Documented description of keepHeader in collectFile doesn't match what happens

[mahesh-panchal]

Bug report

In the documentation, it describes the keepHeader option of collectFile as "Prepend the resulting file with the header fetched in the first collected file", but this is not what happens in practice.

Expected behavior and actual behavior

The expected behaviour should be: "Prepend the resulting file with the header fetched in the first collected file".

The actual behaviour is that it keeps the header of all input files, not just the first.

Steps to reproduce the problem

workflow {
    TASK(Channel.of(1..3))
    ch_metrics = TASK.out
        .collectFile( keepHeader: true, skip: 1 , sort: false )
        .view { it.text }
}

process TASK {
    input:
    val id

    script:
    """
    cat <<-EOF > sample${id}.tsv
    assembly\tnum. contigs\tN50
    $id\t3\t40
    EOF
    """

    output:
    path "*.tsv"
}

Program output

$ nextflow run main.nf

 N E X T F L O W   ~  version 24.10.4

Launching `main.nf` [loving_faggin] DSL2 - revision: f077c6d708

executor >  local (3)
[96/8d2d4d] TASK (1) [100%] 3 of 3 ✔
assembly        num. contigs    N50
3       3       40

assembly        num. contigs    N50
2       3       40

assembly        num. contigs    N50
1       3       40

Environment

• Nextflow version: 24.10.4
• Java version: openjdk 17.0.14-internal 2025-01-21
• Operating system: Linux
• Bash version: GNU bash, version 5.1.16(1)-release (x86_64-pc-linux-gnu)