You probably mean 'add-apt-repository foorepo && apt-get update && apt-get install foopkg' or 'dpkg -i foopkg' (as root) because as is, 'apt-get install' is inn

You probably mean 'add-apt-repository foorepo && apt-get update && apt-get install foopkg' or 'dpkg -i foopkg' (as root) because as is, 'apt-get install' is innocuous given one uses the default trusted, signed and whatnot repos.