## troubleshooting --fakeroot issues $ singularity build --fakeroot ... FATAL: could not use fakeroot: no user entry found for {USER} This probably means that _subuid_ range is not defined in the "/etc/subuid" file. On up-to-date system subuid/subgid ranges are automatically assigned when a new user is added (e.g. `adduser {USER}`) but on systems upgraded from prior Debian releases {USER} created in old environment before upgrade may not have _subuid_ mapping. "usermod" command have "--add-subuids" and "--add-subgids" options but it does not check "/etc/login.defs" for ranges. An awkward solution may be to add a new temporary user, apply her ranges to {USER} (in "/etc/subuid" and in "/etc/subgid") then remove a temporary user (e.g. `deluser --remove-home {USER}`). The following command show the subuids and subgids of the current user: cat /etc/s*id|grep $USER --- $ singularity build --fakeroot ... ERROR : Failed to create container namespaces The above error means that the system has been booted without "kernel.unprivileged_userns_clone=1" option. To enable user namespaces use the following command: sudo sysctl kernel.unprivileged_userns_clone=1 See also https://sylabs.io/guides/3.4/user-guide/fakeroot.html https://github.com/sylabs/singularity/issues/4361 singularity config fakeroot --help