Another version of this page is provided by the shadow-utils project
PASSWD(5) Linux Programmer's Manual PASSWD(5)
passwd - password file
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 'x' 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 'x'. (See pwconv(8) for an
explanation of 'x'.)
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.
/etc/passwd
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).
chfn(1), chsh(1), login(1), passwd(1), su(1), crypt(3), getpwent(3),
getpwnam(3), group(5), shadow(5)
This page is part of release 4.12 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/.
Linux 2015-02-01 PASSWD(5)
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