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Log File


Definition: What Is a Log File?

In computer science a log file is a textual data file that stores events, processes, messages, and other data from applications, operating systems, or devices. They provide information based on the actions performed by users, playing an important role in monitoring IT environments. Not only can you detect if things are working as they should be, but also if the system/network has been compromised.

For example, you can use security logs to check successful or unsuccessful user logins, system logs to investigate server shutdowns, application logs for application failures, and much more. 

Why Are Log Files Important to Monitor?

Log files are important as they store valuable information which can be used to recreate past events, find security flaws, or troubleshoot. Here are the five most important reasons why you should monitor logs.

Better Performance

By monitoring log files, you can decrease downtime, minimize the risk of data loss, and access valuable information like the need for updates or areas where performance may be improved. For example, timestamps in logs show us the time between events. In addition, some logs (e.g., database query logs) provide latency information that can be used to determine where time is lost. 

Faster Troubleshooting

Log information can help determine what went wrong and troubleshoot when encountering an error. For example, if a service goes down, you can check the log files to see why it crashed. Was it because the memory ran out, or did it encounter an unhandled exception or something else?

Businesses employ log monitoring to avoid or rectify operating system errors, improve network observability, and offer transparency and insight into the computing environment.

Security Checks and Pentesting

Security experts use log files as a trustworthy source of audit information since they provide a complete history of system activity such as access attempts, command line input, changes to sensitive information, and more. If the network has been compromised, you can use log files to reverse engineer and go back into the kill chain to determine how someone gained access to the network. 

Understand User Behavior

Companies can use log monitoring to understand how users interact with their products (applications) by going over the log files and looking into user behavior. This helps developers understand the user’s needs better and optimize the application to fit them.

Data Analysis

The data from the log files is often transferred to a secure server that serves as a centralized logging point. Usually, before the system administrator can process it. However, log aggregation – i.e., putting all logs together in a datastore – is often not enough. For instance, when you want to build visualizations, like a world map with countries that target your website. That’s where log management software comes into play as they allow you to easily collect, parse, and analyze log files.

Who Uses Log Files?

Different professionals use log files differently. Here are some examples of the types of professionals and how they use log files.

  • ITOps (Information Technology Operations) uses log files to examine the health of a company’s IT infrastructure, helping them handle job load, reduce downtime, keep operations running smoothly, and reduce financial and operational risk.

  • DevOps engineers utilize log files to keep an eye on CI/CD, keep applications operating smoothly, discover issues before they create downtime, and improve performance.
  • DevSecOps employs log files to create shared ownership of application development and security, saving time and money and reducing risks by discovering possible vulnerabilities before release.
  • White hat hackers and security researchers analyze log files for attack “who,” “when,” and “where” information, which they then use to spot anomalies in blocked and authorized traffic flows.
  • IT analysts use log files for compliance control and reporting on operational expenditures (OpEx) and capital expenditures (CapEx).

Different Types of Log Files

Log files provide a comprehensive history of occurrences throughout time, making them an invaluable resource for security and monitoring purposes. Applications and web browsers use logs in addition to operating system components. Below are some known examples of types of log files in use.

Web Server Logs

Web server log files produced by web applications like Apache and NGINX give an unfiltered picture of website traffic. Web logs include information like “who” visited your website (IP address) and “which” pages were browsed (URLs). You can also identify spider traps, spam content dumped by hackers, broken external links, incorrect server responses, and exploit attempts.

Network Logs

Switches, routers, firewalls, VPN concentrators, and other devices linked to the networking infrastructure provide various logs based on their network activity.  A network log can provide information on failed user login attempts, discover unauthorized attempts to run processes or access locked information, and much more.  

Application Logs

Application log files are records of activities logged by software applications. You can use them for troubleshooting, diagnosis, and auditing giving you a wealth of information about an application’s performance, for example disk space warnings, completed operations, issues that prohibit the application from starting, successful login audit, and login failure audit.

Container Logs

Applications running in containers typically log to stdout or stderr, and you can usually capture that. For example, to capture Docker container logs, you can configure a logging driver to send logged stdout or stderr messages to a remote destination. Container logs can be either all plain text or all JSON files.

System Logs

System log files, also referred to as “server logs”, include detailed information logs about the OS, file system, running applications, and login credentials. They enable administrators to determine if system processes are loading correctly or if there are any problems, such as system errors, warnings, startup messages, system modifications, unexpected shutdowns, etc.

