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

Solution

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 

Comments

[Xiong Chiamiov]

The default (in coreutils 8.22 at least) is to split columns based on whitespace, not tabs.

[northern-bradley]

it might we worth clarifying that the OPTS are the sort command's "-" options e.g. sort -k8rn,8 is equivalent to sort -r -n -k8,8. it allows you to sort each key in a different way rather than globally for the entire sort command (it took me ages before i realised)