Best way to set a conditional output filter (head or output "as is") [duplicate]

Question

For testing purpose, I want to activate or not a filtering with the command head.

For example, the command below could outputs a lot of lines

#!/bin/bash
produce_a_lot_of_lines

I want to set on demand a filter with head, depending on a variable. A way The sample below works:

#!/bin/bash
function filter() {
  [[ "$HEAD" =~ [0-9]+ ]] && head -n $HEAD || cat
}
produce_a_lot_of_lines | filter

Is there a cleaner/more efficient/better way to achieve this?

if [[ "$HEAD" =~ ^[0-9]+$ ]]; then PaLoL | head -n "$HEAD"; else PaLoL; fi — user313992
– user313992, Commented Jan 9, 2020 at 20:51
@mosvy, upvoted comment. It repeats PaLoL, but avoid additional cat. — Amessihel
– Amessihel, Commented Jan 9, 2020 at 20:53

Philippe · Accepted Answer · 2020-01-09 20:59:45Z

1

A slightly more reusable way :

function filter() {
    local nb_lines="$1"; shift
    if  [[ "$nb_lines" =~ ^[0-9]+$ ]]; then
        "$@" | head -n "$nb_lines"
    else
        "$@"
    fi
}

filter "$HEAD" produce_a_lot_of_lines

answered Jan 9, 2020 at 20:59

Philippe

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