sort by key in coreutils

May 8, 2025

This post is about sort tool from GNU coreutils. Until today I foolishly thought that to sort a file by a second (and just second column) you just need to use sort -k2 option.

Indeed that does seem to work for a simple case:

$ printf "1 2\n2 1\n"
1 2
2 1

$ printf "1 2\n2 1\n" | sort -k2
2 1
1 2

But today I attempted a slightly more complicated sort by sorting commit history:

abcd foo: commit z
bcde bar: commit a
cdef foo: commit y
defg bar: commit b

I wanted to sort these by the foo: / bar: component while preserving the order of commit within the component. I want this outcome:

bcde bar: commit a
defg bar: commit b
abcd foo: commit z
cdef foo: commit y

How do you achieve that? My naive attempt was to use sort -k2 --stable:

$ sort -k2 --stable <l
bcde bar: commit a
defg bar: commit b
cdef foo: commit y
abcd foo: commit z

Note how foo: commits were unexpectedly reordered.

Quick quiz: why did excessive reorder happen? How to fix it?

The answer

sort --help has an answer by describing what -k2 key selection actually does. But --debug option is even better at illustrating what is being compared. Let’s use that:

$ LANG=C sort -k2 --stable --debug <l
sort: text ordering performed using simple byte comparison
sort: leading blanks are significant in key 1; consider also specifying 'b'
bcde bar: commit a
    ______________
defg bar: commit b
    ______________
cdef foo: commit y
    ______________
abcd foo: commit z
    ______________

The ______________ underscore shows the actual compared key: it’s not just foo: or bar:. It’s the whole line that starts at the whitespace right before foo: and bar:. The fix is to tweak the selector:

$ LANG=C sort -k2,2 --stable --debug <l
sort: text ordering performed using simple byte comparison
sort: leading blanks are significant in key 1; consider also specifying 'b'
bcde bar: commit a
    _____
defg bar: commit b
    _____
abcd foo: commit z
    _____
cdef foo: commit y
    _____

or with -b if leading spaces look confusing:

$ LANG=C sort -k2,2 --stable --debug -b <l
sort: text ordering performed using simple byte comparison
bcde bar: commit a
     ____
defg bar: commit b
     ____
abcd foo: commit z
     ____
cdef foo: commit y
     ____

This way the sorting works as expected:

$ LANG=C sort -k2,2 --stable -b <l
bcde bar: commit a
defg bar: commit b
abcd foo: commit z
cdef foo: commit y

parting words

sort -k is tricky: it’s not a field number but a field range. --debug option is great at showing used sorting key (or keys of --stable is not used).

Have fun!