This operator is used to provide alternative (or default) values when a particular expression is either null or false.

LHS is defined

Given a sample.yml file of:

a: bridge

then

yq eval '.a // "hello"' sample.yml

will output

bridge

LHS is not defined

Given a sample.yml file of:

{}

then

yq eval '.a // "hello"' sample.yml

will output

hello

LHS is null

Given a sample.yml file of:

a: ~

then

yq eval '.a // "hello"' sample.yml

will output

hello

LHS is false

Given a sample.yml file of:

a: false

then

yq eval '.a // "hello"' sample.yml

will output

hello

RHS is an expression

Given a sample.yml file of:

a: false
b: cat

then

yq eval '.a // .b' sample.yml

will output

cat