How to Use the Journalctl Command to Tail Service Logs in Linux [4 Best Uses]

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Last updated: May 20, 2024

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To learn to use journalctl command to tail service logs in Linux, you can try following methods:

  1. Tail Logs for All Services: Tail all system logs in real time to get a comprehensive overview of system activities.
  2. Tail Logs for Specific Services: Tail the logs of specific services to closely monitor and troubleshoot those services.
  3. Tail Logs by Time Range and Criteria: Tail logs within a specified time range or based on specific criteria to narrow down your log analysis.
  4. Grep Command with Journalctl: Search for specific keywords or patterns within your logs using the Grep command in combination with journalctl.

Continue reading the guide below to learn different methods to use journalctl command to tail service logs in Linux, the benefits of using this command, and common errors that can occur during the process.

Journalctl is a command-line utility that serves as a system journal logger. It is tightly integrated with systemd, the default init system in many Linux distributions. The systemd-journald service collects, and stores log data in a binary format, which can be accessed and analyzed using the Journalctl command. In this comprehensive guide, I will explore various methods to use the Journalctl command to tail service logs in Linux, common errors that can occur during the process, and the benefits of using journalctl command to tail service logs.

How to Use the Journalctl Command to Tail Service Logs in Linux

To use the Journalctl command to tail service logs, follow methods such as tailing logs for all services, specific services, by time range and criteria, and using grep for keyword searches. These methods offer real-time visibility, focused analysis, and efficient log filtering, enabling effective troubleshooting and monitoring of Linux system logs.

1. Tail Logs for All Services

Gain real-time visibility into system activities by tailing logs for all services on your Linux system. This method provides a comprehensive overview of your system’s log events and is ideal for troubleshooting, monitoring system-wide activities, and identifying issues promptly. Follow these steps:

  1. Open a Terminal on your Linux system.
opening terminal 18
  1. Enter the following command: 
<strong>journalctl -f</strong>
  1. This command starts tailing the logs in real-time, displaying the most recent log entries.
  2. The -f flag allows you to continuously follow the logs as new entries are added.
tailing log entries in real time

2. Tail Logs for Specific Services

Focus your log analysis efforts on specific services by tailing logs for those particular units. This method allows you to closely monitor and troubleshoot specific services, ensuring efficient analysis and issue resolution. Here are the steps to do it:

  1. Access your Terminal window and to list all available units (services), use the following command: 
<strong>systemctl list-units --type=service</strong>
  1. This command will display a list of service units present on your system. Identify the service unit you want to monitor from the list.
listing all available services
  1. To tail the logs of a specific service, use the following command: 
<strong>sudo journalctl -u service_unit -f</strong>

Replace service_unit with the actual name of the service unit you want to monitor.

  1. This command will start tailing the logs of the specified service in real-time.
tailing logs of a specified service

3. Tail Logs by Time Range and Criteria

Tail logs within a specified time range or based on specific criteria to narrow down your log analysis. This method enables targeted log analysis and facilitates troubleshooting within specific time frames or under specific conditions. Follow these steps:

  1. Launch your command prompt and to tail logs within a specific time range, use the --since and --until flags. For example: To tail logs from yesterday, run: 
<strong>sudo journalctl --since "yesterday" --until "today" -f</strong>
  1. This command will display the logs within the specified time range and continuously follow new log entries as they occur.
viewing log entries of a specific time period
  1. To filter logs based on log priority, log levels, or specific log fields, use the appropriate flag. For example: To only display logs with a priority of err or higher, use: 
<strong>sudo journalctl -p err -f</strong>
  1. This command will filter the logs and show only those with the specified log priority or higher. The -f flag enables continuous following of new log entries.
viewing log entries with priority of error

4. grep Command with Journalctl

Harness the power of the grep command in combination with Journalctl to search for specific keywords or patterns within your logs. This method is particularly useful when you need to quickly locate and analyze logs related to specific events, errors, or patterns within your system.

  1. Open your command window and and run the command:
<strong>journalctl | grep "keyword"</strong>
<strong>journalctl | grep -E "network|error"</strong>
  1. Replace “keyword” with the actual term you want to search for. This command will display all log entries that contain the specified keyword.
  2. You can also use regular expressions with grep to perform more advanced searches. For example, to search for logs related to network errors, you can use:
  1. This command will display log entries that contain either the word "network" or "error."
viewing log entries containing specified keyword

3 Benefits of Using Journalctl Command to Tail Service Logs

The Journalctl command in Linux provides a powerful and efficient solution for tailing service logs. By leveraging Journalctl, you can streamline log analysis, troubleshoot issues, and gain valuable insights into your system’s behavior.

  • 🔍 Efficient Log Storage and Retrieval: Journalctl utilizes a binary log format that optimizes storage space and enables fast, indexed access to logs. This ensures efficient disk usage and quick retrieval of log entries, saving time and resources during troubleshooting and analysis.
  • ⏱️ Real-time Log Monitoring: With Journalctl, you can tail logs in real-time, enabling you to monitor system activities and identify issues as they occur. By staying up-to-date with the latest log entries, you can promptly address errors, anomalies, or performance issues, minimizing potential downtime and optimizing system reliability.
  • 🔍 Flexible Filtering and Analysis: Journalctl provides advanced filtering options, allowing you to narrow down log output based on various criteria such as time range, log priority, and specific log fields. This flexibility empowers you to focus on relevant logs, extract meaningful insights, and perform targeted analysis, resulting in efficient troubleshooting and precise issue resolution.

