For users not wishing to use a smartphone or to provide a backup device to enable login with TOTP the oathtool command line tool can be used on Linux or MacOS systems to provide a one-time password as an alternative to using an authenticator client on a smartphone.
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Use as:
oathtool -b --digits=6 --totp=sha1 "ABCDEFGHIJKLMNOPQRSTUVWXYZ012345"
blah blah blah