Bug in sort or misunderstanding

Question

Suppose I have this file

b 10 foo
a 10 bar
a 2 bar

I want to sort by the third colum, alphabetically. In case of ties, I want to sort by the second column, numerically. In the man page for the unix sort program you find this:

> sort -k3 -k2n a

Which gives

a 2 bar
a 10 bar
b 10 foo

Which is correct.

Now, what I actually want, is to sort by the first column, alphabetically, and in case of ties, by the second column, numerically. Can anybody explain to me why

> sort -k1 -k2n a

Gives

a 10 bar
a 2 bar
b 10 foo

Which is plain wrong?

Answer 1

The reason your example doesn't work as you expect is that sort considers all positions after the specified one as well. In your example, the dictionary sort of -k1 sorts the string a 10 bar before a 2 bar, and you don't need a tie break.

You need to explicitly specify the limits of the sort key, as described in the man page:

   -k, --key=POS1[,POS2]
          start a key at POS1, end it at POS2 (origin 1)

Example:

$ sort -k1,1 -k2n theFile
a 2 bar
a 10 bar
b 10 foo