_Param’s cannot be set using a mapper or any other means except pipeline param’s, think of it as a global param which you set it at pipeline level, you can use it across pipelines but can be set at a global/pipeline level. Now when it comes to mapper or any other snap that lets you set variables via $varName notation, this are like local vars which can have values set via mapper and are available to the immediate following snap.
when I type “_mypipelineparam” as the Target Path in a Mapper, it gets corrected to “$_mypipelineparam
and this is the expected behavior, now lets say you are getting some value from an external call, ex: you invoke a triggered task and when you do that you pass on a “fileName” param to this task which is then used in the pipeline as a fileName + timeStamp + .extn in your file writer snap. In that case you can use something like this in your File Writer snap settings
‘/some/project/path’ + _fileName + $timeStamp + ‘.json’
where timeStamp is set via a mapper like this
mapper
you could have also created a $fileToWriteTo variable in a mapper which will have this expression
‘/some/project/path’ + __fileName + ‘_’ + Date.now().toLocaleDateTimeString() + ‘.json’
Hope it makes sense!
Sorry my point was that File Reader doesn’t accept a $variable parameter, so I am at a dead end as I can’t write a value to the pipeline _parameter either which it does accept.
