/usr/bin/mcs is in mono-mcs 2.10.8.1-1ubuntu2.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 | #!/bin/sh
exec /usr/bin/mono $MONO_OPTIONS /usr/lib/mono/2.0/mcs.exe -lib:/usr/lib/mono/2.0 -lib:/usr/lib/mono/3.5 "$@"
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