passwd man page on CentOS

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PASSWD(5)		   Linux Programmer's Manual		     PASSWD(5)

       passwd - password file

       Passwd  is  a text file, that contains a list of the system's accounts,
       giving for each account some useful information like user ID, group ID,
       home  directory,	 shell,	 etc.	Often,	it also contains the encrypted
       passwords for each account.  It should  have  general  read  permission
       (many  utilities, like ls(1) use it to map user IDs to user names), 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 asterisks (*) instead of encrypted passwords, and
       the encrypted passwords are in /etc/shadow which	 is  readable  by  the
       superuser only.

       Regardless  of whether shadow passwords are used, many sysadmins 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 the
       Notes below.)

       If you create a new login, first put an asterisk in the password field,
       then use passwd(1) to set it.

       There is one entry per line, and each line has the format:


       The field descriptions are:

	      account	the  name  of  the  user on the system.	 It should not
			contain capital letters.

	      password	the encrypted user password, an asterisk (*),  or  the
			letter	'x'.   (See  pwconv(8)	for  an explanation of

	      UID	the numerical user ID.

	      GID	the numerical primary group ID for this user.

	      GECOS	This field is optional and only used for informational
			purposes.   Usually,  it  contains the full user name.
			GECOS means General Electric  Comprehensive  Operating
			System, which has been renamed to GCOS when GE's large
			systems	 division  was	sold  to  Honeywell.	Dennis
			Ritchie	 has reported: "Sometimes we sent printer out‐
			put or batch jobs to the GCOS machine.	The gcos field
			in the password file was a place to stash the informa‐
			tion for the $IDENTcard.  Not elegant."

	      directory the user's $HOME directory.

	      shell	the program to run at login (if empty,	use  /bin/sh).
			If  set to a non-existing executable, the user will be
			unable to login through login(1).

       If you want to create user groups, their GIDs must be equal  and	 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(1),
       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), group(5), shadow(5)

File formats			  1998-01-05			     PASSWD(5)

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