join command in linux says that files aren't sorted but they are

Question

I have 2 files, I want to join them. they are both sorted. I sorted them by these commands:

$sort -n -k1,1 f1 > t1
$echo $?
0
$mv t1  f1
$sort -n -k1,1 f2 > t1
$echo $?
0
$mv t1 f2

now I run the join command

$join -1 1 -2 1 f1 f2 > fjoin
$echo $?
1

It says those files aren't sorted

$cat f1
0 0
5 0
9 0
10 0 <----- problem is here
$cat f2
0 1
3 1
11 2 <----- problem is here

Answer 1

I suggest to remove sort's option -n .

From man join :

Important: FILE1 and FILE2 must be sorted on the join fields. Eg, use sort -k 1b,1 if join has no options, or use join -t '' if sort has no options. Note, comparisons honor the rules specified by LC_COLLATE . If the input is not sorted and some lines cannot be joined, a warning message will be given.