yq/pkg/yqlib/doc/Assign.md

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This operator is used to update node values. It can be used in either the:
### plain form: `=`
Which will assign the LHS node values to the RHS node values. The RHS expression is run against the matching nodes in the pipeline.
### relative form: `|=`
This will do a similar thing to the plain form, however, the RHS expression is run against _the LHS nodes_. This is useful for updating values based on old values, e.g. increment.
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## Create yaml file
Running
```bash
yq eval --null-input '.a.b = "cat" | .x = "frog"'
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```
will output
```yaml
a:
b: cat
x: frog
```
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## Update node to be the child value
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Given a sample.yml file of:
```yaml
a:
b:
g: foof
```
then
```bash
yq eval '.a |= .b' sample.yml
```
will output
```yaml
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a:
g: foof
```
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## Update node from another file
Note this will also work when the second file is a scalar (string/number)
Given a sample.yml file of:
```yaml
a: apples
```
And another sample another.yml file of:
```yaml
b: bob
```
then
```bash
yq eval-all 'select(fileIndex==0).a = select(fileIndex==1) | select(fileIndex==0)' sample.yml another.yml
```
will output
```yaml
a:
b: bob
```
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## Update node to be the sibling value
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Given a sample.yml file of:
```yaml
a:
b: child
b: sibling
```
then
```bash
yq eval '.a = .b' sample.yml
```
will output
```yaml
a: sibling
b: sibling
```
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## Updated multiple paths
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Given a sample.yml file of:
```yaml
a: fieldA
b: fieldB
c: fieldC
```
then
```bash
yq eval '(.a, .c) |= "potatoe"' sample.yml
```
will output
```yaml
a: potatoe
b: fieldB
c: potatoe
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```
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## Update string value
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Given a sample.yml file of:
```yaml
a:
b: apple
```
then
```bash
yq eval '.a.b = "frog"' sample.yml
```
will output
```yaml
a:
b: frog
```
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## Update string value via |=
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Note there is no difference between `=` and `|=` when the RHS is a scalar
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Given a sample.yml file of:
```yaml
a:
b: apple
```
then
```bash
yq eval '.a.b |= "frog"' sample.yml
```
will output
```yaml
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a:
b: frog
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```
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## Update selected results
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Given a sample.yml file of:
```yaml
a:
b: apple
c: cactus
```
then
```bash
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yq eval '(.a[] | select(. == "apple")) = "frog"' sample.yml
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```
will output
```yaml
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a:
b: frog
c: cactus
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```
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## Update array values
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Given a sample.yml file of:
```yaml
- candy
- apple
- sandy
```
then
```bash
yq eval '(.[] | select(. == "*andy")) = "bogs"' sample.yml
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```
will output
```yaml
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- bogs
- apple
- bogs
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```
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## Update empty object
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Given a sample.yml file of:
```yaml
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{}
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```
then
```bash
yq eval '.a.b |= "bogs"' sample.yml
```
will output
```yaml
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{a: {b: bogs}}
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```
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## Update empty object and array
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Given a sample.yml file of:
```yaml
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{}
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```
then
```bash
yq eval '.a.b.[0] |= "bogs"' sample.yml
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```
will output
```yaml
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{a: {b: [bogs]}}
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```