Mastering Bash Output Redirection: Redirecting Both STDOUT and STDERR to a Single File

Introduction

In Bash scripting, managing standard output (stdout) and standard error (stderr) efficiently is crucial for debugging and logging purposes. By default, stdout and stderr are separate streams that allow scripts to distinguish between regular program outputs and error messages. However, there are scenarios where you might want to redirect both of these streams into a single file or handle them in more sophisticated ways using tools like tee. This tutorial will guide you through various methods for redirecting stdout and stderr within Bash.

Basic Concepts

Standard Streams

Standard Output (stdout): File descriptor 1. It’s where programs write their regular output.
Standard Error (stderr): File descriptor 2. It captures error messages and diagnostics.

I/O Redirection Syntax

Bash uses the > operator for redirection:

command > file redirects stdout to a file.
command 2> file redirects stderr to a file.
command &> file or command >& file is often used in Bash to redirect both stdout and stderr to a single file.

Redirecting Both Streams to a Single File

Method 1: Using `&>`

The simplest way to combine stdout and stderr into a single file:

your_command &> output.log

Here, &> redirects both standard output and error streams of your_command to output.log. This method is convenient but only works in Bash.

Method 2: Redirecting Error to Output

For more control over the streams, you can redirect stderr to stdout and then handle them together:

some_command >output.log 2>&1

The significance of this order lies in how file descriptors are handled:

> redirects stdout to output.log.
2>&1 tells Bash to send stderr (file descriptor 2) to the same place as stdout (file descriptor 1).

Method 3: Using `tee`

When you want to write both streams to a file while still displaying them on the terminal, use tee. This is useful for logging and monitoring:

do_something 2>&1 | tee -a log_file.txt

The command redirects stderr to stdout using 2>&1.
Then, | tee -a log_file.txt appends both streams into the specified file while also displaying them.

Advanced Redirection: Using File Descriptors

In more complex scripts or when daemonizing a script, managing file descriptors directly provides flexibility:

exec 3>&1 4>&2 # Save original stdout and stderr
exec 1>>output.log 2>&1 # Redirect both to log_file while keeping stderr as part of stdout

echo "This will go into the log file"

exec 3>&1 and exec 4>&2 create backups of the original stdout and stderr.
exec 1>>output.log 2>&1 appends output to a file.

Redirecting Streams within Scripts

For scenarios requiring redirection at multiple points in a script, such as logging while keeping terminal outputs intact:

# Create descriptors for real terminal output
exec 3>&1 4>&2

# Log and display using tee
exec 1> >(tee log_file.txt >&3) 2> >(tee -a log_file.txt >&4)

echo "This goes to both the log file and terminal"

exec 1> >(tee ... >&3) uses process substitution with tee for stdout.
It directs output both into log_file.txt and back to original standard output.

Conclusion

Understanding how to manage stdout and stderr in Bash is vital for effective scripting. Whether you’re logging output for diagnostics or monitoring a running script, the techniques demonstrated here equip you with the tools necessary for robust redirection strategies. Practice these methods to enhance your scripts’ reliability and maintainability.