the password file
$ passwd [new_password]
$ passwd [username] [new_password]
$ passwd -S
$ passwd -d
/etc/passwd contains one line for each user account, with seven fields delimited by colons ( . These fields are:'-04' '+03' .sp -1
2.3 login name '-04' '+03' .sp -1
2.3 optional encrypted password '-04' '+03' .sp -1
2.3 numerical user ID '-04' '+03' .sp -1
2.3 numerical group ID '-04' '+03' .sp -1
2.3 user name or comment field '-04' '+03' .sp -1
2.3 user home directory '-04' '+03' .sp -1
2.3 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 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 (Aqs 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 .
/etc/passwd User account information .
/etc/shadow optional encrypted password file
/etc/passwd - Backup file for /etc/passwd .