yq/pkg/yqlib/doc/Anchor and Alias Operators.md

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Use the `alias` and `anchor` operators to read and write yaml aliases and anchors. The `explode` operator normalises a yaml file (dereference 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.
## 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
```
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## 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
```
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## 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
```
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## Explode alias and anchor
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Given a sample.yml file of:
```yaml
f:
a: &a cat
b: *a
```
then
```bash
yq eval 'explode(.f)' sample.yml
```
will output
```yaml
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f:
a: cat
b: cat
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```
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## Explode with no aliases or anchors
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Given a sample.yml file of:
```yaml
a: mike
```
then
```bash
yq eval 'explode(.a)' sample.yml
```
will output
```yaml
a: mike
```
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## Explode with alias keys
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Given a sample.yml file of:
```yaml
f:
a: &a cat
*a: b
```
then
```bash
yq eval 'explode(.f)' sample.yml
```
will output
```yaml
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f:
a: cat
cat: b
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```
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## Explode with merge anchors
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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
a: foo_a
thing: bar_thing
c: foobarList_c
foobar:
c: foo_c
a: foo_a
thing: foobar_thing
```