Siphon
Intercept input/output (stdin/stdout/stderr) for any process, even where said output is sent to /dev/null or elsewhere.
It can also be used to spy on another users shell:
Currently Siphon works on Linux, with amd64, arm64, arm, and 386. Adding support for more architectures is pretty simple, feel free to raise an issue.
It uses ptrace which means you'll likely need to run it as root for the ptrace privilege.
Installation
Grab a binary from the latest release.