I/O Redirection

In Linux, every process that runs in a shell interacts with three essential data streams: input, output, and error. These streams are the foundation of I/O redirection, which allows control over how data flows into and out of commands.

The Three Standard Streams

Linux associates three standard streams with every process:

1. Standard Input (STDIN) – File Descriptor 0

STDIN is the source of input data for commands. By default, this comes from the keyboard. For example, running cat without any arguments reads from STDIN.

cat

Typing text followed by Ctrl+D (end-of-file) will send input to the command.

2. Standard Output (STDOUT) – File Descriptor 1

STDOUT is where a command sends its regular output. By default, this is displayed on the terminal screen.

Example:

echo "Hello"

prints “Hello” to STDOUT.

3. Standard Error (STDERR) – File Descriptor 2

STDERR is used exclusively for error messages or diagnostics, separate from STDOUT. This ensures that errors do not get mixed with regular output unless explicitly redirected.

Example:

ls non_existing_file

generates an error message sent to STDERR.

Redirection Operators

Redirection changes the default flow of these streams.

Examples:

command > output.txt            # STDOUT to file
command 2> errors.txt           # STDERR to file
command > out.txt 2>&1          # Both to same file

Pipes: Chaining Commands

A pipe (|) connects the STDOUT of one command directly to the STDIN of another, enabling powerful command chains.

cat file.txt | grep "pattern"

This setup sends the contents of file.txt to grep, which searches for lines matching “pattern”.

Useful Tools and Tricks

Using tee to Save and Display Output

The tee command reads from STDIN and writes to both STDOUT and a file:

echo "Log entry" | tee logfile.txt

This prints “Log entry” to the terminal and saves it in logfile.txt.

Add -a to append:

command | tee -a logfile.txt

Here Documents (<<) and Here Strings (<<<)

Here documents allow multi-line input directly in the shell:

cat << EOF
This is
multi-line input
EOF

Here strings provide a single-line string as STDIN:

cat <<< "This is a single line"

These are especially useful in scripts to feed blocks of text to commands.