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Input and output redirection: write to a file what was originally output to the screen. This operation is called output redirection.

Pipeline command: the information that the previous command was supposed to output to the screen is given to the later command for secondary processing.

Wildcard characters on the command line

An asterisk (*) can match a null value or infinity

Say hello (? ) one character. (there must be something to match.)

[amurz] matches a single lowercase letter

[Amurz] matches a single capital letter

[0-9] matches a single number

[1BZ 3P5] matches the specified number

[aMagnec _ e] matches the specified lowercase letter

[AmenthCpence E] matches the specified uppercase letters

Commonly used escape characters

The four most commonly used escape characters are shown below.

Backslash (\): changes a variable after a backslash to a simple string.

Single quotation mark (''): escapes all variables as simple strings.

Double quotation marks (""): the variable attributes are retained without escape processing.

Back quotation mark (``): returns the result after the command is executed.

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