According to the standards, the default sort order matches the order of characters in the ASCII code set. So:

Code:

sort sort.txt

should give you:

Code:

AAA
GGG
aaa
bbb
ddd
eee
ggg

And, the command:

Code:

sort -f sort.txt

(with -f used to fold lowercase characters to uppercase for comparisons) should yield:

Code:

AAA
aaa
bbb
ddd
eee
GGG
ggg

What OS are you using? What is the output from the commands:

Code:

type sort
alias sort

on your system?