sina.com.cn/s/blog_406127500102uxrs.html

http://blog.csdn.net/jfkidear/article/details/7823486