hypot.3: srcfix: add some notes about the underflow case

[thirdparty/man-pages.git] / man5 / passwd.5

.\" Copyright (c) 1993 Michael Haardt (michael@moria.de),
.\"     Fri Apr  2 11:32:09 MET DST 1993
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.\" Modified Sun Jul 25 10:46:28 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Sun Aug 21 18:12:27 1994 by Rik Faith (faith@cs.unc.edu)
.\" Modified Sun Jun 18 01:53:57 1995 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Mon Jan  5 20:24:40 MET 1998 by Michael Haardt
.\"  (michael@cantor.informatik.rwth-aachen.de)
.TH PASSWD 5 2018-04-30 "Linux" "Linux Programmer's Manual"
.SH NAME
passwd \- password file
.SH DESCRIPTION
The
.IR /etc/passwd
file is a text file that describes user login accounts for the system.
It should have read permission allowed for all users (many utilities, like
.BR ls (1)
use it to map user IDs to usernames), but write access only for the
superuser.
.PP
In the good old days there was no great problem with this general
read permission.
Everybody could read the encrypted passwords, but the
hardware was too slow to crack a well-chosen password, and moreover the
basic assumption used to be that of a friendly user-community.
These days many people run some version of the shadow password suite, where
.I /etc/passwd
has an \(aqx\(aq character in the password field,
and the encrypted passwords are in
.IR /etc/shadow ,
which is readable by the superuser only.
.PP
If the encrypted password, whether in
.I /etc/passwd
or in
.IR /etc/shadow ,
is an empty string, login is allowed without even asking for a password.
Note that this functionality may be intentionally disabled in applications,
or configurable (for example using the "nullok" or "nonull" arguments to
pam_unix.so).
.PP
If the encrypted password in
.I /etc/passwd
is "\fI*NP*\fP" (without the quotes),
the shadow record should be obtained from an NIS+ server.
.PP
Regardless of whether shadow passwords are used, many system administrators
use an asterisk (*) in the encrypted password field to make sure
that this user can not authenticate themself using a
password.
(But see NOTES below.)
.PP
If you create a new login, first put an asterisk (*) in the password field,
then use
.BR passwd (1)
to set it.
.PP
Each line of the file describes a single user,
and contains seven colon-separated fields:
.PP
.in +4n
.EX
name:password:UID:GID:GECOS:directory:shell
.EE
.in
.PP
The field are as follows:
.TP 12
.I name
This is the user's login name.
It should not contain capital letters.
.TP
.I password
This is either the encrypted user password,
an asterisk (*), or the letter \(aqx\(aq.
(See
.BR pwconv (8)
for an explanation of \(aqx\(aq.)
.TP
.I UID
The privileged
.I root
login account (superuser) has the user ID 0.
.TP
.I GID
This is the numeric primary group ID for this user.
(Additional groups for the user are defined in the system group file; see
.BR group (5)).
.TP
.I GECOS
This field (sometimes called the "comment field")
is optional and used only for informational purposes.
Usually, it contains the full username.
Some programs (for example,
.BR finger (1))
display information from this field.
.IP
GECOS stands for "General Electric Comprehensive Operating System",
which was renamed to GCOS when
GE's large systems division was sold to Honeywell.
Dennis Ritchie has reported: "Sometimes we sent printer output or
batch jobs to the GCOS machine.
The gcos field in the password file was a place to stash the
information for the $IDENTcard.
Not elegant."
.TP
.I directory
This is the user's home directory:
the initial directory where the user is placed after logging in.
The value in this field is used to set the
.B HOME
environment variable.
.TP
.I shell
This is the program to run at login (if empty, use
.IR /bin/sh ).
If set to a nonexistent executable, the user will be unable to login
through
.BR login (1).
The value in this field is used to set the
.B SHELL
environment variable.
.SH FILES
.I /etc/passwd
.SH NOTES
If you want to create user groups, there must be an entry in
.IR /etc/group ,
or no group will exist.
.PP
If the encrypted password is set to an asterisk (*), the user will be unable
to login using
.BR login (1),
but may still login using
.BR rlogin (1),
run existing processes and initiate new ones through
.BR rsh (1),
.BR cron (8),
.BR at (1),
or mail filters, etc.
Trying to lock an account by simply changing the
shell field yields the same result and additionally allows the use of
.BR su (1).
.SH SEE ALSO
.BR chfn (1),
.BR chsh (1),
.BR login (1),
.BR passwd (1),
.BR su (1),
.BR crypt (3),
.BR getpwent (3),
.BR getpwnam (3),
.BR group (5),
.BR shadow (5),
.BR vipw (8)