Build an OSX .pkg installer from Linux using mkbom and xar

build_mac.sh

#!/bin/bash

# change the values below to match your system.

# target the BUILD_DIR to output from an nw.io build process. nwjs-shell-builder recommended!
# https://github.com/Gisto/nwjs-shell-builder
# BASE_DIR is the target directory for this script, where files will be gathered and packaged to 

BUILD_DIR="/var/www/deploy/TMP/osx-ia32/latest-git"
BASE_DIR="/var/www/deploy/osx" 
VERSION="your_app_version"
IDENTIFIER="your.app.desc.com"
APPNAME="Your ApplicationName"
BACKGROUND="/var/www/yourApp/img/installerbackground.png"

rm -rf "$BASE_DIR/darwin"
mkdir -p "$BASE_DIR/darwin/flat/Resources/en.lproj"
mkdir -p "$BASE_DIR/darwin/flat/base.pkg/"
mkdir -p "$BASE_DIR/darwin/root/Applications"
cp -R $BUILD_DIR/*.* $BASE_DIR/darwin/root/Applications
cp $BACKGROUND $BASE_DIR/darwin/flat/Resources/en.lproj/background
NUM_FILES=$(find ${BASE_DIR}/darwin/root | wc -l)
INSTALL_KB_SIZE=$(du -k -s ${BASE_DIR}/darwin/root | awk '{print $1}')

cat <<EOF > ${BASE_DIR}/darwin/flat/base.pkg/PackageInfo
<?xml version="1.0" encoding="utf-8" standalone="no"?>
<pkg-info overwrite-permissions="true" relocatable="false" identifier="${IDENTIFIER}" postinstall-action="none" version="${VERSION}" format-version="2" generator-version="InstallCmds-502 (14B25)" auth="root">
 <payload numberOfFiles="${NUM_FILES}" installKBytes="${INSTALL_KB_SIZE}"/>
 <bundle-version>
 <bundle id="${IDENTIFIER}" CFBundleIdentifier="${IDENTIFIER}" path="./Applications/${APPNAME}.app" CFBundleVersion="1.3.0"/>
 </bundle-version>
 <update-bundle/>
 <atomic-update-bundle/>
 <strict-identifier/>
 <relocate/>
 <scripts/>
</pkg-info>
EOF

cat <<EOF > ${BASE_DIR}/darwin/flat/Distribution
<?xml version="1.0" encoding="utf-8"?>
<installer-script minSpecVersion="1.000000" authoringTool="com.apple.PackageMaker" authoringToolVersion="3.0.3" authoringToolBuild="174">
 <title>${APPNAME} ${VERSION}</title>
 <options customize="never" allow-external-scripts="no"/>
 <background file="background" alignment="bottomleft" scaling="none"/>
 <domains enable_anywhere="true"/>
 <installation-check script="pm_install_check();"/>
 <script>function pm_install_check() {
 if(!(system.compareVersions(system.version.ProductVersion,'10.5') >= 0)) {
 my.result.title = 'Failure';
 my.result.message = 'You need at least Mac OS X 10.5 to install ${APPNAME}.';
 my.result.type = 'Fatal';
 return false;
 }
 return true;
 }
 </script>
 <choices-outline>
 <line choice="choice1"/>
 </choices-outline>
 <choice id="choice1" title="base">
 <pkg-ref id="${IDENTIFIER}.base.pkg"/>
 </choice>
 <pkg-ref id="${IDENTIFIER}.base.pkg" installKBytes="${INSTALL_KB_SIZE}" version="${VERSION}" auth="Root">#base.pkg</pkg-ref>
</installer-script>
EOF

PKG_LOCATION="${BASE_DIR}/${APPNAME}-${VERSION}.pkg"

( cd ${BASE_DIR}/darwin/root && find . | cpio -o - format odc - owner 0:80 | gzip -c ) > ${BASE_DIR}/darwin/flat/base.pkg/Payload
mkbom -u 0 -g 80 ${BASE_DIR}/darwin/root ${BASE_DIR}/darwin/flat/base.pkg/Bom
( cd ${BASE_DIR}/darwin/flat/ && xar - compression none -cf "${PKG_LOCATION}" * )
echo "osx package has been built: ${PKG_LOCATION}"

Note: The cpio and xar commands are broken here with missing -, probably due to copy/pasta.
The commands should be cpio -o --format odc --owner 0:80 and xar --compression none -cf "${PKG_LOCATION}" *