Easiest way to join two files from the unix command line, inserting zero entries for missing keys

Question

I'm trying to join two files each of which contains rows of the form <key> <count>. Each file contains a few lines that are missing from the other, and I would like to have zero inserted for all such values rather than omitting these lines (I've seen -a, but this isn't quite what I'm looking for). Is there a simple way to accomplish this?

Here is some sample input:

a.txt

apple 5
banana 7

b.txt

apple 6
cherry 4

expected output:

apple 5 6
banana 7 0
cherry 0 4

Answer 1

join -o 0,1.2,2.2 -e 0 -a1 -a2 a.txt b.txt

• -o 0,1.2,2.2 → output join field, then 2nd field of 1st file, then 2nd field of 2nd file.
• -e 0 → Output 0 on empty input fields.
• -a1 -a2 → Show all values from file 1 and file 2.