How to create a bootable El Capitan Installer from MacOS. See https://bootableinstaller.com

create-bootable-installer-for-os-x-el-capitan.sh

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#                           See bootableinstaller.com                          # 
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# set strict and verbose modes for bash
set -e
set -u

# If the script already ran once successfully, don't re-run
if [ -f "el-capitan.iso" ]; then
    echo "'el-capitan.iso' already exists"
    exit 0
fi

# Test for variations of InstallESD.dmg
my_installesd="./InstallESD.dmg"
if [ -f "./ElCapitanInstallESD.img" ]; then
    my_installesd="./ElCapitanInstallESD.img"
fi
if [ -f "./ElCapitanInstallESD.dmg" ]; then
    my_installesd="./ElCapitanInstallESD.dmg"
fi
if [ -f "./InstallESD.img" ]; then
    my_installesd="./InstallESD.img"
fi

# Prompt the user to download InstallMacOSX.dmg if it doesn't exist
if ! [ -f "$my_installesd" ]; then
    if ! [ -f "InstallMacOSX.dmg" ]; then
        echo "Error: Did not find 'InstallMacOSX.dmg'"
        echo "    1. Go to https://support.apple.com/en-us/HT206886 (How to upgrade to OS X El Capitan)"
        echo "    2. Click the 'Download OS X El Capitan' link in Step 4"
        echo "    3. Copy 'InstallMacOSX.dmg' to this folder ("$(pwd)")"
        exit 1
    fi

    # show output of each command
    set -x

    # Mount the Full Installer and extract the Install System 
    hdiutil attach InstallMacOSX.dmg -noverify -nobrowse -mountpoint '/Volumes/Install OS X/'
    pkgutil --expand '/Volumes/Install OS X/InstallMacOSX.pkg' './Install OS X/'
    hdiutil detach '/Volumes/Install OS X'
    mv './Install OS X/InstallMacOSX.pkg/InstallESD.dmg' "$my_installesd"

    # Cleanup
    rm -rf './Install OS X/'
else
    # show output of each command
    set -x
fi

# Mount DMG Installer
hdiutil attach "$my_installesd" -noverify -nobrowse -mountpoint '/Volumes/OS X Install ESD/'

# Create a DMG Disk
# (this exact file created by this command is included at bootableinstaller.com as 'empty.7900m.dmg.bz2')
# This is 656mb bigger than it needs to be, might might as well leave the excess for potential variation
if ! [ -f "el-capitan.dmg" ]; then
    hdiutil create -o ./el-capitan -size 7900m -volname el-capitan -layout SPUD -fs HFS+J
fi

# Mount el-capitan.dmg disk to your macOS
hdiutil attach ./el-capitan.dmg -noverify -mountpoint /Volumes/el-capitan

# Create Installer Disk
# (in recovery mode the system is already mounted as 'OS X Base System', hence the weird stuff)
asr restore -source '/Volumes/OS X Install ESD/BaseSystem.dmg' -target /Volumes/el-capitan -noprompt -noverify -erase
hdiutil detach '/Volumes/OS X Base System' || true
hdiutil detach '/Volumes/OS X Base System 1' || true
hdiutil detach '/Volumes/el-capitan' || true
hdiutil attach ./el-capitan.dmg -noverify -mountpoint /Volumes/el-capitan
rm '/Volumes/el-capitan/System/Installation/Packages'
cp -rp '/Volumes/OS X Install ESD/Packages' '/Volumes/el-capitan/System/Installation/'
cp -rp '/Volumes/OS X Install ESD/BaseSystem.chunklist' '/Volumes/el-capitan'
cp -rp '/Volumes/OS X Install ESD/BaseSystem.dmg' '/Volumes/el-capitan'
hdiutil detach '/Volumes/el-capitan'
hdiutil detach '/Volumes/OS X Install ESD'

# Convert DMG Disk to ISO File
# (this turns out to be a straight copy, as the image was created in the correct format)
# hdiutil convert ./el-capitan.dmg -format UDTO -o ./el-capitan

# Move DMG to ISO, here
mv ./el-capitan.dmg el-capitan.iso

# Cleanup
# rm -f ./el-capitan.dmg

# Many thanks to https://techsviewer.com/install-macos-elcapitan-virtualbox-windows/