tail: Displaying the End of Files and Real-time Monitoring

Overview

The tail command outputs the end portion of a file to standard output. By default, it displays the last 10 lines, and the number of lines to output can be adjusted using the -n option. It is a very powerful tool for real-time log monitoring.

Key Features

Display the last N lines of a file
Real-time monitoring of file changes (-f)
Process multiple files simultaneously

Key Options

The main options for the tail command control the output format and monitoring capabilities.

Output Control

Monitoring

Other

Generated command:

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Description:

`tail` Executes the command.

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Usage Examples

Various examples of how to use the tail command.

View the last 10 lines of a file

tail -n 10 /var/log/syslog

Outputs the last 10 lines of the specified file.

View the last 50 bytes of a file

tail -c 50 /etc/passwd

Outputs the last 50 bytes of the specified file.

View from the 3rd line to the end of a file

tail -n +3 my_file.txt

Outputs all content from the 3rd line to the end of the file, including the 3rd line.

Real-time monitoring of a log file

tail -f /var/log/nginx/access.log

Outputs new content to the log file as it is added, allowing for real-time monitoring.

Real-time monitoring robust to log rotation

tail -F /var/log/myapp/error.log

Continues to track the log file even if it is renamed or recreated. Suitable for environments with log rotation.

View the last 5 lines of multiple files

tail -n 5 file1.txt file2.txt

Outputs the last 5 lines of each of the specified files, including file name headers.

Real-time filtering of specific logs with grep

tail -f /var/log/auth.log | grep "Failed password"

Monitors a log file in real-time and filters to output only lines containing the specific keyword ('Failed password').

Tips & Considerations

Tips and points to consider for more effective use of the tail command.

Useful Combinations

tail -f is essential for log monitoring. It becomes more powerful when piped (|) with grep to filter specific keywords.
tail -F provides more stable log tracking than -f in environments with frequent log rotation.
The -n option defaults to 10 lines if omitted. tail file.log is equivalent to tail -n 10 file.log.

Performance Considerations

When using tail -n +NUM on very large files, it may impact performance as it needs to read from the beginning of the file up to the specified line. In such cases, other tools like sed or awk might be more efficient.