Update 2

src/fasta_sampler.nf

 Channel
   .fromPath( "data/tiny_dataset/fasta/*.fasta" )
   .set { fasta_file }
+
+
+
+process sample_fasta {
+publishDir "results/sampling/", mode: 'copy'
+  input:
+file fasta from fasta_file
+
+  output:
+file "*_sample.fasta" into fasta_sample
+
+  script:
+"""
+head ${fasta} > ${fasta.baseName}_sample.fasta
+"""
+}
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