Redirecting Command Output to Both a File and Standard Output in Bash

When working with command-line tools in Unix-based systems, you often need to capture their output for logging or processing while simultaneously displaying it on the screen. This is particularly useful when monitoring long-running processes where you want real-time feedback without losing any data due to terminal buffer overflow. In this tutorial, we will explore how to achieve dual redirection of command outputs to both a file and standard output using Bash.

Understanding Redirection in Unix

Redirection is a fundamental concept in Unix-based systems that allows users to control where the input and output of commands are directed. By default, a command writes its output to the terminal screen (standard output) and sends error messages to an error stream (standard error). However, you can redirect these streams to files or other processes.

Basic Redirection

• Standard Output: The > operator redirects standard output (stdout) of a command to a file.

  command > output.txt
  

• Appending Output: The >> operator appends the command’s standard output to a file instead of overwriting it.

  command >> output.txt
  

Using the tee Command

The tee command is particularly useful when you need both on-screen display and logging. It reads from standard input and writes to both standard output (to be displayed) and one or more files.

Basic Usage of tee

command | tee output.txt

This command will execute command, displaying its output on the terminal while simultaneously writing it to output.txt.

Including Standard Error

To capture both standard output and standard error, use:

command 2>&1 | tee output.txt

Here, 2>&1 redirects standard error (stderr) to standard output so that tee can handle both streams.

Appending Output with tee

If you want to append the output rather than overwrite it, use the -a option:

command 2>&1 | tee -a output.txt

This ensures all new data is appended to output.txt.

Advanced Redirection Techniques

Sometimes buffering issues arise due to how streams are flushed. The unbuffer command can mitigate these problems by forcing immediate flushing of both standard output and error.

Using unbuffer

unbuffer command 2>&1 | tee output.txt

This ensures outputs remain in sync, which is crucial for processes with infrequent updates.

Shorthand Redirection

Bash provides a shorthand method for redirecting both standard output and error simultaneously using the pipe operator:

command |& tee output.txt

In this syntax, |& combines stdout and stderr, making it equivalent to 2>&1 |.

Practical Examples

• Listing Directory Contents:

  ls -lR / | tee directory_contents.log
  

  This captures the detailed listing of all files in the root directory, showing them on-screen and saving them into directory_contents.log.

• Java Process Logging:
  If you have a Java process script (my_java_process_run_script.sh), use:

  my_java_process_run_script.sh |& tee java_output.log
  

  This will display the log output as it runs and save it in java_output.log.

Best Practices

• Error Handling: Always consider both standard output and error streams when redirecting to ensure comprehensive logging.

• Buffer Management: Use unbuffer for processes with slow or infrequent outputs to avoid out-of-order logs.

• Resource Management: When dealing with large amounts of data, be mindful of disk space usage, especially if using the append mode indiscriminately.

By mastering these redirection techniques, you can effectively manage and monitor command outputs in Unix-based systems. These skills are essential for scripting, debugging, and ensuring reliable logging practices.