How to merge files after using split command from terminal?
cat xaa &gt; newfile
cat xab &gt;&gt; newfile
cat xac &gt;&gt; newfile


Basically using a single '>' operand send the output to a new file.  using a double '>>' operand makes it append the contents to the end of an existing file (and also out of interest create it if it does not already exist.

If all your files are definitely in a neat alphabetical order, then you could use:

cat x* &gt; newfile


or

cat xaa xab xac &gt; newfile


in case the filenames aren't in alphabetical order.