06-17-2010
- A user without a login shell is a user that can't log into a system, and thus can't run any commands interactively on that system. Still, processes can run as that user.
- Imagine a server process running as root with a bug that a remote attacker can use to gain local access. Any processes spawned by that process, even those unwanted, will also run as root with full system access.
- See 2. It ain't physical security, obviously.
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WHO(1) BSD General Commands Manual WHO(1)
NAME
who -- display who is logged in
SYNOPSIS
who [-abdHlmqrstTuv] [file]
who am i
DESCRIPTION
The who utility displays a list of all users currently logged on, showing for each user the login name, tty name, the date and time of login,
and hostname if not local.
Available options:
-a Same as --bdlprTtuv.
-b Time of last system boot.
-d Print dead processes.
-H Write column headings above the regular output.
-l Print system login processes.
-m Only print information about the current terminal. This is the POSIX way of saying who am i.
-p Print active processes spawned by init(8).
-q ``Quick mode'': List only the names and the number of users currently logged on. When this option is used, all other options are
ignored.
-r Print the current runlevel. Supported runlevels are:
d (DEATH) The system has halted.
s (SINGLE_USER) The system is running in single user mode.
r (RUNCOM) The system is executing /etc/rc.
t (READ_TTYS) The system is processing /etc/ttys.
m (MULTI_USER) The system is running in multi-user mode.
T (CLEAN_TTYS) The system is in the process of stopping processes associated with terminal devices.
c (CATATONIA) The system is in the process of shutting down and will not create new processes.
-s List only the name, line and time fields. This is the default.
-T Print a character after the user name indicating the state of the terminal line: '+' if the terminal is writable; '-' if it is not; and
'?' if a bad line is encountered.
-t Print last system clock change.
-u Print the idle time for each user, and the associated process ID.
-v When printing of more information is requested with -u, this switch can be used to also printed process termination signals, process
exit status, session id for windowing and the type of the entry, see documentation of ut_type in getutxent(3).
am I Returns the invoker's real user name.
file By default, who gathers information from the file /var/run/utmpx. An alternative file may be specified which is usually /var/log/wtmpx
(or /var/log/wtmp, or /var/log/wtmpx.[0-6] or /var/log/wtmp.[0-6] depending on site policy as wtmpx can grow quite large and daily ver-
sions may or may not be kept around after compression by ac(8)). The wtmpx and wtmp file contains a record of every login, logout,
crash, shutdown and date change since wtmpx and wtmp were last truncated or created.
If /var/log/wtmpx or /var/log/wtmp are being used as the file, the user name may be empty or one of the special characters '|', '}' and '~'.
Logouts produce an output line without any user name. For more information on the special characters, see utmp(5).
FILES
/var/run/utmp
/var/run/utmpx
/var/log/wtmp
/var/log/wtmp.[0-6]
/var/log/wtmpx
/var/log/wtmpx.[0-6]
SEE ALSO
last(1), mesg(1), users(1), getuid(2), utmp(5), utmpx(5)
STANDARDS
The who utility is expected to conform to IEEE Std 1003.2-1992 (``POSIX.2'').
HISTORY
A who utility appeared in Version 6 AT&T UNIX.
BSD
January 17, 2007 BSD