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This article mainly analyzes the relevant knowledge points of Shell how to input or output redirection, the content is detailed and easy to understand, the operation details are reasonable, and has a certain reference value. If you are interested, you might as well follow the editor to have a look, and follow the editor to learn more about "how to redirect Shell input or output".

Most UNIX system commands accept input from your terminal and send the resulting output back to your terminal. A command usually reads input from a place called standard input, which happens to be your terminal by default. Similarly, commands usually write their output to standard output, which is also your terminal by default.

The list of redirect commands is as follows:

The command instructs command > file to redirect the output to file. Command redirects input to file. Command > > file redirects the output to file as an append. N > file redirects the file with the file descriptor n to file. N > > file redirects the file with the file descriptor n to file as an append. N > & m merges the output files m and n. N merges the input files m and n.

Take the content between the opening tag tag and the closing tag tag as input.

It should be noted that the file descriptor 0 is usually standard input (STDIN), 1 is standard output (STDOUT), and 2 is standard error output (STDERR).

Output redirection

Redirection is typically achieved by inserting specific symbols between commands. In particular, the syntax of these symbols is as follows:

Command1 > file1

The above command executes command1 and then stores the output in file1.

Note that any existing content in the file1 will be replaced by the new content. If you want to add new content to the end of the file, use the > > operator.

Example

Execute the following who command, which redirects the complete output of the command in the user file (users):

$who > users

After execution, no information is output at the terminal because the output has been redirected from the default standard output device (terminal) to the specified file.

You can use the cat command to view the contents of the file:

$cat users_mbsetupuser console Oct 31 17:35 tianqixin console Oct 31 17:35 tianqixin ttys000 Dec 1 11:33

Output redirection overwrites the contents of the file, as shown in the following example:

$echo "Rookie tutorial: www.runoob.com" > users$ cat users Rookie tutorial: www.runoob.com$

If you do not want the contents of the file to be overwritten, you can use > > to append to the end of the file, for example:

$echo "Rookie tutorial: www.runoob.com" > > users$ cat users Rookie tutorial: www.runoob.com Rookie tutorial: www.runoob.com$ input Redirection

Like output redirection, the Unix command can also get input from a file with the syntax:

Command1 file1

In this way, the commands that would otherwise need to get input from the keyboard are transferred to the file to read.

Note: output redirection is greater than sign (>), input redirection is less than sign (

Example

Following the above example, we need to count the number of lines in the users file and execute the following command:

$wc-l users 2 users

You can also redirect input to a users file:

$wc-l users 2

Note: the results of the above two examples are different: the first example outputs the file name; the second does not, because it only knows how to read from standard input.

Command1 infile > outfile

Replace both input and output, execute command1, read from the file infile, and then write the output to outfile.

Redirect in-depth explanation

In general, three files are opened when each Unix/Linux command runs:

Standard input file (stdin): the file descriptor of stdin is 0Magna Unix program that reads data from stdin by default.

Standard output file (stdout): the file descriptor of stdout is 1Magna Unix program to output data to stdout by default.

Standard error file (stderr): the file descriptor of stderr is 2. The Unix program writes an error message to the stderr stream.

By default, command > file redirects stdout to file,command

If you want stderr to be redirected to file, you can write:

$command 2 > file

If you want stderr to be appended to the end of the file file, you can write:

$command 2 > > file

2 represents a standard error file (stderr).

If you want to merge stdout and stderr and redirect to file, you can write:

$command > file 2 > & 1 or $command > > file 2 > & 1

If you want to redirect both stdin and stdout, you can write:

$command file1 > file2

The command command redirects stdin to file1 and stdout to file2.

Here Document

Here Document is a special redirect method in Shell that redirects input to an interactive Shell script or program.

Its basic form is as follows:

Command delimiter documentdelimiter

Its purpose is to pass the content (document) between two delimiter as input to command.

Note:

The delimiter at the end must be written at the top, with no characters before it and no characters after it, including spaces and tab indentation.

Spaces before and after the initial delimiter are ignored.

Example

Calculate the number of lines of Here Document from the command line through the wc-l command:

$wc-l EOF Welcome to the rookie tutorial www.runoob.comEOF3 # the output is 3 lines $

We can also use Here Document in scripts, such as:

#! / bin/bash# author: rookie tutorial # url:www.runoob.comcat EOF Welcome to rookie tutorial www.runoob.comEOF

Execute the above script and output the result:

Welcome to the rookie tutorial www.runoob.com/dev/null file

If you want to execute a command but do not want the output to be displayed on the screen, you can redirect the output to / dev/null:

$command > / dev/null

/ dev/null is a special file, and everything written to it is discarded; if you try to read from it, you won't be able to read anything. However, the / dev/null file is very useful, and redirecting the output of the command to it has the effect of "suppressing output".

If you want to block stdout and stderr, you can write:

$command > / dev/null 2 > & 1

Note: 0 is standard input (STDIN), 1 is standard output (STDOUT), and 2 is standard error output (STDERR).

There can be no space between 2 and >, and error output is indicated only when 2 > is one.

This is the end of the introduction on "how to redirect Shell input or output". More related content can be searched for previous articles, hoping to help you answer questions, please support the website!

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