What is a log file?

A log file, also called a log file or log file, is a file in which computer processes log various events. Logfiles are important sources of information to make the processes on a system traceable. For example, they can be used for problem analysis or the reconstruction of lost data

The log file, often referred to as the log file or log file, is a file in which events on computer systems or networks are logged. They are a useful tool to document or archive the many different processes in a traceable way. Log files can be used, for example, as a source of information for error analysis when a system crashes or to investigate user behavior. Often it is also possible to use log files to reconstruct lost data. Log files are automatically created, filled and continuously stored on the system. Logfiles are often used in the web environment to investigate the access to web pages and to obtain information about the visitors of web pages.

Other typical areas of application for log files are transaction logging in databases, event logs of operating systems or the logging of network events on a network computer. The evaluation of log files is called log file analysis.

In which areas are log files generated?

Many different processes of common operating systems such as Windows, MacOS or Linux continuously generate log files on the computer, unnoticed by the user. Examples are the system logbook of Linux or the event viewer of Windows. Among other things, user logins, process events or error messages from software and hardware are recorded in these log files.

Other programs that generate log files are e-mail servers, firewalls, virus scanners or installation routines. Web servers or FTP servers also continuously store log files in which information about users and data accesses are logged.

Purpose and use of log files

A log file can serve different purposes. Usually log files are used for the following actions:

  • Problem analysis of computer systems (investigation of a computer crash)
  • Problem analysis in networks
  • Traceability of data transactions
  • Reconstruction of lost data
  • Investigation of data accesses
  • Investigation of the user behaviour
  • Logging of security relevant events
  • Fulfilment of compliance requirements

In the web environment, search engine optimization (SEO) often relies on log files. The stored data provide clues about the origin of visitors and the pages called up on an Internet presence. This information can be used to optimize the website in terms of user-friendliness and information content. Other information that can be obtained from the log files of a web server are website performance data, accessibility information and error data.

Structure, content and information elements of a log file

Usually log files are structured line-oriented. Each event fills one line of the log file and begins or ends with the time and date (timestamp) of the entry. Since log files are continuously updated, the entries are usually in chronological order. The newest entry is at the top of the list, the oldest entry at the bottom. To prevent log files from growing in size, many recording processes limit the maximum number of entries. When the maximum size is reached, the oldest entries are overwritten. Another important information often found in every line of a log file is a rating of the importance of the recorded event. This classification makes it easier to search log files for relevant information.

Common categories for the importance of events are

  • Debug or Trace: Information about the program flow
  • Information: Information about normal transactions of a process
  • Warning: Important information about transactions that may lead to errors or security events
  • Error: application or process errors that have occurred
  • critical or fatal error: error that has occurred that leads to a crash or termination of a process
  • Besides timestamps and classification of the importance of events, the following information is often found in log files:
  • Name of the event that occurred with any additional information
  • State of the system at the time of the event
  • executed command
  • affected user of the event
  • IP addresses of users or associated processes
  • affected files and file paths
  • used transmission protocols
  • accessed URLs
  • database transaction performed
  • data volume transferred
  • the operating system or browser versions used by the user
  • current screen resolution

The different formats of log files

In many cases, log files are simple text files that use character encodings like ASCII. These log files can be called, read and edited with any word processing program. However, there are also standardised log file formats such as CLF (Common Log Format), ELF (Extended Log Format) or NCSA, which are also based on text data. In the various formats, the arrangement and the individual information elements of the log file events are defined. This standardization allows a more efficient evaluation of log files with special analysis tools. Especially in the web server environment, standardized formats are often used for logging and analyzing web traffic.