LinuxQuestions.org - Question abt entry in shadow file

Quoted from

Code:

man shadow

Quote:

DESCRIPTION
shadow contains the encrypted password information for user’s accounts
and optional the password aging information. Included is:

· login name

· encrypted password

[...some lines cut...]

The password field must be filled. The encrypted password consists of 13 to 24
characters from the 64 characters alphabet a thru z, A thru Z, 0 thru 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.

Refer to crypt(3) for details on how this string is interpreted.

If the password field contains some string that is not valid result of
crypt(3), for instance ! or *, the user will not be able to use a unix
password to log in, subject to pam(7).

According to that the user could not login if the password field includes '!', so that can be understood as the account being locked. I'm not sure if that's the proper or usual way of locking an account, though.