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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, beit a path .a.b.c | select(. == "cat"), or an update .a.b.c = "gogo".
Tip: This can be useful way parameterise complex scripts.
Note that versions prior to 4.18 require the 'eval/e' command to be specified.
yq e <exp> <file>

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
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Dynamically evaluate a path
Dynamically update a path from an environment variable