Channel.fromFilePairs in Netflow does not work

I’m doing a small Nextflow script for practice. My working directory is:

.
├── metadata_a.txt
├── metadata_b.txt
├── metadata_c.txt
└── parallel.nf

0 directories, 4 files

With my Nextflow script (parallel.nf), I’m trying to cat each metadata file and show the name of each one.

#!/usr/bin/env nextflow

process READFILE{
    input:
    tuple val(name), path(file)

    output:
    stdout

    """
    echo \$(cat $file) of $name
    """
}

workflow {
    input_ch = Channel.fromFilePairs('./metadata_*.txt')
    READFILE(input_ch).view()
}

But my pipeline even actually didn’t run:

Launching `parallel.nf` [romantic_austin] DSL2 - revision: 4a80e4e585
[-        ] process > READFILE -

Does anyone know about this situation? Please give me some help, thanks.

>Solution :

The workflow ran, but your input channel was empty. The fromFilePairs factory method just needs a size parameter to specify the number of expected input files. Otherwise a pair of files is expected. I think what you want is:

workflow {
    input_ch = Channel.fromFilePairs('./metadata_*.txt', size: 1)

    READFILE(input_ch)

    READFILE.out.view()
}