Shell input/output redirection, shell output redirection

Shell input/output redirection, shell output redirection
Output redirection is generally achieved by inserting a specific symbol between commands. In particular, the syntax for these symbols is as follows:

command1 > file1

Run command1 in the preceding command and save the output content to file1. Note that existing content in any file1 will be replaced by new content. To add new content to the end of the file, use the> operator. The instance executes the following who command, which redirects the complete output of the command to the User File (users)

who > users

After execution, no information is output on 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 File Content

cat usersh tty7 2017-08-10 11:48 (:0)

The same as the output redirection, the Unix Command can also get the input from the file. The syntax is

command1 < file1

In this way, the command that needs to be obtained from the keyboard will be transferred to the file to read the content. Note: The output redirection is greater than the number (>), the input redirection is less than the number (<) instance, and then the above instance, we need to count the number of rows in the users file and execute the following command

$ wc -l users1 users

The wc (Word Count) command is used to Count the number of bytes, number of words, and number of rows in a specified file. The statistical result is displayed as follows:-l the number of statistical rows can also be redirected to the users file.

$  wc -l < users1 

Note: The results of the above two examples are different: In the first example, the file name is output; In the second example, the file name is not, because it only knows to read content from the standard input.

command1 < infile > outfile

Replace the input and output, execute command1, read the content from the file infile, and write the output to the outfile. In general, every Unix/Linux Command opens three files when running:

• Standard input file (stdin): The file descriptor of stdin is 0, and Unix programs read data from stdin by default.
• Standard output file (stdout): The file descriptor of stdout is 1, and the Unix program outputs data to stdout by default.
• Standard Error file (stderr): The file descriptor of stderr is 2. Unix programs write error messages to the stderr stream.

By default, command> file redirects stdout to file, and command <file redirects stdin to file.

If you want stderr to redirect to file, you can write as follows:

command 2 > file

If you want stderr to append to the end of the file, you can write as follows:

command 2 >> file

2 indicates the standard error file (stderr ). If you want to merge stdout and stderr and then redirect to file, you can write as follows:

$ Command> file 2> & 1 # Or $ command> file 2> & 1

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

command < file1 > file2

The command redirects stdin to file1 and stdout to file2Here using enthere Document is a special redirection method in Shell to redirect input to an interactive Shell script or program. Its basic form is as follows:

command << delimiter    documentdelimiter

It is used to pass the content (document) between two delimiter to the command as input. Note: delimiter at the end must be written in the top level. There cannot be any character at the front and no character at the end, including spaces and tab indentation. Spaces before and after the start of delimiter are ignored. The instance uses the wc-l command in the command line to calculate the number of lines in the Here Document:

$ Wc-l <EOF welcome to frethers www.cnblogs.com/freescienceEOF3 #3 rows of output result

We can also use the Here Document in a script, for example:

#! /Bin/bashcat <EOF welcome to freases www.cnblogs.com/freescienceEOF

Run the above script and output the result: Welcome to the frethers www.cnblogs.com/freescience/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, and the content written to it will be discarded. If you try to read the content from the file, nothing can be read. However, the/dev/null file is very useful. Redirecting the command output to it will disable output. If you want to block stdout and stderr, you can write as follows:

command > /dev/null 2>&1

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