KEYDEF is F[.C][OPTS][,F[.C][OPTS]] for start and stop position, where F is a field number and C a character position in the field; both are origin
1, and the stop position defaults to the line’s end. If neither -t nor -b is in effect, characters in a field are counted from the beginning of
the preceding whitespace. OPTS is one or more single-letter ordering options [bdfgiMhnRrV], which override global ordering options for that key.
If no key is given, use the entire line as the key.
An example input file:
123 233
214 176
341 325
sort on the first field:
sort -t' ' -k1 input
Gives:
123 233
214 176
341 325
The second field:
sort -t' ' -k2 input
Gives:
214 176
123 233
341 325
Second and third digits of the first field:
sort -t' ' -k1.2 input
Gives:
214 176
123 233
341 325
Last digit of the second field:
sort -t' ' -k2.3 input
Gives:
123 233
341 325
214 176