yq/pkg/yqlib/doc/operators/eval.md

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# Eval
Use `eval` to dynamically process an expression - for instance from an environment variable.
`eval` takes a single argument, and evaluates that as a `yq` expression. Any valid expression can be used, be it a path `.a.b.c | select(. == "cat")`, or an update `.a.b.c = "gogo"`.
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Tip: This can be a useful way to parameterise complex scripts.
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## Dynamically evaluate a path
Given a sample.yml file of:
```yaml
pathExp: .a.b[] | select(.name == "cat")
a:
b:
- name: dog
- name: cat
```
then
```bash
yq 'eval(.pathExp)' sample.yml
```
will output
```yaml
name: cat
```
## Dynamically update a path from an environment variable
The env variable can be any valid yq expression.
Given a sample.yml file of:
```yaml
a:
b:
- name: dog
- name: cat
```
then
```bash
pathEnv=".a.b[0].name" valueEnv="moo" yq 'eval(strenv(pathEnv)) = strenv(valueEnv)' sample.yml
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```
will output
```yaml
a:
b:
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- name: moo
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- name: cat
```