Run one-time setup tasks with systemd Type=oneshot, RemainAfterExit and ExecStartPre
domain: systemd · 7 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Type=oneshot is for run-to-completion tasks: systemd waits for the process to finish before considering the unit started, so dependents (After=/Requires=) start only after it succeeded:
[Unit]
Description=prepare data dir
Before=myapp.service
[Service]
Type=oneshot
ExecStart=/usr/local/bin/prepare-data
RemainAfterExit=yes
[Install]
WantedBy=multi-user.target
RemainAfterExit=yes keeps the unit 'active (exited)' after the process ends, so it is not re-run every time a dependent starts and `systemctl status` reflects that it ran. Without it the unit goes directly to inactive/dead after running.
oneshot allows MULTIPLE ExecStart= lines executed serially (the only Type that does); any non-zero exit aborts the sequence and fails the unit. Prefix a command with '-' to ignore its failure: `ExecStart=-/usr/bin/cleanup-optional`.
ExecStartPre= lines (any service type) run serially before ExecStart with the same '-' semantics — use for preflight checks; they cannot set environment variables for ExecStart.
Bind it to the consumer: in myapp.service add `Wants=prepare-data.service` and `After=prepare-data.service` (or Requires= for a hard dependency).
Re-run an already 'active (exited)' oneshot with `systemctl restart prepare-data.service` — `systemctl start` on an active unit is a no-op.
For long oneshots raise TimeoutStartSec (default 90s), or set `TimeoutStartSec=infinity`.
Known gotchas
Default oneshot without RemainAfterExit reports 'inactive (dead)' after success — monitoring that checks for 'active' will false-alarm.
Restart=always is not allowed with plain oneshot semantics you might expect: systemd refuses Restart=always for Type=oneshot units (`Service has Restart= setting other than no, which isn't allowed for Type=oneshot services. Refusing.`); use a timer for periodic runs.
Multiple ExecStart lines in non-oneshot services are refused at start: `Service has more than one ExecStart= setting, which is only allowed for Type=oneshot services.`
ExecStartPre runs in a fresh process; `export FOO=1` there does not reach ExecStart — use EnvironmentFile written by the pre step if you must pass data.
With RemainAfterExit=yes, ExecStop= (if present) runs at `systemctl stop`, which may be at shutdown — do not put one-time logic there accidentally.
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