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Linux 2.6 - man page for passwd (linux section 5)

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.  Every-
       body 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-com-
       munity.	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  aster-
       isk  (*)  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:


       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.   Den-
		   nis	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.


       If  you want to create user groups, there must be an entry in /etc/group, or no group will

       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).

       login(1), passwd(1), su(1), getpwent(3), getpwnam(3), crypt(3), group(5), shadow(5)

       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

Linux					    2012-05-03					PASSWD(5)

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