What does the k parameter do in the sort function (Linux Bash Scripting)?

Question

From Linux manual: sort via a key; KEYDEF gives location and type.

I have no idea what that means but I saw it being used like this:

cut -f 2 *ptt | tail -n +4 | sort | uniq -c | sort -k1 -rn 

And then again like this:

ls -1 *\.flv | sort -n -k1.2 

Answer 1

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