Patch MTU on KVM uCernVM image

patch-ucvm-mtu.sh

#!/bin/bash
cd $(dirname "$0")
ucvm=$PWD/ucernvm-prod.1.18-2.cernvm.x86_64.hdd
[ ! -e ${ucvm}.orig ] && cp $ucvm ${ucvm}.orig
t=$(mktemp -d /tmp/cvm-initrd-XXXXX)
cd $t
echo starting guestfish
eval "`guestfish --listen`" || exit 1
echo mounting fs
guestfish --remote add $ucvm : run : mount /dev/sda1 / || exit 1
echo getting file
guestfish --remote download /cernvm/initrd.img $PWD/initrd.img || exit 1
echo uncompressing
mkdir fs && cd fs || exit 1
cat ../initrd.img | xz -dc | cpio -id || exit 1
rm -f ../initrd.img
echo patching
sed -e 's/mtu 1460/mtu 1400/' -i init || exit 1
echo compressing
find . | cpio --create --format=newc | xz -9 > ../initrd.img || exit 1
echo putting file
cd ..
guestfish --remote upload initrd.img /cernvm/initrd.img || exit 1
echo stopping guestfish
guestfish --remote exit
cd /
rm -rf $t
echo sha256 checksums
echo "original file: $(sha256sum ${ucvm}.orig)"
echo "modified file: $(sha256sum $ucvm)"