redirection

Input Redirection Methods in Linux (Including tee)

Input redirection in Linux allows commands to receive input from sources other than the keyboard (stdin). Below are the main methods, including tee for capturing output while redirecting.

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1. Standard Input Redirection (<)

Redirects input from a file instead of the keyboard.

Example: Read from a file

command < input.txt

Example with cat:

cat < file.txt

This is equivalent to cat file.txt and displays the file's contents.

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2. Here Document (<<)

A here document (<<) provides multiple lines of input inline.

Example: Pass multi-line input to cat

cat << EOF
Hello
This is a test
EOF

This prints:

Hello
This is a test

EOF is a delimiter; you can use any word (e.g., END, DATA).

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3. Here String (<<<)

A here string (<<<) sends a single string as input.

Example: Count words in a string

wc -w <<< "Hello world"

Output:

2

(wc -w counts words, and "Hello world" has 2 words.)

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4. Input Redirection from a Process (|, Pipe)

A pipe (|) sends the output of one command as input to another.

Example: Count lines in ls output

ls -l | wc -l

• ls -l lists files.
• wc -l counts lines.

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5. Using /dev/stdin or /proc/self/fd/0

These refer to standard input and can be used as an alternative to <.

Example: Read input from /dev/stdin

cat /dev/stdin

Type some text, then press Ctrl+D to end input.

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6. Using tee for Input & Output Redirection

The tee command reads input from stdin, writes it to a file, and also passes it to the next command.

Example: Save and display output simultaneously

ls -l | tee output.txt

• tee output.txt writes the output to output.txt and displays it.

Example: Append instead of overwriting

ls -l | tee -a output.txt

(-a appends instead of replacing.)

Example: Using tee in Input Redirection

You can also use tee to store input while passing it:

cat << EOF | tee input_copy.txt | wc -w
Hello
Linux input redirection
EOF

• Saves input to input_copy.txt.
• Counts words and prints the result.

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Comparison of Methods

Method	Usage	Best for
< file	Redirect file input	Using a file as input
<< EOF	Here document	Multi-line input
<<< "text"	Here string	Single-line input
`	`	Pipe
/dev/stdin	Read from standard input	Alternative method
tee file	Save & pass output	Logging while executing

Would you like more practical examples? 🚀