passwd - password file
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
, which is readable by the superuser only.
If the encrypted password, whether in /etc/passwd
, 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+
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.)
If you create a new login, first put an asterisk (*) in the password field, then
(1) to set it.
Each line of the file describes a single user, and contains seven
The field are as follows:
- This is the user's login name. It should not contain capital letters.
- This is either the encrypted user password, an asterisk (*), or the letter
'x'. (See pwconv(8) for an explanation of 'x'.)
- The privileged root login account (superuser) has the user ID
- 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)).
- 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.
- 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.
- 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
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
existing processes and initiate new ones through rsh
(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