Você pode ler em man smbclient
:
-U|--user=username[%password]
Sets the SMB username or username and password.
If %password is not specified, the user will be prompted. The client will first check the USER environment variable, then the LOGNAME variable and if
either exists, the string is uppercased. If these environmental variables are not found, the username GUEST is used.
A third option is to use a credentials file which contains the plaintext of the username and password. This option is mainly provided for scripts where
the admin does not wish to pass the credentials on the command line or via environment variables. If this method is used, make certain that the
permissions on the file restrict access from unwanted users. See the -A for more details.
Portanto, será melhor usar a opção -A
e definir as permissões corretas no arquivo para que ele NÃO seja legível