PASSWD
Section: Linux Programmer's Manual (5)
Updated: 2015-02-01
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NAME
passwd - password file
DESCRIPTION
The
/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
ls(1)
use it to map user IDs to usernames), but write access only for the
superuser.
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
/etc/passwd
has an aqxaq character in the password field,
and the encrypted passwords are in
/etc/shadow,
which is readable by the superuser only.
If the encrypted password, whether in
/etc/passwd
or in
/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).
If the encrypted password in
/etc/passwd
is "*NP*" (without the quotes),
the shadow record should be obtained from an NIS+ server.
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 him- or herself using a
password.
(But see NOTES below.)
If you create a new login, first put an asterisk (*) in the password field,
then use
passwd(1)
to set it.
Each line of the file describes a single user,
and contains seven colon-separated fields:
name:password:UID:GID:GECOS:directory:shell
The field are as follows:
- name
-
This is the user's login name.
It should not contain capital letters.
- password
-
This is either the encrypted user password,
an asterisk (*), or the letter aqxaq.
(See
pwconv(8)
for an explanation of aqxaq.)
- UID
-
The privileged
root
login account (superuser) has the user ID 0.
- GID
-
This is the numeric primary group ID for this user.
(Additional groups for the user are defined in the system group file; see
group(5)).
- 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,
finger(1))
display information from this field.
-
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."
- 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
HOME
environment variable.
- shell
-
This is the program to run at login (if empty, use
/bin/sh).
If set to a nonexistent executable, the user will be unable to login
through
login(1).
The value in this field is used to set the
SHELL
environment variable.
FILES
/etc/passwd
NOTES
If you want to create user groups, there must be an entry in
/etc/group,
or no group will exist.
If the encrypted password is set to an asterisk (*), the user will be unable
to login using
login(1),
but may still login using
rlogin(1),
run existing processes and initiate new ones through
rsh(1),
cron(8),
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
su(1).
SEE ALSO
chfn(1),
chsh(1),
login(1),
passwd(1),
su(1),
crypt(3),
getpwent(3),
getpwnam(3),
group(5),
shadow(5)
COLOPHON
This page is part of release 4.13 of the Linux
man-pages
project.
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- DESCRIPTION
-
- FILES
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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