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Alternative (Default value)

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 '.a // "hello"' sample.yml
will output
bridge

LHS is not defined

Given a sample.yml file of:
{}
then
yq '.a // "hello"' sample.yml
will output
hello

LHS is null

Given a sample.yml file of:
a: ~
then
yq '.a // "hello"' sample.yml
will output
hello

LHS is false

Given a sample.yml file of:
a: false
then
yq '.a // "hello"' sample.yml
will output
hello

RHS is an expression

Given a sample.yml file of:
a: false
b: cat
then
yq '.a // .b' sample.yml
will output
cat

Update or create - entity exists

This initialises a if it's not present
Given a sample.yml file of:
a: 1
then
yq '(.a // (.a = 0)) += 1' sample.yml
will output
a: 2

Update or create - entity does not exist

This initialises a if it's not present
Given a sample.yml file of:
b: camel
then
yq '(.a // (.a = 0)) += 1' sample.yml
will output
b: camel
a: 1