Runs a program in a program within a netns that temporary steals a host interface

run-in-netns

#!/bin/bash
  
#This script "steals" an interface from the host to a temporary network namespace
#Then, Linux will release the interface back to the host
#Teardown is guaranteed by kernel when all pid with iface/netns are dead

iface="$1"
shift

cmd=""
while [ "$#" -ge 1 ];do
    cmd="$cmd $(printf '%q' "$1")"
    shift
done

pidfile=$(mktemp)
(
    if ! unshare -nfp --mount-proc bash -c "
        #This is the pid of the (), not unshared!&
        echo $$ > $pidfile
        while [ -f $pidfile ];do sleep .1;done
        $cmd || true
        kill -9 -1 || true
    ";then
        echo shit
        echo shit > $pidfile
    fi
) &

while [ ! -s "$pidfile" ];do
    sleep .1
done

#Pid of subshell
pid=$(cat $pidfile)
if [ "$pid" = "shit" ];then
    echo shit...
    exit 1
fi

#Pid of unshare
pid=$(pgrep -P $pid)
#Pid of shell inder unshare
pid=$(pgrep -P $pid)
ip link set dev "$iface" netns $(pgrep -P $pid)
rm -f $pidfile

wait