The error: Running a script by hand works perfectly. The identical command in a crontab entry silently never produces results, with no obvious error anywhere.
Environment: Any Linux system using cron.
Linux
apt or dnf Is Locked by Another Process and Won't Run
The error: Running apt/apt-get or dnf/yum fails with a message about a lock file already being held, even though no package operation appears to be running.
Environment: Any Debian/Ubuntu (apt) or RHEL/Fedora-family (dnf/yum) system.
High Load Average but Low CPU Usage — Where's the Bottleneck?
The error: uptime or top shows a high load average, but the CPU usage percentage itself looks perfectly normal or even low — which seems contradictory.
Environment: Any Linux system.
SSH Connection Refused or Hangs, But the Server Is Definitely Up
The error: SSH connection attempts either get refused outright or hang indefinitely with no response, even though the server is confirmed running and reachable via ping.
Environment: Any Linux server running sshd.
A systemd Service Fails to Start and journalctl Shows Nothing Useful
The error: systemctl start myservice reports failure, but the default systemctl status output is too short to show the actual cause, and it's not obvious where else to look.
Environment: Any modern Linux distribution using systemd.
Why this happens: systemctl status deliberately truncates output for readability — the real error is almost always present, just cut off or living in a separate log stream than the one being checked first.
The fix:
"No Space Left on Device" but df Shows Plenty Free
The error: A process fails to write a file with "No space left on device," but df -h clearly shows the filesystem has free space available.
Environment: Any Linux system, most commonly seen on servers handling large numbers of small files (mail queues, session caches, log directories).
SELinux Is Blocking My Service and the Permissions Look Fine
The error: A service fails to start, or a process gets "Permission denied" trying to read/write a file — despite ls -l showing exactly the ownership and permissions you'd expect. chmod, chown, double- and triple-checking the obvious — nothing changes.
Environment: Any RHEL/CentOS/Rocky/Alma system with SELinux in Enforcing mode (the default).
SELinux Is Blocking My Service and the Permissions Look Fine
The error: A service fails to start, or a process gets "Permission denied" trying to read/write a file — despite ls -l showing exactly the ownership and permissions you'd expect. chmod, chown, double- and triple-checking the obvious — nothing changes.
Environment: Any RHEL/CentOS/Rocky/Alma system with SELinux in Enforcing mode (the default).