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Linux in a Nutshell

This directory of Linux commands is from Linux in a Nutshell, 5th Edition.

Click on any of the 687 commands below to get a description and list of available options. All links in the command summaries point to the online version of the book on Safari Bookshelf.

Buy it now, or read it online on Safari Bookshelf.


who [options] [file] who am i

Show who is logged into the system. With no options, list the names of users currently logged in, their terminal, the time they have been logged in, and the name of the host from which they have logged in. An optional system file (default is /etc/utmp) can be supplied to give additional information.


-a, --all

Equivalent to -b -d --login -p -r -t -T -u.

am i

Print the username of the invoking user.

-b, --boot

Print time of last system boot.

-d, --dead

Print a list of dead processes.


Print a help message and then exit.

-i, --idle

Include idle times. An idle time of . indicates activity within the last minute; one of old indicates no activity in more than a day.

-l, --login

Print list of system login processes.


Attempt to include canonical hostnames via DNS.


Same as who am i.

-p, --process

Print active processes spawned by init.

-q, --count

"Quick." Display only the usernames and total number of users.

-r, --runlevel

Print the current runlevel.

-s, --short

Print only name, line, and time. This is the default behaviour.

-t, --time

Print the last system clock change.

-u, --users

Print a list of the users who are logged in.


Print version information and then exit.

-w, -T, --mesg, --message, --writable

Include user's message status in the output:


mesg y (write messages allowed)


mesg n (write messages refused)


Cannot find terminal device

-H, --heading

Print headings.


This sample output was produced at 8 a.m. on April 17:

$ who -uH NAME LINE TIME IDLE PID COMMENTS Earvin ttyp3 Apr 16 08:14 16:25 2240 Larry ttyp0 Apr 17 07:33 . 15182

Since Earvin has been idle since yesterday afternoon (16 hours), it appears that he isn't at work yet. He simply left himself logged in. Larry's terminal is currently in use.

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