What is sort doing

Question

This is a followup to this question : What does sort -n do with non numeric values?

Here is my input:

1-----***
R----****
1---*****

And I run:

sort -k 1.2,1.9n input.txt > output.txt

Here is my output:

1-----***
1---*****
R----****

And my expected output:

1---*****
R----****
1-----***

I expect this output since the ASCII value of * (42) is smaller than the value of - (45). However, it appears the R is being considered in the sort, but I thought 1.2,1.9 would exclude it. What am I doing wrong?

Answer 1

You are using an invalid key for sort (invalid because it is not matching your input text). The info manual for sort suggests using the --debug option to investigate the behavior of what they key is matching. Using your command key, 1.2,1.9n, sort produces the following output:

% sort -k 1.2,1.9n --debug input.txt 
sort: using ‘en_US.UTF-8’ sorting rules
1-----***
 ^ no match for key
_________
1---*****
 ^ no match for key
_________
R----****
 ^ no match for key
_________

and appears to be falling back on a default sort using the whole field. If you omit the n from your key and just use 1.2,1.9 you instead get the following:

% sort -k 1.2,1.9 --debug input.txt 
sort: using ‘en_US.UTF-8’ sorting rules
1-----***
 ________
_________
R----****
 ________
_________
1---*****
 ________
_________

which is properly using the 2nd through 9th characters of the first field for the sort operation.