Quick-Reference: How to combine multiple pdfs in Ubuntu using command-line

sudo apt-get install gs pdftk

Use

gs -dNOPAUSE -sDEVICE=pdfwrite -sOUTPUTFILE=combinedpdf.pdf -dBATCH 1.pdf 2.pdf 3.pdf

I must admit this is not an original finding. Please refer to to This Post  for the original instruction list. I am basically putting it down here for my quick-reference