The password file
/etc/passwd contains one line for each user account, with seven fields delimited by colons (“:”). These fields are:
login name
optional encrypted password
numerical user ID
numerical group ID
user name or comment field
user home directory
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.
/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.
crypt(3), getent(1), getpwnam(3), login(1), passwd(1), pwck(8), pwconv(8), pwunconv(8), shadow(5), su(1), sulogin(8).