Features used to implement the example

• Write field on issues returned by JQL query or issue listUpdate issue fields
• Copy parsed text to a field

Example: Transition only a sub-task among several ones

Could you please advise how I can transit only first sub-task on parent issue transition.

It will require JIRA Workflow Toolbox 2.1.28 or higher. It can be done using 3 post-functions:

Copy a parsed text to a field post-function to write the name of a transition into virtual field "Ephemeral string 1" virtual field. In this example I use transition "Start Progress":

Copy a parsed text to a field post-function to write the issue key of first sub-task order by a certain field into virtual field "Ephemeral string 2". In this example I order sub-tasks by creation date-time in ascending order:

Note that:

• Text to be parsed is: toString(first(sort(subtasks(), {00009}, ASC)))