Understanding /etc/shadow file

Can you explain /etc/shadow file format used under Linux or UNIX-like system?

The /etc/shadow file stores actual password in encrypted format (more like the hash of the password) for user’s account with additional properties related to user password. Basically, it stores secure user account information. All fields are separated by a colon (:) symbol. It contains one entry per line for each user listed in /etc/passwd file. Generally, shadow file entry looks as follows (click to enlarge image):

/etc/shadow file fields

(Fig.01: /etc/shadow file fields)

  1. Username : It is your login name.
  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. $1$ is MD5
    2. $2a$ is Blowfish
    3. $2y$ is Blowfish
    4. $5$ is SHA-256
    5. $6$ 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.

The last 6 fields provides password aging and account lockout features. You need to use the chage command to setup password aging. According to man page of shadow – the password field must be filled. The encrypted password consists of 13 to 24 characters from the 64 character alphabet a through z, A through Z, 0 through 9, . and /. Optionally it can start with a “$” character. This means the encrypted password was generated using another (not DES) algorithm. For example if it starts with “$1$” it means the MD5-based algorithm was used. Please note that a password field which starts with a exclamation mark (!) means that the password is locked. The remaining characters on the line represent the password field before the password was locked.

How do I change the password?

Use the following syntax to change your own password:
$ passwd
See passwd command tutorial page for more information.

How do I change the password for other users?

You must be root to change the password for all other users:
# passwd userNameHere
$ sudo passwd userNameHere

How do I change or set password ageing information?

To change user password expiry information use the chage command on Linux. The syntax is (again you must be root to set the password again) as follows:

chage username
chage [options] username 
chage vivek
chage -l tom

The options are as follows:

  -d, --lastday LAST_DAY        set date of last password change to LAST_DAY
  -E, --expiredate EXPIRE_DATE  set account expiration date to EXPIRE_DATE
  -h, --help                    display this help message and exit
  -I, --inactive INACTIVE       set password inactive after expiration
                                to INACTIVE
  -l, --list                    show account aging information
  -m, --mindays MIN_DAYS        set minimum number of days before password
                                change to MIN_DAYS
  -M, --maxdays MAX_DAYS        set maximim number of days before password
                                change to MAX_DAYS
  -R, --root CHROOT_DIR         directory to chroot into
  -W, --warndays WARN_DAYS      set expiration warning days to WARN_DAYS

How do I verify integrity of password files?

Use the pwck command verifies the integrity of the users and authentication information. It checks that all entries in /etc/passwd and /etc/shadow have the proper format and contain valid data. The user is prompted to delete entries that are improperly formatted or which have other uncorrectable errors. The syntax is:

pwck -r /etc/passwd
pwck -r /etc/shadow
pwck [options] /etc/shadow

The options are as follows:

  -h, --help                    display this help message and exit
  -q, --quiet                   report errors only
  -r, --read-only               display errors and warnings
                                but do not change files
  -R, --root CHROOT_DIR         directory to chroot into
  -s, --sort                    sort entries by UID
This entry is 2 of 3 in the Linux / UNIX System’s Accounts Files Tutorial series. Keep reading the rest of the series:

  1. Understanding /etc/passwd File Format
  2. Understanding /etc/shadow file
  3. Understanding /etc/group file

Posted by: SXI ADMIN

The author is the creator of SXI LLC and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.