3 Common Errors When Using Journalctl Command to Tail Service Logs

While the Journalctl command is a powerful tool for tailing service logs in Linux, it’s important to be aware of common errors that may occur during usage. Understanding these errors will help you troubleshoot and overcome potential issues, ensuring a smooth log analysis experience.

  • 🚫 Insufficient Permissions: One common error is encountering “Permission Denied” when attempting to access journal files. This typically occurs when running Journalctl as a non-root user without sufficient privileges. To resolve this, use the sudo command to run Journalctl with administrative privileges or configure appropriate permissions to access the journal files.
  • ⚠️ Incomplete Log Retrieval: In some cases, you may encounter situations where Journalctl fails to retrieve complete logs, resulting in missing or truncated entries. This can happen due to limited journal storage capacity or when logs rotate or get cleared. To address this, ensure sufficient disk space for journal storage and consider adjusting log rotation settings to retain desired log history.
  • Inaccurate Filtering Results: Incorrect filtering can lead to inaccurate or incomplete log output. This error can occur when specifying incorrect filters or using incorrect syntax. Double-check your filtering options, such as time range, log levels, or field values, to ensure they align with the intended criteria and syntax guidelines.

In a Nutshell

By utilizing the various methods available to use the Journalctl command to tail service logs, you can effectively analyze and troubleshoot Linux systems. However, it is crucial to be aware of common errors that may arise, such as insufficient permissions, incomplete log retrieval, and inaccurate filtering. By overcoming these challenges, you can unlock the benefits of efficient log storage and retrieval, real-time log monitoring, flexible filtering, and analysis.

To further enhance your Linux log analysis skills, consider exploring related topics, such as: Advanced systemd features for log rotation and retention, Best practices for interpreting and extracting insights from service logs, and Optimizing system performance through effective log monitoring and analysis. Remember, the journey to becoming an expert in Linux log management is an ongoing process. Continuously expand your knowledge and fine-tune your skills to maximize the potential of your Linux environment.

Frequently Asked Questions

How can I exclude certain log entries from the output?

To exclude specific log entries from the Journalctl output, you can utilize the --invert or --grep flags in combination with suitable filters. For instance, suppose you want to exclude logs containing the word debug. In that case, you can execute the following command: journalctl --invert --grep "debug" -f. By using the --invert flag, you invert the matching logic, displaying all log entries that do not contain the specified pattern. The --grep flag allows you to filter the logs based on a specific search pattern, such as excluding logs containing certain words or phrases. This technique provides fine-grained control over the log output, allowing you to focus on relevant information while excluding specific entries.

Can I save the Journalctl output to a file for future reference?

Absolutely! You can save the Journalctl output to a file for future reference by leveraging the output redirection feature in the Linux command line. To accomplish this, use the > operator to redirect the command’s output to your chosen file. For instance, if you want to save the Journalctl logs to a file named logs.txt, execute the following command: journalctl > logs.txt. This command directs the Journalctl output to the logs.txt file, which will be created (or overwritten if it already exists) in the current working directory. You can then access this file anytime to review the saved logs, perform further analysis, or share them with others. This functionality is particularly useful when you need to retain log data for auditing purposes or for referencing specific events in the future.

How do I filter logs from multiple units simultaneously?

To filter logs from multiple units simultaneously, you can use the --unit flag followed by a comma-separated list of unit names. For example, suppose you want to tail logs from both service1 and service2. In that case, execute the following command: journalctl -u service1,service2 -f. By specifying multiple unit names separated by commas after the -u flag, you instruct Journalctl to display logs related to all those units. This allows you to monitor and analyze logs from multiple services or units simultaneously in a single output stream. Whether you need to troubleshoot interdependent services or monitor multiple components simultaneously, this method ensures efficient log analysis and a comprehensive view of your system’s behavior.

Ojash

Author

Ojash is a skilled Linux expert and tech writer with over a decade of experience. He has extensive knowledge of Linux's file system, command-line interface, and software installations. Ojash is also an expert in shell scripting and automation, with experience in Bash, Python, and Perl. He has published numerous articles on Linux in various online publications, making him a valuable resource for both seasoned Linux users and beginners. Ojash is also an active member of the Linux community and participates in Linux forums.

Akshat

Reviewer

Akshat is a software engineer, product designer and the co-founder of Scrutify. He's an experienced Linux professional and the senior editor of this blog. He is also an open-source contributor to many projects on Github and has written several technical guides on Linux. Apart from that, he’s also actively sharing his ideas and tutorials on Medium and Attirer. As the editor of this blog, Akshat brings his wealth of knowledge and experience to provide readers with valuable insights and advice on a wide range of Linux-related topics.

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