yq/pkg/yqlib/doc/anchor-and-alias-operators.md

# Anchor and Alias Operators

Use the `alias` and `anchor` operators to read and write yaml aliases and anchors. The `explode` operator normalises a yaml file (dereference (or expands) aliases and remove anchor names).

`yq` supports merge aliases (like `<<: *blah`) however this is no longer in the standard yaml spec (1.2) and so `yq` will automatically add the `!!merge` tag to these nodes as it is effectively a custom tag.


## Merge one map
see https://yaml.org/type/merge.html

Given a sample.yml file of:
```yaml
- &CENTER
  x: 1
  y: 2
- &LEFT
  x: 0
  y: 2
- &BIG
  r: 10
- &SMALL
  r: 1
- !!merge <<: *CENTER
  r: 10
```
then
```bash
yq eval '.[4] | explode(.)' sample.yml
```
will output
```yaml
x: 1
y: 2
r: 10
```

## Merge multiple maps
see https://yaml.org/type/merge.html

Given a sample.yml file of:
```yaml
- &CENTER
  x: 1
  y: 2
- &LEFT
  x: 0
  y: 2
- &BIG
  r: 10
- &SMALL
  r: 1
- !!merge <<:
    - *CENTER
    - *BIG
```
then
```bash
yq eval '.[4] | explode(.)' sample.yml
```
will output
```yaml
r: 10
x: 1
y: 2
```

## Override
see https://yaml.org/type/merge.html

Given a sample.yml file of:
```yaml
- &CENTER
  x: 1
  y: 2
- &LEFT
  x: 0
  y: 2
- &BIG
  r: 10
- &SMALL
  r: 1
- !!merge <<:
    - *BIG
    - *LEFT
    - *SMALL
  x: 1
```
then
```bash
yq eval '.[4] | explode(.)' sample.yml
```
will output
```yaml
r: 10
x: 1
y: 2
```

## Get anchor
Given a sample.yml file of:
```yaml
a: &billyBob cat
```
then
```bash
yq eval '.a | anchor' sample.yml
```
will output
```yaml
billyBob
```

## Set anchor
Given a sample.yml file of:
```yaml
a: cat
```
then
```bash
yq eval '.a anchor = "foobar"' sample.yml
```
will output
```yaml
a: &foobar cat
```

## Set anchor relatively using assign-update
Given a sample.yml file of:
```yaml
a:
  b: cat
```
then
```bash
yq eval '.a anchor |= .b' sample.yml
```
will output
```yaml
a: &cat
  b: cat
```

## Get alias
Given a sample.yml file of:
```yaml
b: &billyBob meow
a: *billyBob
```
then
```bash
yq eval '.a | alias' sample.yml
```
will output
```yaml
billyBob
```

## Set alias
Given a sample.yml file of:
```yaml
b: &meow purr
a: cat
```
then
```bash
yq eval '.a alias = "meow"' sample.yml
```
will output
```yaml
b: &meow purr
a: *meow
```

## Set alias to blank does nothing
Given a sample.yml file of:
```yaml
b: &meow purr
a: cat
```
then
```bash
yq eval '.a alias = ""' sample.yml
```
will output
```yaml
b: &meow purr
a: cat
```

## Set alias relatively using assign-update
Given a sample.yml file of:
```yaml
b: &meow purr
a:
  f: meow
```
then
```bash
yq eval '.a alias |= .f' sample.yml
```
will output
```yaml
b: &meow purr
a: *meow
```

## Explode alias and anchor
Given a sample.yml file of:
```yaml
f:
  a: &a cat
  b: *a
```
then
```bash
yq eval 'explode(.f)' sample.yml
```
will output
```yaml
f:
  a: cat
  b: cat
```

## Explode with no aliases or anchors
Given a sample.yml file of:
```yaml
a: mike
```
then
```bash
yq eval 'explode(.a)' sample.yml
```
will output
```yaml
a: mike
```

## Explode with alias keys
Given a sample.yml file of:
```yaml
f:
  a: &a cat
  *a: b
```
then
```bash
yq eval 'explode(.f)' sample.yml
```
will output
```yaml
f:
  a: cat
  cat: b
```

## Explode with merge anchors
Given a sample.yml file of:
```yaml
foo: &foo
  a: foo_a
  thing: foo_thing
  c: foo_c
bar: &bar
  b: bar_b
  thing: bar_thing
  c: bar_c
foobarList:
  b: foobarList_b
  !!merge <<:
    - *foo
    - *bar
  c: foobarList_c
foobar:
  c: foobar_c
  !!merge <<: *foo
  thing: foobar_thing
```
then
```bash
yq eval 'explode(.)' sample.yml
```
will output
```yaml
foo:
  a: foo_a
  thing: foo_thing
  c: foo_c
bar:
  b: bar_b
  thing: bar_thing
  c: bar_c
foobarList:
  b: bar_b
  thing: foo_thing
  c: foobarList_c
  a: foo_a
foobar:
  c: foo_c
  a: foo_a
  thing: foobar_thing
```

## Dereference and update a field
`Use explode with multiply to dereference an object

Given a sample.yml file of:
```yaml
item_value: &item_value
  value: true
thingOne:
  name: item_1
  !!merge <<: *item_value
thingTwo:
  name: item_2
  !!merge <<: *item_value
```
then
```bash
yq eval '.thingOne |= explode(.) * {"value": false}' sample.yml
```
will output
```yaml
item_value: &item_value
  value: true
thingOne:
  name: item_1
  value: false
thingTwo:
  name: item_2
  !!merge <<: *item_value
```