Security Logs

Many devices maintain security log data that lets you see what kinds of network traffic are allowed or denied on your network. For example, audit logs and access controls can help identify suspicious users misusing their access privileges. And possibly prevent a potential brute force attack. 

Another example is authentication log files capturing user attempts to access a network resource. This helps debug access issues and alter authentication policies. It also logs high-level security events for auditing purposes.

Security logs are often a subset of system logs recording events that are particular to the security and safety of your IT infrastructure.

Examples of Log Files Generated by Operating Systems

Log files keep track of all the events generated by an operating system, helping developers of both software and hardware have an easier time troubleshooting. 


Windows log types in the Event viewer are categorized into five folders for easier access.

  • Application events log: Log files generated by each application that writes to Windows Event Log. 
  • Security log:  Logs related to security, such as unsuccessful logins, authentication failures, file deletion, password changes and more. Typically written by Windows itself.
  • Setup log: Windows Setup logs store all the actions during the installation of Windows components and help troubleshoot any installation problems.
  • System log: System logs record operating system events such as driver faults, sign-ins, sign-outs, and other activities.
  • Forwarded events logs: Windows allows events to forward from one host to another and be monitored in case the system is compromised or data is lost at the source. It also helps create aggregate log files to help with log analysis. 


Linux logs are time stamped records of what the server, kernel, services, and applications are doing. In Linux, logs come from different sources, such as systemd journal, Linux kernel, syslog, audit logs, and non-system application logs.  

Linux kernel only writes its activity in a buffer that can be picked up by the syslog or journal daemon. While other services that are often associated with Linux, such as DNS client or even DNS server, typically go to the systemd journal or syslog. 

Which Directory Typically Contains Log Files?

The location of your log files will vary depending on the OS you use. Here’s how you can access them:

*nix Systems

This includes all Unix- and Linux-based operating systems, such as Ubuntu, Debian, RedHat, Fedora, macOS, BSD, etc.

  • Most Linux system log files are in the /var/log directory and sub-directories.
  • Most Linux distros also use journald for system logging.
  • On MacOS, log files can be viewed using the console app, which is the Mac equivalent of the Event Viewer for Windows. Alternatively, Mac application logs are kept in /Library/Logs, while Mac system logs are kept in /var/log.


The Event Viewer in Windows allows users to access log files. 

Log File Monitoring with Sematext

Sematext Logs is a log monitoring solution that allows you to collect and analyze logs from various sources across your infrastructure in a single pane of glass. It auto-discovers logs, which enables you to automatically start forwarding and monitoring logs directly through the user interface.

It comes with pre-built dashboards and alerts, which saves you a ton of time, but you can also build custom dashboards that connect logs, metrics, and infrastructure data for real-time system visibility and you can set up custom alerts to be notified about things that matter to you and your team.

Frequently Asked Questions

What is in a log file?

The exact content of a log file can vary depending on the specific system or application being logged. However, in general, a log file will contain a timestamp indicating when each event occurred, as well as information about the event itself.

How do I open a log file?

To open a log file, you can use a text editor to open the file directly, or use a log viewer built into the software application you’re using. You can also use a command-line interface to view the file or use a specialized log analysis tool for more complex log files. The specific method will depend on the operating system, software, and format of the log file.

Where are the log files?

The location of log files can vary depending on the specific operating system and application being used. In general, log files are often stored in a subdirectory within the application’s installation directory or in a system directory such as /var/log on Linux or macOS. Some applications may allow you to configure the location of log files or provide options to view them within the application’s interface. It’s also possible to search for log files using the operating system’s built-in search functionality or by using a command-line interface to search for log files.

What is log file used for?

Log files are used to record events or activities that occur within a computer system, application, or program. They serve as a detailed record of what has happened within a system, which can be used for troubleshooting problems or investigating security incidents. Log files can also provide insights into the performance of a system or application, allowing administrators to identify bottlenecks or areas where optimization is needed. Additionally, log files can be used to track user activity or audit compliance with regulatory requirements.

What format is a log file?

The forma of a log file depends on the specific system or application that generated them. Some common formats include plain text files, CSV files, XML files, and JSON files. Text-based log files are among the most common and are human-readable, making them easy to view and search. Other formats like CSV, XML, and JSON are machine-readable and can be easily parsed and processed by other software applications.

What is the difference between log file and data file?

Log files and data files serve different purposes in computing. A log file is used to record events or activities that occur within a system or application, while a data file is used to store data that an application needs to access or manipulate. Log files are typically used for troubleshooting or auditing purposes, while data files are used for storing information needed by the application to function.

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