Question

# comm -12 /tmp/src /tmp/txt | wc -l
  10338
# join /tmp/src /tmp/txt | wc -l
  10355

Both the files are single columns of alphanumeric strings and sort-ed. Shouldn’t they be the same?

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Updated following @Kevin-s answer below:

cat /tmp/txt | sed 's/^[:space:]*//' > /tmp/stxt
cat /tmp/src | sed 's/^[:space:]*//' > /tmp/ssrc

and the result:

#join /tmp/ssrc /tmp/stxt | wc -l
516
# comm -12 /tmp/ssrc /tmp/stxt | wc -l
513

On manual inspection of the diff-s … the results differ due to some whitespaces that were not taken out by the sed.

Answer 1

I haven’t used either extensively, but from a quick look at the man pages and test input, it seems that if the two files differ, comm prints both and join only prints matching lines. The -12 took care of that. You could store the output of the two into files and do a diff to see how they differ.

$ echo -e '1\n2\n3\n5' > a
$ echo -e '1\n2\n4\n5' > b
$ comm a b
                1
                2
3
        4
                5
$ join a b
1
2
5
$

Edit:
Join only compares the first whitespace-separated field but comm compares the whole line. Any whitespace on the line will therefore make the output differ.