process-when-empty.adoc

Execute when empty

Problem

You need to execute a process if a channel is empty.

Solution

Use the ifEmpty operator to emit a marker value to trigger the execution of the process.

Example

process foo {
  input:
  val x from ch.ifEmpty { 'EMPTY' }
  when:
  x == 'EMPTY'

  script:
  '''
  your_command
  '''
}

Run it

The command run the script with an empty channel:

nextflow run patterns/process-when-empty.nf

Use the following command to provide the same script some input files, that prevents the process to be executed:

nextflow run patterns/process-when-empty.nf --inputs ../data/prots/\*