Check: getting started with Linux

Linux is the kernel. Ubuntu, Alpine, and RHEL are distributions: the kernel plus a shell, utilities, a package manager, and configs. That's why uname -s answers the same way on any of them.

docker run --rm alpine:3.20.3 uname -s
docker run --rm ubuntu:24.04 uname -s

Linux
Linux

Configs live in /etc. /var holds logs and data that change while the system runs. /opt holds large third-party packages as a whole. /proc isn't on disk at all: it's a virtual filesystem the kernel generates on the fly.

docker run --rm alpine:3.20.3 ls /etc

Among the files you'll see os-release, passwd, hostname, and the apk folder with the package manager's settings.

apk runs on Alpine. Debian and Ubuntu install packages with apt, RHEL and Rocky with dnf, Arch with pacman.

docker run --rm alpine:3.20.3 sh -c 'command -v apk; command -v apt'
docker run --rm ubuntu:24.04 sh -c 'command -v apk; command -v apt'

/sbin/apk
/usr/bin/apt

Each system found only its own manager; the other one isn't there.

The contents of /tmp aren't guaranteed to survive a reboot. On some systems /tmp is mounted as tmpfs in memory and empties on every start; on others, systemd-tmpfiles removes old files on a timer. /var/log isn't cleared on reboot, so logs pile up there and can fill the whole disk. /var/lib holds service data (databases, for example), and /etc holds configs. The system leaves those alone too.

You can't check this one in a container: a container has no reboot of its own. On a Linux machine with systemd, check how /tmp is mounted:

findmnt /tmp

If the FSTYPE column shows tmpfs, the directory lives in memory. Empty output means /tmp sits on the root disk and a timer removes old files from there.

du (disk usage) counts space on disk. The -s flag (summarize) prints one total per directory instead of a line for every nested folder. -h (human-readable) prints the size in K, M, G instead of blocks. The command only reads; it changes nothing.

docker run --rm ubuntu:24.04 du -sh /var/log

288K	/var/log

Your number may differ; what matters is the shape: one line, a size with a letter.