Linux
Services, systemd, filesystems and the everyday plumbing under everything else.
Notes
Why open ports do not prove an NFS server is alive, why a working mount on another host proves nothing, and how a failed automount quietly sends writes to local disk.
How I took a Raspberry Pi back to a fresh-install state without reflashing it: reconstruct what was added from /var/log/apt/history.log, unmount NFS before touching the mountpoint, and reboot to clear leftover Docker networking.
Guard rails that stop a routine static-site deploy from turning into an account wipe: sentinel files, exclusion lists and deletion ceilings.
System services run without HOME unless the unit sets User=, DynamicUser=, PAMName= or SetLoginEnvironment=. Go tools that look up the home directory then fail instantly, and a health check that throws its output away turns that into an abort with no reason attached.
Relaying through a cPanel mail server with strict TLS fails with "hostname mismatch" even though the certificate is valid. Postfix does not send SNI by default; smtp_tls_servername = nexthop fixes it.
Without pipefail, a backup pipeline reports the exit status of gzip, not of pg_dump. Dump to a temporary file in custom format, check it with pg_restore -l, and only then rename it.