[Shell] basic shell function: Input/Output redirection

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@ Blackeye poet <www. chenwei. ws>

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Role of input/output redirection:

Output redirection saves the command execution result to a file for easy viewing.

Input redirection is to change the original keyboard input to file input.

 

First, let's take a look at the standard input and output:

[Device] [device file name] [file descriptor] [type]

Keyboard/dev/stdin 0 standard input

Display/dev/stdout 1 standard output

Display/dev/stderr2Standard Error output

 

Concepts (> Overwrite,> append)

[Correct output redirection]

Command> file # overwrite the command and output it to a file or device.

Command> file # append the command to the correct output to a file or device.

 

[Error output redirection]

Error command 2> file # overwrite the output of command errors to the specified file or device

Error command 2> file # output the command error to a specified file or device in append Mode

(Note: To save the error message, you must add 2 before the single sign or double sign, with no space in the middle)

 

[Correct output and error output are saved at the same time]

1. Command> File2> & 1# Store both the correct and error outputs to the same file in overwrite Mode

2. Command> file 2> & 1 # Save both the correct and error outputs to the same file in append mode.

3. Commands&> File # is equivalent to the first one (& the role is equivalent to 2> & 1)

4. Command &> file # is equivalent to the second

5.Command> file a 2> file B# Append the correct output to file a and append the wrong output to file 2 (useful during regular backup)

 

Ls&>/Dev/null# Save the execution result to/dev/null (devices that do not exist are about to clear meaningless results)

 

Input redirection:

[WC for keyboard input statistics]

WC [-C] file name # count the number of bytes

WC [-W] file name # count the number of words

WC [-L] file name # Number of Statistics rows

(After WC is input, other characters will be counted. CTRL + D ends)

(4 rows, 7 words, 40 characters)

 

[WC Statistical File: a unit smaller than the number]

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