The gecos field, or GECOS field is an entry in the /etc/passwd file on Unix

Unix

Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

, and similar operating system

Operating system

An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...

s.

It is typically used to record general information about the account or its user(s) such as their real name and phone number.

Format

The typical format for the GECOS field is a comma-delimited list with this order:

1. User's full name (or application name, if the account is for a program)
2. Building and room number or contact person
3. Office telephone number
   Telephone number
   A telephone number or phone number is a sequence of digits used to call from one telephone line to another in a public switched telephone network. When telephone numbers were invented, they were short — as few as one, two or three digits — and were given orally to a switchboard operator...
   
   
4. Any other contact information (pager
   Pager
   A pager is a simple personal telecommunications device for short messages. A one-way numeric pager can only receive a message consisting of a few digits, typically a phone number that the user is then requested to call...
   
    number, fax
   Fax
   Fax , sometimes called telecopying, is the telephonic transmission of scanned printed material , normally to a telephone number connected to a printer or other output device...
   
   , etc.)

In most UNIX systems this informations can be changed by normal users using the chfn command from the shell

History

Some early Unix systems at Bell Labs used GECOS

General Comprehensive Operating System

General Comprehensive Operating System is a family of operating systems oriented toward mainframe computers.The original version of GCOS was developed by General Electric from 1962; originally called GECOS...

 machines for print spooling and various other services, so this field was added to carry information on a user's GECOS identity.