Merged envsubst into env operators

pkg/yqlib/doc/operators/env-variable-operators.md

@@ -1,6 +1,21 @@
 # Env Variable Operators
 
-This operator is used to handle environment variables usage in path expressions. While environment variables can, of course, be passed in via your CLI with string interpolation, this often comes with complex quote escaping and can be tricky to write and read. Note that there are two forms, `env` which will parse the environment variable as a yaml (be it a map, array, string, number of boolean) and `strenv` which will always parse the argument as a string.
+These operators are used to handle environment variables usage in expressions and documents. While environment variables can, of course, be passed in via your CLI with string interpolation, this often comes with complex quote escaping and can be tricky to write and read. 
+
+There are three operators:
+-  `env` which takes a single environment variable name and parse the variable as a yaml node (be it a map, array, string, number of boolean) 
+- `strenv` which also takes a single environment variable name, and always parses the variable as a string.
+- `envsubst` which you pipe strings into and it interpolates environment variables in strings using [envsubst](https://github.com/a8m/envsubst). 
+
+
+## Tip
+To replace environment variables across all values in a document, `envsubst` can be used with the recursive descent operator
+as follows:
+
+```bash
+yq eval '(.. | select(tag == "!!str")) |= envsubst' file.yaml
+```
+
 
 ## Read string environment variable
 Running
@@ -77,3 +92,47 @@ will output
 meow
 ```
 
+## Replace strings with envsubst
+Running
+```bash
+myenv="cat" yq eval --null-input '"the ${myenv} meows" | envsubst'
+```
+will output
+```yaml
+the cat meows
+```
+
+## Replace strings with envsubst, missing variables
+Running
+```bash
+myenv="cat" yq eval --null-input '"the ${myenvnonexisting} meows" | envsubst'
+```
+will output
+```yaml
+the  meows
+```
+
+## Replace strings with envsubst, missing variables with defaults
+Running
+```bash
+myenv="cat" yq eval --null-input '"the ${myenvnonexisting-dog} meows" | envsubst'
+```
+will output
+```yaml
+the dog meows
+```
+
+## Replace string environment variable in document
+Given a sample.yml file of:
+```yaml
+v: ${myenv}
+```
+then
+```bash
+myenv="cat meow" yq eval '.v |= envsubst' sample.yml
+```
+will output
+```yaml
+v: cat meow
+```
+