How does -m option work in sort command?

Question

As sort's man page says:

-m, --merge
    merge already sorted files; do not sort

Here are my two simple text files and the result of sort command with -m option:

soroush@pop-os:~/Desktop$ cat a_file.txt 
aa
ff
hh
bb
soroush@pop-os:~/Desktop$ cat b_file.txt 
gg
tt
ss
ii
cc
soroush@pop-os:~/Desktop$ sort -m a_file.txt b_file.txt 
aa
ff
gg
hh
bb
tt
ss
ii
cc

I expected to see this output:

aa
ff
hh
bb
gg
tt
ss
ii
cc

Could anyone explain this behavior please?

Answer 1

Merging assumes the files are already sorted: "merge already sorted files; do not sort", so will attempt to merge them into alphabetic order. It is not a simple concatination. So in your example:

1. aa < gg : print aa move on a_file
2. ff < gg : print ff move on a_file
3. hh > gg : print gg move on b_file
4. hh < tt : print hh move on a_file
5. bb < tt : print bb move on a_file
6. No a_file left, so print the rest of b_file.