lxc enter pipe error

Hello , I have some strange behaviour using "lxc enter VMID" command. I use this command to generate passwords into a bash script : "cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 20 | head -n 1" This works on almost any machine, but not into an "lxc enter" session. This command justs freeze...