The sort command in linux does not work "correctly"

Question

I'm working on Linux, the sort command returns not as expected.

Input text:

$ cat input.txt
rep1_1.fq
rep1_2.fq
rep12_1.fq
rep12_2.fq

Command and output:

$ sort input.txt
rep1_1.fq
rep12_1.fq
rep12_2.fq
rep1_2.fq


$ sort --version
sort (GNU coreutils) 8.28
Copyright (C) 2017 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Written by Mike Haertel and Paul Eggert.

After sorting, I expected rep1_2.fq would be after rep1_1.fq, but the result is different.

Solved

according to @Federico klez Culloca's advice, use LC_ALL=C

$ LC_ALL=C sort input.txt
rep12_1.fq
rep12_2.fq
rep1_1.fq
rep1_2.fq

Edited

use LC_ALL=C also fix sorting files in a directory.

in case there are four files in current directory:

$ LC_ALL= ls
rep1_1.fq  rep12_1.fq  rep12_2.fq  rep1_2.fq

$ LC_ALL=C ls
rep12_1.fq  rep12_2.fq  rep1_1.fq  rep1_2.fq

Answer 1

Try with version-sort. From the manual:

       -V, --version-sort
          natural sort of (version) numbers within text

This is the output using your example:

$ sort -V input.txt 
rep1_1.fq
rep1_2.fq
rep12_1.fq
rep12_2.fq