The Sort Keys operator sorts maps by their keys (based on their string value). This operator does not do anything to arrays or scalars (so you can easily recursively apply it to all maps).

Sort is particularly useful for diffing two different yaml documents:

yq eval -i 'sortKeys(..)' file1.yml
yq eval -i 'sortKeys(..)' file2.yml
diff file1.yml file2.yml

Sort keys of map

Given a sample.yml file of:

c: frog
a: blah
b: bing

then

yq eval 'sortKeys(.)' sample.yml

will output

a: blah
b: bing
c: frog

Sort keys recursively

Note the array elements are left unsorted, but maps inside arrays are sorted

Given a sample.yml file of:

bParent:
  c: dog
  array:
    - 3
    - 1
    - 2
aParent:
  z: donkey
  x:
    - c: yum
      b: delish
    - b: ew
      a: apple

then

yq eval 'sortKeys(..)' sample.yml

will output

aParent:
  x:
    - b: delish
      c: yum
    - a: apple
      b: ew
  z: donkey
bParent:
  array:
    - 3
    - 1
    - 2
  c: dog