yq/pkg/yqlib/doc/headers/File Operators.md

File operators are most often used with merge when needing to merge specific files together. Note that when doing this, you will need to use eval-all to ensure all yaml documents are loaded into memory before performing the merge (as opposed to eval which runs the expression once per document).

Merging files

Note the use of eval-all to ensure all documents are loaded into memory.

yq eval-all 'select(fileIndex == 0) * select(filename == "file2.yaml")' file1.yaml file2.yaml