Export multiple variables from shell script into a Nextflow output channel

Question

Within a Nextflow workflow, some process runs a shell script that creates multiple variables, e.g.

var_a=$(echo "1")
var_b=$(echo "2")
var_c=$(echo "3")
var_d=$(echo "4")

These variables contain statistics (e.g. number of input reads, quality scores, ...) that I want to keep until the end of the pipeline, to then save them and perform QC. So I'd like to export them out of this process and carry them to the end of the workflow. To be clear, the number of variables is not unknown, it's just too many for convenience if I need to list them all every time.

I understand that I can use the env qualifier or an intermediate file to export the variables individually:

process get_vars {
  output:
    tuple env(var_a), env(var_b), env(var_c), env(var_d)
  
  shell:
  '''
  var_a=$(echo "1")
  var_b=$(echo "2")
  var_c=$(echo "3")
  var_d=$(echo "4")
  '''
}

workflow{
  get_vars | view
}

with output:

[ea/a145b4] process > get_vars [100%] 1 of 1 ✔
[1, 2, 3, 4]

However that becomes very impractical if I have a lot of such variables (that I do want to continue passing along to later processes, along with a much longer tuple of other relevant variables).

My question: is there a convenient way to "encapsulate" all these variables in the output channel, for example with a Groovy map similar to the meta map in nf-core? So I could directly write something like:

process get_vars {
  output:
    val(my_vars)
  
  script:
  '''
  var_a=$(echo "1")
  var_b=$(echo "2")
  var_c=$(echo "3")
  var_d=$(echo "4")
  '''
  
  my_vars = [:]
  my_vars.a = var_a
  my_vars.b = var_b
  my_vars.c = var_c
  my_vars.d = var_d
}

workflow{
  get_vars | view
}

which obviously fails because var_a is not defined outside of the ''' block.

Answer 1

I am not sure if i get what you mean by encapsulate, you can give your tuple a name using emit?

process get_vars {
  output:
    tuple env(var_a), env(var_b), env(var_c), env(var_d), emit: vars

  script:
  '''
  var_a=$(echo "1")
  var_b=$(echo "2")
  var_c=$(echo "3")
  var_d=$(echo "4")
  '''
}

workflow {
  get_vars()
  get_vars.out.vars.view()
}

That gives:

 [1, 2, 3, 4]