stable sort in linux

Question

I have a file

ksh$ cat test
b d
b c
a b
a a

I want to sort on first field but I want stable sort i.e. order of 2nd field should remain the

Answer 1

You must specify the end field:

sort -k1,1 -s test

Answer 2

You forgot to constrain the key fields. By default it uses until the end of the line.

sort -k1,1 -s t.txt