The standard

Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...

command who displays a list of users who are currently logged into the computer. The who command is related to the command , which provides the same information but also displays additional data and statistics.

History

A command that displays the names of users logged in was first implemented for the CTSS operating system by ''Noel Morris'' in 1965. This inspired ''Tom Van Vleck'' to write a

Multics Multics ("MULTiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of t ...

version. Later, it appeared in Version 1 Unix and became part of the

X/Open X/Open group (also known as the Open Group for Unix Systems and incorporated in 1987 as X/Open Company, Ltd.) was a consortium founded by several European UNIX systems manufacturers in 1984 to identify and promote open standards in the field of info ...

Portability Guide since issue 2 of 1987. It was inherited by the first version of POSIX.1 and the

Single Unix Specification The Single UNIX Specification (SUS) is a standard for computer operating systems, compliance with which is required to qualify for using the "UNIX" trademark. The standard specifies programming interfaces for the C language, a command-line shell, ...

. The existence of presupposes the existence of an online group of users in a shared collegial environment with a need to communicate with each other. The version of who bundled in GNU

coreutils The GNU Core Utilities or coreutils is a collection of GNU software that implements many standard, Unix-based shell commands. The utilities generally provide POSIX compliant interface when the environment variable is set, but otherwise offers ...

was written by Joseph Arceneaux, David MacKenzie, and Michael Stone.

Specification

The

Single UNIX Specification The Single UNIX Specification (SUS) is a standard for computer operating systems, compliance with which is required to qualify for using the "UNIX" trademark. The standard specifies programming interfaces for the C language, a command-line shell, ...

(SUS) specifies that who should list information about accessible users. The XSI extension also specifies that the data of the username, terminal, login time, process ID, and time since last activity occurred on the terminal, furthermore, an alternate system database used for user information can be specified as an optional argument to . The command can be invoked with the arguments am i or am I (so it is invoked as who am i or who am I), showing information about the current terminal only (see the command and the -m option below, of which this invocation is equivalent).

Usage

The SUS without extensions only specifies the following -m, -T, and -u options, all other options are specified in the XSI extension. :-a, process the system database used for user information with the -b, -d, -l, -p, -r, -t, -T and -u. :-b, show time when system was last rebooted :-d, show

zombie process On Unix and Unix-like computer operating systems, a zombie process or defunct process is a process that has completed execution (via the exit system call) but still has an entry in the process table: it is a process in the " terminated stat ...

es and details :-H, show column headers :-l, show terminals where a user can log in :-m, show information about the current terminal only :-p, show active processes :-q, quick format, show only names and the number of all users logged on, disables all other options; equivalent to users

command line A command-line interface (CLI) is a means of interacting with software via command (computing), commands each formatted as a line of text. Command-line interfaces emerged in the mid-1960s, on computer terminals, as an interactive and more user ...

utility :-r, show runlevel of the

init In Unix-based computer operating systems, init (short for ''initialization'') is the first process started during booting of the operating system. Init is a daemon process that continues running until the system is shut down. It is the direc ...

process. :-s, (default) show only name, terminal, and time details :-t, show when system clock was last changed :-T, show details of each terminal in a standard format (see note in Examples section) :-u, show idle time; XSI shows users logged in and displays information whether the terminal has been used recently or not Other Unix and

Unix-like A Unix-like (sometimes referred to as UN*X, *nix or *NIX) operating system is one that behaves in a manner similar to a Unix system, although not necessarily conforming to or being certified to any version of the Single UNIX Specification. A Uni ...

operating systems may add extra options. GNU includes a -i option behaving similarly to -u and a -w option displaying whether the user listed accepts messages (the SUS displays this when -T is specified), yet GNU who and

BSD The Berkeley Software Distribution (BSD), also known as Berkeley Unix or BSD Unix, is a discontinued Unix operating system developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Berkeley, beginni ...

who both omit a number of the above options (such as -a, -b, -d, and others); GNU who instead uses -l to perform DNS lookups on hostnames listed.

Output

The SUS without extensions specifies that the output format is to be "implementation-defined". The XSI extension specifies a format, but notes that it is not fully specified; delimiters and field lengths are not precisely specified. Thus, the format of the output differs considerably among Unix implementations.

References

External links

*
who
—

manual page A man page (short for manual page) is a form of software documentation found on Unix and Unix-like operating systems. Topics covered include programs, system libraries, system calls, and sometimes local system details. The local host administra ...

from GNU