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".

Tip: This can be a useful way to parameterise complex scripts.

Dynamically evaluate a path

Given a sample.yml file of:

pathExp: .a.b[] | select(.name == "cat")
a:
  b:
    - name: dog
    - name: cat

then

yq 'eval(.pathExp)' sample.yml

will output

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:

a:
  b:
    - name: dog
    - name: cat

then

pathEnv=".a.b[0].name"  valueEnv="moo" yq 'eval(strenv(pathEnv)) = strenv(valueEnv)' sample.yml

will output

a:
  b:
    - name: moo
    - name: cat