I'm trying to use an Ubuntu 18.04 LiveUSB to recover a lost file. I've booted into the LiveUSB. Now, I need to install the 'testdisk' package.
I can run $ sudo apt update and $ sudo apt upgrade with normal output and no errors.  However, I can't install packages.  If I use apt-cache show to find a package, I always get a "No packages found" error.  For instance:
$ apt-cache show testdisk
N: Unable to locate package testdisk
E: No packages found
$ apt-cache show cowsay
N: Unable to locate package cowsay
E: No packages found
If I try to install, I always get results like
$ sudo apt install testdisk
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package testdisk.
I am connected to the network and can browse the Internet normally from the LiveUSB.
This AskUbuntu answer says that I ought to be able to install packages to the LiveUSB.  I don't need it to be persistent, I just need testdisk long enough to recover a file.
How can I install packages in an Ubuntu 18.04 LiveUSB?
 
     
     
    