check Linux password from /etc/shadow and C language

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LEARN ABOUT LINUX

passwd

PASSWD(5)						   File Formats and Conversions 						 PASSWD(5)

NAME

       passwd - the password file

DESCRIPTION

       /etc/passwd contains one line for each user account, with seven fields delimited by colons (":"). These fields are:

       o   login name

       o   optional encrypted password

       o   numerical user ID

       o   numerical group ID

       o   user name or comment field

       o   user home directory

       o   optional user command interpreter

       The encrypted password field may be blank, in which case no password is required to authenticate as the specified login name. However, some
       applications which read the /etc/passwd file may decide not to permit any access at all if the password field is blank. If the password
       field is a lower-case "x", then the encrypted password is actually stored in the shadow(5) file instead; there must be a corresponding line
       in the /etc/shadow file, or else the user account is invalid. If the password field is any other string, then it will be treated as an
       encrypted password, as specified by crypt(3).

       The comment field is used by various system utilities, such as finger(1).

       The home directory field provides the name of the initial working directory. The login program uses this information to set the value of
       the $HOME environmental variable.

       The command interpreter field provides the name of the user's command language interpreter, or the name of the initial program to execute.
       The login program uses this information to set the value of the $SHELL environmental variable. If this field is empty, it defaults to the
       value /bin/sh.

FILES

       /etc/passwd
	   User account information.

       /etc/shadow
	   optional encrypted password file

       /etc/passwd-
	   Backup file for /etc/passwd.

	   Note that this file is used by the tools of the shadow toolsuite, but not by all user and password management tools.

SEE ALSO

       crypt(3), getent(1), getpwnam(3), login(1), passwd(1), pwck(8), pwconv(8), pwunconv(8), shadow(5), su(1), sulogin(8).

File Formats and Conversions					    06/24/2011								 PASSWD(5)

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