Linux_passwd_shadow_file

Password

1. Username (1-32 chars)
2. x indicates that it's an encrypted password in the /etc/shadow file.
3. uid - UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
4. gid - stored in /etc/group
5. Comment field.
6. User's ~ directory.
7. Absolute path to command/shell. Doesn't need to be a shell.

Shadow

1. Username.

2. Password : It is your encrypted password. The password should be minimum 8-12 characters long including special characters, digits, lower case alphabetic and more. Usually password format is set to $id$salt$hashed, The $id is the algorithm used On GNU/Linux as follows:

   $1$ is MD5

   $2a$ is Blowfish

   $2y$ is Blowfish

   $5$ is SHA-256

   $6$ is SHA-512

3. Last password change (lastchanged) : Days since Jan 1, 1970 that password was last changed

4. Minimum : The minimum number of days required between password changes i.e. the number of days left before the user is allowed to change his/her password

5. Maximum : The maximum number of days the password is valid (after that user is forced to change his/her password)

6. Warn : The number of days before password is to expire that user is warned that his/her password must be changed

7. Inactive : The number of days after password expires that account is disabled

8. Expire : days since Jan 1, 1970 that account is disabled i.e. an absolute date specifying when the login may no longer be used.