Complete Guide to Redirecting Bash Output to File and Stdout

To redirect command output to both a file and stdout simultaneously in bash, you can use the tee command. The most common approach is to pipe your command’s output to tee, which will then write to both the terminal and the specified file. For example: command | tee filename redirects stdout to both places, while command 2>&1 | tee filename redirects both stdout and stderr to both destinations.

Contents

• Using the tee Command
• Redirecting Both stdout and stderr
• Advanced Techniques
• Script-Level Redirection
• Common Use Cases
• Troubleshooting Tips

Using the tee Command

The tee command is specifically designed for this purpose. It reads from standard input and writes to both standard output and one or more files simultaneously.

Basic Syntax

command | tee filename

Example:

ls -la | tee output.txt

This command will list directory contents and display them in the terminal while also writing the same output to output.txt.

Key tee Options:

• -a or --append: Append to the file instead of overwriting it
• -i or --ignore-interrupts: Ignore interrupt signals

Example with append:

long_running_process | tee -a app.log

Redirecting Both stdout and stderr

To redirect both standard output and standard error to both the terminal and a file, you need to combine the file descriptors first.

Method 1: Combine stderr to stdout first

command 2>&1 | tee filename

Example:

find . -name "*.txt" 2>&1 | tee search_results.txt

Method 2: Use the |& shorthand (Bash 4+)

command |& tee filename

Example:

curl -s https://example.com |& tee curl_output.log

Method 3: Using process substitution for separate handling

(command > >(tee stdout.log)) 2> >(tee stderr.log)

Advanced Techniques

Process Substitution for Complex Scenarios

Process substitution >(...) creates a FIFO that can be read by one command and written to by another.

Example: Separate stdout and stderr to different files while also showing both on terminal

(echo 'ok'; echo 'error' >&2) 2> >(tee err.log) > >(tee out.log) | tee all.log

Multiple Files with tee

You can write to multiple files simultaneously:

command | tee file1.txt file2.txt file3.txt

Conditional Redirection

You can make redirection conditional based on whether output is going to a terminal:

if [ -t 1 ]; then
    command | tee "$logfile"
else
    command > "$logfile"
fi

Script-Level Redirection

For entire scripts, you can use exec to redirect all output:

Redirect all output (stdout and stderr) for the entire script

#!/bin/bash
exec &> >(tee -a script.log)
echo "This goes to both terminal and log file"
some_command_with_errors

Conditional script-level redirection

#!/bin/bash
if [ -t 1 ]; then
    exec &> >(tee -a "$logfile")
else
    exec &> "$logfile"
fi

Common Use Cases

Logging Long-Running Processes

./long_running_script.sh | tee -a process.log

Development and Debugging

make 2>&1 | tee build.log

System Administration Tasks

sudo apt update 2>&1 | tee apt_update.log

Monitoring Commands

tail -f /var/log/syslog | tee -a monitor.log

Pipeline Processing

cat largefile.txt | grep "pattern" | tee filtered.txt | wc -l

Troubleshooting Tips

Common Issues and Solutions

Issue: Missing stderr output
Solution: Make sure to redirect stderr to stdout first: command 2>&1 | tee file

Issue: File permissions errors
Solution: Ensure you have write permissions to the target file/directory

Issue: Output appears delayed
Solution: Use tee -a instead of tee to avoid overwriting issues

Issue: Different behavior in scripts vs interactive shell
Solution: Use set -o pipefail to handle errors properly in pipelines

Performance Considerations

• For very high-volume output, consider using tee -a to avoid file truncation issues
• Multiple tee commands can impact performance significantly
• For production logging, consider dedicated logging tools

Conclusion

Key Takeaways

1. The tee command is the primary tool for redirecting output to both file and stdout
2. Use command 2>&1 | tee filename to capture both stdout and stderr
3. Process substitution offers advanced capabilities for complex redirection scenarios
4. exec can redirect all output at the script level
5. Understanding file descriptors (1=stdout, 2=stderr) is crucial for proper redirection

Practical Recommendations

• For simple logging: Use command | tee filename
• For comprehensive logging: Use command 2>&1 | tee filename
• For scripts: Consider exec &> >(tee -a logfile) for complete redirection
• Always verify file permissions before redirecting to files
• Use -a option with tee when you need to append to existing log files

Related Questions

• How to redirect only stderr to both file and terminal?
• Can I use tee with gzipped output?
• What’s the difference between tee and tee -a?
• How to handle color codes when using tee with log files?

For more advanced scenarios, consider exploring bash process substitution and advanced file descriptor manipulation techniques.

Sources

1. Stack Overflow - How to redirect output to a file and stdout
2. Server Fault - Capturing STDERR and STDOUT to a file using ‘tee’
3. Linux Bash - tee: Redirect output to files and stdout
4. Baeldung on Linux - Redirect Output of a Process to a File and Standard Streams
5. Unix & Linux Stack Exchange - Using exec and tee to redirect logs
6. Super User - How to redirect output to both a file and stdout
7. Reddit - Command tee file tee stderr and stdout issue
8. Linux Hint - Redirecting stderr Using the tee Command