To append the standard output from a command to a file use two >> (greater than) symbols followed by the name of the file. If the file does not exist it is created. For example:
cat part1 part2 part3 part4 >> chapt2
This reads files part1 through part4 in turn and appends them to the file chapt2. The contents of part2 are appended to part1 and so on.