Original: https://www.runoob.com/linux/linux-shell-io-redirections.html
Most UNIX systems command accepts input and output generated from your terminal sends back to your terminal. A command normally read input from a place called standard input by default, which happens to be your terminal. Similarly, a command is usually writes its output to standard output, by default, this is your terminal.
Redirect command list is as follows
| command | Explanation |
|---|---|
| command > file | Redirect the output to a file. |
| command < file | Redirect input to file. |
| command >> file | The additional output redirected to the way file. |
| n > file | The file descriptor n file redirected to file. |
| n >> file | The file descriptor n to file additional way redirected to file. |
| n >& m | M and n are the combined output file. |
| n <& m | M and n are the combined input file. |
| << tag | It will mark the beginning of the content between the tag and the end tag tag as input. |
0 Note that the file descriptor is typically standard input (STDIN), 1 is a standard output (STDOUT), 2 is the standard error output (STDERR).
Redirect depth explanation
Under normal circumstances, each Unix / Linux command is running will open three files:
- Standard input file (stdin): stdin file descriptor is 0, Unix default program data read from stdin.
- Standard output file (stdout): stdout file descriptor 1, Unix program default data output to stdout.
- The standard error file (stderr): stderr file descriptor 2, Unix program writes error information to stderr stream.
By default, command> file redirect stdout to the file, command <file to redirect stdin to file.
If you want to redirect stderr to file, you can write:
$ command 2 > file
If you want to append to the end of file stderr file can be written // 2 standard error file (stderr)
$ command 2 >> file
If you want to merge after the stdout and stderr redirected to a file, you can write:
$ command > file 2>&1 或者 $ command >> file 2>&1
If you want to redirect stdin and stdout, you can write:
$ command < file1 >file2
command command stdin redirected to file1, redirect stdout to file2.
Here Document
Here Document Shell is in a special manner redirection, to redirect input to an interactive Shell scripts or programs.
Its basic form is as follows:
command << delimiter
document
delimiter
It is the role of the content between the two delimiter (document) is transmitted as input to the command.
note:
- ending delimiter must be the top grid write, can not have any characters in front of the back can not have any characters, including spaces and tab indent.
- Space before and after the start of the delimiter is ignored.
Examples
The number of lines in the command line by calculating Here Document wc -l command:
$ WC the -l << EOF Welcome to the rookie tutorial www.runoob.com EOF 3 # output is 3 lines $
We can also Here Document used in a script, for example:
! # / bin / bash # author: rookie Tutorial # url: www.runoob.com CAT << EOF Welcome to the rookie tutorial www.runoob.com EOF
The implementation of the above script, output:
Welcome to the
rookie tutorial
www.runoob.com
/ Dev / null file
If you want to execute a command, but do not want to display the output on the screen, you can redirect the output to / dev / null:
$ command > /dev/null
/ Dev / null is a special file, its contents will be written to be discarded; if you try to read from the file, then nothing can not read. However, / dev / null file is very useful to redirect the output of a command to it, it will play the effect of "No Output".
If you want to shield stdout and stderr, can be written:
$ command > /dev/null 2>&1
Note: 0 is the standard input (STDIN), 1 is a standard output (STDOUT), 2 is the standard error output (STDERR).