For computer log management, the Common Log Format, also known as the NCSA Common log format, (after NCSA HTTPd) is a standardized text file format used by web servers when generating server log files. Because the format is standardized, the files can be readily analyzed by a variety of web analysis programs, for example Webalizer and Analog. Each line in a file stored in the Common Log Format has the following syntax: host ident authuser date request status bytes The format is extended by the Combined Log Format with referer and user-agent fields.

Example

127.0.0.1 user-identifier frank [10/Oct/2000:13:55:36 -0700] "GET /apache_pb.gif HTTP/1.0" 200 2326 A dash (-) in a field indicates missing data.

Usage

Log files are a standard tool for computer systems developers and administrators. They record the "what happened, when, by whom" of the system. This information can record faults and help their diagnosis. It can identify security breaches and other computer misuse. It can be used for auditing. It can be used for accounting purposes. The information stored is only available for later analysis if it is stored in a form that can be analysed. This data can be structured in many ways for analysis. For example, storing it in a relational database would force the data into a query-able format. However, it would also make it more difficult to retrieve if the computer crashed, and logging would not be available unless the database was available. A plain text format minimises dependencies on other system processes, and assists logging at all phases of computer operation, including start-up and shut-down, where such processes might be unavailable.