Unix sort treatment of underscore character

This is likely caused by a difference in locale. In the en_US.UTF-8 locale, underscores (_) sort after letters and numbers, whereas in the POSIX C locale they sort after uppercase letters and numbers, but before lowercase letters.

# won't change LC_COLLATE=C after execution
$ LC_COLLATE=C sort filename

You can also use sort --debug to show more information about the sorting behavior in general:

$ (echo 'foo_bar'; echo 'fooAbar'; echo 'foo0bar'; echo 'fooabar') |
      LC_COLLATE=en_US.UTF-8 sort --debug
sort: using ‘en_US.UTF-8’ sorting rules
foo0bar
fooabar
fooAbar
foo_bar

$ (echo 'foo_bar'; echo 'fooAbar'; echo 'foo0bar'; echo 'fooabar') | 
      LC_COLLATE=C sort --debug
sort: using simple byte comparison
foo0bar
fooAbar
foo_bar
fooabar

As also shown in this answer, you can use the above formula to force LC_COLLATE=C for a single command, without modifying your shell environment:

I really liked the answer above with the useful example, i'd just add another string to its list to show how strange the sorting behavior can be:

$ (echo 'foo_bar'; echo 'fooAbar'; echo 'foo0bar'; echo 'fooabar'; echo 'foobbar'; echo 'foobar') | LC_COLLATE=en_US.UTF-8 sort --debug
sort: using ‘en_US.UTF-8’ sorting rules
foo0bar
_______
fooabar
_______
fooAbar
_______
foobar
______
foo_bar
_______
foobbar
_______

Seems crazy right ? Found the explanation here, in this case it's because the unicode collation algorithm is being used in this locale : https://unix.stackexchange.com/questions/252419/unexpected-sort-order-in-en-us-utf-8-locale

HOWEVER, even the 'sort --debug' option is not able to easily demonstrate the subtleties that go into the strcoll() function's rules for obeying the locale sorting specification.

POSIX stipulates that locale authors (for all but the C locale) have absolute control over all sorts of fiddly aspects of how strcoll() behaves, and the fact that two vendors declare that their locale is named en_US.UTF-8 does NOT imply/require those two vendors to have the same locale definition. So the collation rules between two different platforms are very likely different, based on whoever wrote the locale file for that platform, and what bug fixes have been incorporated into the locale definition over time.

Thank you Eric Blake at Red Hat for this insight.

You can set LC_COLLATE to traditional sort order just for your command:

env LC_COLLATE=C sort tmp

This won't change the current environment just the one in which the sort command executes. You should have the same behaviour with this.

sort order depends on the current value of the environment variable LC_COLLATE. Check your local documentation for 'locale', 'setlocale', etc. Set LC_COLLATE to 'POSIX' on both machines, and the results should match.