Shell Script by Example: Redirection
Controlling input and output streams with this code example demonstrating stdout redirection with > and >>, stderr redirection with 2>, combining streams with &>, input redirection with <, and here documents for multi-line input.
Code
#!/bin/bash
# Redirect stdout to file (overwrite)
echo "Hello" > output.txt
# Redirect stdout to file (append)
echo "World" >> output.txt
# Redirect stderr to file
ls /nonexistent 2> error.log
# Redirect both stdout and stderr to same file
ls /tmp &> all_output.log
# Redirect stderr to stdout
ls /nonexistent > output.log 2>&1
# Input redirection (read from file)
wc -l < output.txt
# Here Document (feed multi-line input)
cat <<EOF > config.txt
Host=localhost
Port=8080
EOFExplanation
Redirection controls where commands read input from and write output to. Every process has three standard streams: stdin (file descriptor 0) for input, stdout (file descriptor 1) for normal output, and stderr (file descriptor 2) for error messages. The > operator redirects stdout to a file, overwriting existing content, while >> appends to the file. The 2> operator specifically redirects stderr, allowing you to separate error messages from normal output.
To capture both stdout and stderr in the same file, use &> filename or the older > filename 2>&1 syntax. The latter works by first redirecting stdout to the file, then redirecting stderr to wherever stdout is currently going. Order matters: 2>&1 > file doesn't work as expected because stderr is redirected before stdout is redirected to the file. This is common when logging script execution or debugging.
Input redirection < allows commands to read from files instead of the keyboard.
Here Documents using <<DELIMITER provide a convenient way to pass multi-line
text blocks to commands. The block continues until the delimiter (commonly EOF,
END, or similar) appears alone on a line. Here Documents are frequently used for
generating configuration files, feeding input to interactive programs within scripts, or
creating SQL queries. You can use <<-DELIMITER to allow leading tabs in the
here document, which is useful for keeping code indented inside scripts.
Code Breakdown
> redirects stdout to file, overwriting existing content.>> appends to file instead of overwriting previous content.2> redirects file descriptor 2 (stderr) to error.log.< creates here document feeding multi-line input until EOF delimiter.