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In this lesson, we explore various methods for redirecting input and output in Linux. Understanding how Linux programs handle standard streams is crucial for efficient command-line operations.

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Overview of Standard Streams

Linux programs use three primary standard streams:
  • Standard Input (stdin): Receives input data.
  • Standard Output (stdout): Sends processed output.
  • Standard Error (stderr): Outputs error messages.
By default, most programs display output on the terminal. However, output redirection allows you to save results to files or combine multiple streams.
The image illustrates the flow of standard input, output, and error in a command-line environment, showing how data from "file.txt" is processed by the "sort" command, with standard output directed to the terminal and standard error to "errors.txt".

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Redirecting Output

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Redirecting to a File

Consider the utility sort which arranges text after reading it from a file. For example, given a file named file.txt: 
$ cat file.txt
6
5
1
3
4
2
Sorting the file outputs the ordered numbers to the terminal: 
$ sort file.txt
1
2
3
4
5
6
To save the sorted output to a new file instead of displaying it, you can use the redirection operator >: 
$ sort file.txt > sortedfile.txt
The target file (sortedfile.txt) is created automatically if it does not exist.

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Overwriting Versus Appending

Using a single > operator overwrites the contents of the target file. For example: 
$ date > file.txt
$ date > file.txt
$ cat file.txt
Mon Nov  8 18:50:30 CST 2021
If you want to preserve existing content and append new output, use the >> operator: 
$ date >> file.txt
$ date >> file.txt
$ date >> file.txt
$ date >> file.txt
$ date >> file.txt
$ cat file.txt
Mon Nov  8 18:50:30 CST 2021
Mon Nov  8 18:50:31 CST 2021
Mon Nov  8 18:50:32 CST 2021
Mon Nov  8 18:50:34 CST 2021
Mon Nov  8 18:50:35 CST 2021
You can also explicitly target the standard output file descriptor with 1>: 
$ date > file.txt
$ date 1>file.txt
Both commands will redirect the standard output to file.txt.

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Input Redirection

Sometimes, rather than receiving input directly from the keyboard, programs can read data from a file using the < symbol. For example, to send the contents of emailcontent.txt to the sendemail command: 
$ sendemail [email protected] < emailcontent.txt
This operation makes it appear as though you have manually input the email content.

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Redirecting Error Messages

Linux allows you to handle error messages separately by using the file descriptor 2. For example, to redirect error messages to a file named errors.txt: 
$ command 2>errors.txt
Be sure to differentiate between standard output and error messages. This helps in troubleshooting issues effectively.
For instance, running: 
$ grep -r '^The' /etc/
might display many “Permission denied” errors. To suppress these errors, they can be redirected to /dev/null, a special file that discards input: 
$ grep -r '^The' /etc/ 2>/dev/null

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Redirecting Both Standard Output and Standard Error

At times, you might want to capture both standard output and error messages into a single file. You can achieve this by redirecting stdout to a file and then sending stderr to the same location: 
$ grep -r '^The' /etc/ 1>all_output.txt 2>&1
The 2>&1 operator redirects stderr to the current destination of stdout. Ensure you place 2>&1 after the stdout redirection to avoid errors being displayed in the terminal.

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Here Documents and Here Strings

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Here Documents

A here document is a way to provide inline input to commands. For example, sorting numbers using an inline document: 
$ sort <<EOF
6
3
2
5
1
4
EOF
Here, EOF serves as a delimiter indicating where the input ends. You can choose any delimiter, but EOF is widely used.

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Here Strings

A here string is used to pass a single line of input into a command: 
$ bc <<< "1+2"
This operator makes the string "1+2" available as input to bc.

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Piping Output Between Commands

Linux’s philosophy of composing small utilities allows you to perform complex tasks by linking commands together using the pipe | operator. For example, to filter and format configuration settings: 
$ grep -v '^#' /etc/login.defs | sort | column -t
In this pipeline:
  • grep -v '^#' /etc/login.defs removes lines that start with #.
  • sort arranges the remaining output.
  • column -t formats the output into aligned, easy-to-read columns.
This method of chaining commands not only improves workflow efficiency but also enhances readability.

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Summary

This guide introduced essential redirection and piping techniques in Linux:
OperationDescriptionExample Command
Overwriting OutputRedirect output and overwrite existing filesort file.txt > sortedfile.txt
Appending OutputAdd output to the end of an existing filedate >> file.txt
Redirecting Error MessagesRedirect stderr to a filecommand 2>errors.txt
Combining Output and ErrorSend both stdout and stderr to a single filecommand 1>output.txt 2>&1
Input RedirectionDirect input from a file instead of the keyboardsendemail [email protected] < emailcontent.txt
Here DocumentsProvide multiline input directly in the terminalsort <<EOF ... EOF
Here StringsPass a single line of input directlybc <<< "1+2"
Piping CommandsChain commands to process and format data`grep -v ’^#’ /etc/login.defssortcolumn -t`
That concludes our discussion on input and output redirection in Linux. With these techniques, you can manipulate data streams to suit various operational needs. Now it’s time to apply this knowledge in your hands-on labs and command-line experiments.

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