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Redirection

Master Bash redirection and pipes. These essential Linux features are critical for efficient system administration. Every command handles input, output, and errors through File Descriptors (FDs):

• STDIN (0): Standard Input – receives data.
• STDOUT (1): Standard Output – sends regular data.
• STDERR (2): Standard Error – sends error messages.

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Pipes vs. Redirection

Both manage data streams, but with a key difference:

• Redirection routes a command's input/output to or from a file.
• Pipes (|) connect the output of one command directly to the input of another command.

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STDIN (Standard Input)

Commands often expect input. By default, this is your keyboard, but you can redirect it from other sources.

Redirecting from a file (<): Use < to feed a file's content as input to a command. Create input.txt:

touch input.txt

Open input.txt and add:

Line 1
Line 2

Save input.txt. Then run:

cat < input.txt

This prints input.txt content, similar to cat input.txt.

Here-Documents (<< DELIMITER): For multi-line input directly in your script or terminal, use a here-document with a custom delimiter (e.g., EOF).

cat << EOF
Hello World!
How are you?
EOF

This will print:

Hello World!
How are you?

Similarly, with wc -l to count lines:

wc -l << EOF
Hello World!
How are you?
EOF

Output:

2

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STDOUT (Standard Output)

Regular command output typically goes to your terminal. Redirect it to a file.

Overwrite (>): Use > to send output to a file. If the file exists, its content is overwritten. Create file.txt (or ensure it's empty):

echo "Hello World!" > file.txt
cat file.txt # Output: Hello World!

Running again overwrites:

echo "How are you?" > file.txt
cat file.txt # Output: How are you? (Previous content is gone)

Append (>>): Use >> to add output to the end of a file without overwriting existing content.

echo "Hello World!" > file.txt # Start fresh
echo "How are you?" >> file.txt
cat file.txt

Output:

Hello World!
How are you?

You can also explicitly specify the STDOUT file descriptor (1): echo "Hello" 1> file.txt.

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STDERR (Standard Error)

Error messages, distinct from standard output, are sent to STDERR. Redirect them using file descriptor 2.

Redirecting Errors (2>): Use 2> to redirect error messages. For example, ls --invalid-flag generates an error.

ls --invalid-flag 2> error.txt
cat error.txt # Contains the error message

Use 2>> to append error messages to a file.

Discarding Errors (2> /dev/null): Send error output to /dev/null to completely suppress it. /dev/null is a special 'black hole' device that discards all data written to it.

ls --invalid-flag 2> /dev/null # Error message is hidden

Redirecting Both STDOUT and STDERR: You can manage both streams simultaneously.

To separate regular output and errors into different files:

# Assuming 'install_package.sh' generates both output and errors
./install_package.sh > output.txt 2> error.txt

To send both to the same file (concise syntax):

./install_package.sh > combined_output.txt 2>&1

The 2>&1 redirects file descriptor 2 (STDERR) to the same location as file descriptor 1 (STDOUT), which is combined_output.txt. Alternatively, an even shorter Bash 4+ syntax: &> combined_output.txt.

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Piping (|)

Pipes connect the standard output of one command directly to the standard input of another, creating powerful command chains.

Basic Example: Find .txt files in a directory listing.

ls | grep ".txt"

This sends ls's output to grep for filtering.

Chaining Commands: Analyze file ownership in a directory.

ls -l /projects/bash_scripts | tail -n +2 | sed 's/\s\s*/ /g' | cut -d ' ' -f 3 | sort | uniq -c

This sequence lists files, removes the header, cleans spacing, extracts owner names, sorts them, and counts unique occurrences.

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Here-Documents (<< DELIMITER)

Here-documents (<<) provide multi-line input directly within your script or terminal, ideal for commands that read from STDIN without needing a separate temporary file.

Specify a custom delimiter (e.g., EOF):

cat << END_MESSAGE
This text will be passed as input to cat.
It spans multiple lines.
END_MESSAGE

Output:

This text will be passed as input to cat.
It spans multiple lines.

You can also pipe the output of a here-document to another command:

wc -l << MESSAGE_LINES
Line one.
Line two.
Line three.
MESSAGE_LINES

Output: 3 (counts the lines). Variables are expanded within here-documents.

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Here-Strings (<<<)

For piping a single string into a command's STDIN, use a here-string (<<<). This is cleaner than a here-document for single lines.

wc -w <<<"This is a sample string for word count."

Output: 7 (counts words). Variables are also supported in here-strings.

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Summary of Redirection and Piping Operators

Operator	Description
>	Redirect STDOUT to a file, overwriting existing content.
>>	Redirect STDOUT to a file, appending to existing content.
<	Redirect STDIN from a file.
2>	Redirect STDERR to a file, overwriting existing content.
`	`
<<	Here-document: Provide multi-line STDIN from the script.
<<<	Here-string: Provide single-line STDIN from the command line.