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Redirections

Redirection in Linux allows you to control where the output of a command goes and where the input for a command comes from. This is a fundamental aspect of shell scripting and command-line operations, enabling users to connect and manipulate data streams efficiently.

This page will cover the basics of standard input/output (I/O), redirection operators, and how to combine them with pipes to perform powerful data manipulations.

Standard Input, Output, and Error

In Linux, commands use three standard data streams:

  1. Standard Input (stdin): The data stream that provides input to commands, usually from the keyboard. By default, it is file descriptor 0.

  2. Standard Output (stdout): The data stream where a command sends its output, usually to the terminal screen. By default, it is file descriptor 1.

  3. Standard Error (stderr): The data stream where a command sends its error messages, also usually to the terminal screen. By default, it is file descriptor 2.

Redirecting Standard Output (> and >>)

Redirecting output is one of the most common forms of redirection.

Basic Redirection (>)

  • Redirect Output to a File:

    command > file.txt

    This command sends the output to file.txt, overwriting it if the file exists.

Append Output (>>)

  • Append Output to a File:

    command >> file.txt

    This command appends the output to file.txt without overwriting the existing content.

Redirecting Standard Input (<)

Redirecting input allows a command to take input from a file instead of the keyboard.

  • Use a File as Input:

    command < file.txt

    This feeds the contents of file.txt as input to the command.

Redirecting Standard Error (2>)

Sometimes you may want to separate error messages from normal output.

  • Redirect Errors to a File:

    command 2> error.log

    This sends error messages to error.log instead of the terminal.

Combining Standard Output and Error (>& and 2>&1)

You can combine standard output and standard error into a single stream.

  • Redirect Both Output and Errors to the Same File:

    command > file.txt 2>&1

    This sends both the standard output and standard error to file.txt.

  • Redirect Output and Error Separately:

    command > output.txt 2> error.log

    This sends standard output to output.txt and errors to error.log.

Using Pipes (|)

Pipes allow you to use the output of one command as the input for another.

  • Basic Pipe Usage:

    command1 | command2

    This takes the output of command1 and uses it as the input for command2.

Example: Using Pipes with Redirection

  • Search for a Word in a File and Count Occurrences:

    grep "word" file.txt | wc -l

    This command finds all occurrences of "word" in file.txt and then counts the number of matching lines.

Here Documents (<<)

A here document is a type of redirection that allows you to pass a block of text as input to a command.

  • Using a Here Document:

    command << EOF
Line 1
Line 2
EOF

    Everything between the two EOF markers is treated as input to command.

Here Strings (<<<)

A here string is similar to a here document but allows you to pass a single string as input.

  • Using a Here String:

    command <<< "This is a single input string"

    This passes the string "This is a single input string" as input to command.

Conclusion

Understanding redirections in Linux is key to mastering the command-line interface. By controlling where your commands take input from and where they send output, you can automate tasks, create complex workflows, and handle errors more effectively. Pipes, combined with redirections, provide even more powerful ways to manipulate data streams and command outputs.

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