Hosting Operations
Read-onlyCheck Failed Dependencies for a Service
A service fails after startup, but the real blocker may be a dependent unit such as Redis, networking, a mount, or a socket.
Command
systemctl list-dependencies app-worker --failed --no-pagerBefore you run this
System impact: Read-only. Low when scoped to the shown target.
When not to use it: Do not assume every dependency failure is causal; inspect the dependency logs and timeline.
Expected output
A dependency tree containing failed units under the target service.
System impact
Read-only. Nothing changes. systemctl prints failed dependencies for the requested unit.
Recovery / rollback: no state is changed.
When to use it
Use when a service failure mentions connection refused, missing mounts, network targets, sockets, or required local services.
When not to use it
Do not assume every dependency failure is causal; inspect the dependency logs and timeline.
next steps
Related commands
Print Runtime Paths and User From systemd
Confirm the user, working directory, env file, and ExecStart systemd is actually using.
systemctl show app-worker --property=FragmentPath,DropInPaths,EnvironmentFiles,ExecStart,User,WorkingDirectory --no-pager
Find the First Failure Line for One Unit
The first failure line is often more useful than the last restart message.
journalctl -u app-worker -b --no-pager -o short-iso | grep -m1 -E 'ERROR|Failed|status='
Read Warning and Error Logs for One Failed Unit
Filter a failed unit's journal to the lines most likely to explain the stop.
journalctl -u app-worker -b -p warning..alert --no-pager -n 80
Build a Restart Loop Timeline
Restart loops make more sense when you line up starts, failures, and counters.
journalctl -u app-worker -b --no-pager -o short-iso | grep -E 'Started|Failed|Scheduled restart|Main process exited'
Spot Stale systemd Timers
The suspicious timer is the one with no next run.
systemctl list-timers --all --no-pager --plain | awk 'NR==1 || $1=="n/a" || /backup\.timer|logrotate\.timer/'
Study mapping
Use this as independent command practice: read the notes, predict the output, then compare it with the example before using a real shell.
Independent study support only. No affiliation, endorsement, exam dumps, or real exam questions.