Freebsd – How to merge pre-sorted files into a single BIG file, without excessive memory or temporary disk use

The PyPdf library makes this sort of things easy if you're willing to write a bit of Python. Save the code below in a script called pdf-cat-even (or whatever you like), make it executable (chmod +x pdf-cat-even), and run it as a filter (./pdf-cat-even a.pdf b.pdf >concatenated.pdf). You need pyPdf ≥1.13 for the addBlankPage method.

#!/usr/bin/env python
import copy, sys
from pyPdf import PdfFileWriter, PdfFileReader
output = PdfFileWriter()
output_page_number = 0
alignment = 2           # to align on even pages
for filename in sys.argv[1:]:
    # This code is executed for every file in turn
    input = PdfFileReader(open(filename))
    for p in [input.getPage(i) for i in range(0,input.getNumPages())]:
        # This code is executed for every input page in turn
        output.addPage(p)
        output_page_number += 1
    while output_page_number % alignment != 0:
        output.addBlankPage()
        output_page_number += 1
output.write(sys.stdout)

You can—the program zcat (or gzip -cd) to write the decompressed data to stdout. From there, you can use bash's process substitution. 1.gz is the numbers 1 through 3; a.gz is the letters a through c; and z.gz is the number 4 and letter d:

$ sort -m <(zcat 1.gz) <(zcat a.gz) <(zcat z.gz)
1
2
3
4
a
b
c
d

Hopefully your data is somewhat more interesting.