Backup [option...] -from directory -to directory >commands >&progress

Files in a source (from) directory are copied to a destination (to) directory
based on the modification dates.  By default, only files that already exist in
both directories are candidates for copying.  Normally, Backup produces a
series of shell Duplicate commands which can be redirected into a script file
for later execution.  This can be overridden by using the -doit option.

Note: At least -from or -to must be specified

    -a          # copy all files from source to destination
    -c          # create "to" directories if they do not exist
    -doit       # actually perform the Duplicate commands instead of writing
                # them to standard output
    -from name  # specify source directory.  If omitted, the current directory
                # is assumed
    -p          # display copyright and progress information
    -r          # recursively process subdirectories
    -to name    # specify destination directory.  If omitted, the current
                # directory is assumed
