build_image function from mkinitcpio

packinitcpio.sh

#!/bin/bash

build_image() {
    local out=$1 compress=$2 errmsg pipestatus

    case $compress in
        cat)
            msg "Creating uncompressed initcpio image: %s" "$out"
            unset COMPRESSION_OPTIONS
            ;;
        *)
            msg "Creating %s-compressed initcpio image: %s" "$compress" "$out"
            ;;&
        xz)
            COMPRESSION_OPTIONS=('--check=crc32' "${COMPRESSION_OPTIONS[@]}")
            ;;
        lz4)
            COMPRESSION_OPTIONS=('-l' "${COMPRESSION_OPTIONS[@]}")
            ;;
        zstd)
            COMPRESSION_OPTIONS=('-T0' "${COMPRESSION_OPTIONS[@]}")
            ;;
    esac

    pushd "$BUILDROOT" >/dev/null

    # Reproducibility: set all timestamps to 0
    find . -mindepth 1 -execdir touch -hcd "@0" "{}" +

    # If this pipeline changes, |pipeprogs| below needs to be updated as well.
    find . -mindepth 1 -printf '%P\0' |
            sort -z |
            LANG=C bsdtar --null -cnf - -T - |
            LANG=C bsdtar --uid 0 --gid 0 --null -cf - --format=newc @- |
            $compress "${COMPRESSION_OPTIONS[@]}" > "$out"

    pipestatus=("${PIPESTATUS[@]}")
    pipeprogs=('find' 'sort' 'bsdtar (step 1)' 'bsdtar (step 2)' "$compress")

    popd >/dev/null

    for (( i = 0; i < ${#pipestatus[*]}; ++i )); do
        if (( pipestatus[i] )); then
            errmsg="${pipeprogs[i]} reported an error"
            break
        fi
    done

    if (( _builderrors )); then
        warning "errors were encountered during the build. The image may not be complete."
    fi

    if [[ $errmsg ]]; then
        error "Image generation FAILED: %s" "$errmsg"
    elif (( _builderrors == 0 )); then
        msg "Image generation successful"
    fi
}

alias msg=echo
if [[ $# != 2 ]]; then
	echo "Usage: $0 initramfs.img compression_method"
	exit 1
fi
build_image "$1" "$2"