Wrong behavior of sort command?

I tried to sort the content of a file in Ubuntu desktop 14.04 (Trusty Tahr). In my case, the expected result should be same as original content, but the actual result is not. Why?

# cat test.txt
a++-a
a++-b
a++-c
ab
ac

# cat test.txt | sort
a++-a
ab
a++-b
ac
a++-c

Solution 1:

You could use LC_ALL variable, set it to LC_ALL=C before calling sort

$ LC_ALL=C sort test.txt
a++-a
a++-b
a++-c
ab
ac

Read this answer, if you want to know what is this magically LC_ALL=C. Here is short summary:

The C locale is a special locale that is meant to be the simplest locale. You could also say that while the other locales are for humans, the C locale is for computers. In the C locale, characters are single bytes, the charset is ASCII, the sorting order is based on the byte values.

Also, as @KenMollerup pointed, quote from man sort

   ***  WARNING  ***  The locale specified by the environment affects sort
   order.  Set LC_ALL=C to get the traditional sort order that uses native
   byte values.

So when using sort with LC_ALL=C, sort compare symbols bytewise. Otherwise sort will ignore all non alphanumerical characters.

Solution 2:

Sort uses alphabetical and numeric sorting, same as us, special characters like + - < > ... are ignored, numbers are treated numerical so 1, 2, 3.. comes before 11, 12 1066 1104 -- see!

So your list is seen as: aa, ab, ab, ac, ac