This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
who [-mTu] [-abdHlprt] [file] who [-mu] -s [-bHlprt] [file] who -q [file] who am i who am I
The who utility shall list various pieces of information about accessible users. The domain of accessibility is implementation-defined.
Based on the options given, who can also list the user's name, terminal line, login time, elapsed time since activity occurred on the line, and the process ID of the command interpreter for each current system user.
The who utility shall conform to the Base Definitions volume of POSIX.1-2017, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported. The metavariables, such as <line>, refer to fields described in the Stdout section.
Process the implementation-defined database or named file with the -b, -d, -l, -p, -r, -t, -T and -u options turned on.
Write the time and date of the last system reboot. The system reboot time is the time at which the implementation is able to commence running processes.
Write a list of all processes that have expired and not been respawned by the init system process. The <exit> field shall appear for dead processes and contain the termination and exit values of the dead process. This can be useful in determining why a process terminated.
Write column headings above the regular output.
(The letter ell.) List only those lines on which the system is waiting for someone to login. The <name> field shall be LOGIN in such cases. Other fields shall be the same as for user entries except that the <state> field does not exist.
Output only information about the current terminal.
List any other process that is currently active and has been previously spawned by init.
(Quick.) List only the names and the number of users currently logged on. When this option is used, all other options shall be ignored.
Write the current run-level of the init process.
List only the <name>, <line>, and <time> fields. This is the default case.
Indicate the last change to the system clock.
Show the state of each terminal, as described in the Stdout section.
Write “idle time” for each displayed user in addition to any other information. The idle time is the time since any activity occurred on the user's terminal. The method of determining this is unspecified. This option shall list only those users who are currently logged in. The <name> is the user's login name. The <line> is the name of the line as found in the directory /dev. The <time> is the time that the user logged in. The <activity> is the number of hours and minutes since activity last occurred on that particular line. A dot indicates that the terminal has seen activity in the last minute and is therefore “current”. If more than twenty-four hours have elapsed or the line has not been used since boot time, the entry shall be marked <old>. This field is useful when trying to determine whether a person is working at the terminal or not. The <pid> is the process ID of the user's login process.
The following operands shall be supported:
- am i, am I
In the POSIX locale, limit the output to describing the invoking user, equivalent to the -m option. The am and i or I must be separate arguments.
Specify a pathname of a file to substitute for the implementation-defined database of logged-on users that who uses by default.
The following environment variables shall affect the execution of who:
Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of POSIX.1-2017, Section 8.2, Internationalization Variables for the precedence of internationalization variables used to determine the values of locale categories.)
If set to a non-empty string value, override the values of all the other internationalization variables.
Determine the locale for the interpretation of sequences of bytes of text data as characters (for example, single-byte as opposed to multi-byte characters in arguments).
Determine the locale that should be used to affect the format and contents of diagnostic messages written to standard error.
Determine the locale used for the format and contents of the date and time strings.
Determine the location of message catalogs for the processing of LC_MESSAGES.
Determine the timezone used when writing date and time information. If TZ is unset or null, an unspecified default timezone shall be used.
The who utility shall write its default format to the standard output in an implementation-defined format, subject only to the requirement of containing the information described above.
XSI-conformant systems shall write the default information to the standard output in the following general format:
For the -b option, <line> shall be "systemboot". The <name> is unspecified.
The following format shall be used for the -T option:
"%s %c %s %s\n" <name>, <terminal state>, <terminal name>, <time of login>
where <terminal state> is one of the following characters:
The terminal allows write access to other users.
The terminal denies write access to other users.
The terminal write-access state cannot be determined.
This entry is not associated with a terminal.
In the POSIX locale, the <time of login> shall be equivalent in format to the output of:
date +"%b %e %H:%M"
If the -u option is used with -T, the idle time shall be added to the end of the previous format in an unspecified format.
The standard error shall be used only for diagnostic messages.
The following exit values shall be returned:
An error occurred.
Consequences of Errors
The following sections are informative.
The name init used for the system process is the most commonly used on historical systems, but it may vary.
The “domain of accessibility” referred to is a broad concept that permits interpretation either on a very secure basis or even to allow a network-wide implementation like the historical rwho.
Due to differences between historical implementations, the base options provided were a compromise to allow users to work with those functions. The standard developers also considered removing all the options, but felt that these options offered users valuable functionality. Additional options to match historical systems are available on XSI-conformant systems.
It is recognized that the who command may be of limited usefulness, especially in a multi-level secure environment. The standard developers considered, however, that having some standard method of determining the “accessibility” of other users would aid user portability.
No format was specified for the default who output for systems not supporting the XSI option. In such a user-oriented command, designed only for human use, this was not considered to be a deficiency.
The format of the terminal name is unspecified, but the descriptions of ps, talk, and write require that they use the same format.
It is acceptable for an implementation to produce no output for an invocation of who mil.
The Base Definitions volume of POSIX.1-2017, Chapter 8, Environment Variables, Section 12.2, Utility Syntax Guidelines
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
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id(1p), logname(1p), talk(1p), write(1p).