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who
The `who` command
Developer(s)	AT&T Bell Laboratories
Initial release	November 3, 1971; 52 years ago (1971-11-03)
Operating system	Multics, Unix, Unix-like, Plan 9
Platform	Cross-platform
Type	Command
License	coreutils: GPLv3+

The standard Unix command who displays a list of users who are currently logged into the computer.

The who command is related to the command w, which provides the same information but also displays additional data and statistics.

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History

A who command that displays the names of users logged in was first implemented within Multics.^[1] Later, it appeared in Version 1 Unix^[2] and became part of the X/Open Portability Guide since issue 2 of 1987. It was inherited into the first version of POSIX.1 and the Single Unix Specification.^[3]

The version of who bundled in GNU coreutils was written by Joseph Arceneaux, David MacKenzie, and Michael Stone.^[4]

Specification

The Single UNIX Specification (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 who.

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 tty 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 processes 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 utility

-r, show runlevel of the init 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 operating systems may add extra options. GNU who 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 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

^ Multics Commands
^ who(1) – FreeBSD General Commands Manual
^ who – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group
^ who(1) – Linux User Manual – User Commands

External links

The Wikibook Guide to Unix has a page on the topic of: Commands

who – Shell and Utilities Reference, The Single UNIX Specification, Version 4 from The Open Group
who — manual page from GNU coreutils
who(1) – OpenBSD General Commands Manual
who(1) – Plan 9 Programmer's Manual, Volume 1

v t e Unix command-line interface programs and shell builtins
File system	cat chattr chmod chown chgrp cksum cmp cp dd du df file fuser ln ls mkdir mv pax pwd rm rmdir split tee touch type umask
Processes	at bg crontab fg kill nice ps time
User environment	env exit logname mesg talk tput uname who write
Text processing	awk basename comm csplit cut diff dirname ed ex fold head iconv join m4 more nl paste patch printf read sed sort strings tail tr troff uniq vi wc xargs
Shell builtins	alias cd echo test unset wait
Searching	find grep
Documentation	man
Software development	ar ctags lex make nm strip yacc
Miscellaneous	bc cal expr lp od sleep true and false
Categories Standard Unix programs Unix SUS2008 utilities List

v t e Plan 9 command-line interface programs and shell builtins
File system	chmod chgrp cmp cp dd du file gzip ls mkdir pwd rm split tee touch
Processes	kill ps
User environment	passwd who
Text processing	awk basename comm diff ed eqn join sed sort spell strings tail tr troff uniq wc
Shell builtins	echo test
Networking	ip/ipconfig ip/ping netstat
Searching	grep
Software development	ar hoc lex nm strip yacc
Miscellaneous	bc cal fortune sleep
Category

v t e GNU Core Utilities command-line interface programs
File system	chcon chmod chown chgrp cksum cp dd df dir dircolors install ln ls mkdir mkfifo mknod mktemp mv realpath rm rmdir shred sync touch truncate vdir
Text utilities	b2sum base32 base64 cat cksum comm csplit cut expand fmt fold head join md5sum nl numfmt od paste ptx pr sha1sum shuf sort split sum tac tail tr tsort unexpand uniq wc
Shell utilities	arch basename chroot date dirname du echo env expr factor false groups hostid id link logname nice nohup nproc pathchk pinky printenv printf pwd readlink runcon seq sleep stat stdbuf stty tee test timeout true tty uname unlink uptime users who whoami yes

This page was last edited on 7 April 2023, at 11:30

who (Unix)

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