useradd: unable to lock password file

Useradd: unable to lock password file

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Problem description:

Under Red Hat Linux release 9, use root to add a user, get error message as below:

    useradd: unable to lock password file


Solution:

Needed to remove a lock file

Under the /etc directory, there are some lock files as:
    group.lock
    gshadow.lock
    passwd.lock
   shadow.lock

or
   .pwck.lock
   .pwd.lock

Remove them.