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What is the difference between systemd service Type oneshot and simple? This link states to use simple instead of oneshot for timers. I am not able to understand it correctly.

sjy
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Dinesh P.R.
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2 Answers2

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The Type=oneshot service unit:

  • blocks on a start operation until the first process exits, and its state will be reported as "activating";

  • once the first process exits, transitions from "activating" straight to "inactive", unless RemainAfterExit=true is set (in which case it becomes "active" with no processes!);

  • may have any number (0 or more) of ExecStart= directives which will be executed sequentially (waiting for each started process to exit before starting the next one);

  • may leave out ExecStart= but have ExecStop= (useful together with RemainAfterExit=true for arranging things to run on system shutdown).

The Type=simple service unit:

  • does not block on a start operation (i. e. becomes "active" immediately after forking off the first process, even if it is still initializing!);

  • once the first process exits, transitions from "active" to "inactive" (there is no RemainAfterExit= option);

  • is generally discouraged because there is no way to distinguish situations like "exited on start because of a configuration error" from "crashed after 500ms of runtime" and suchlike.

Both Type=oneshot and Type=simple units:

  • ignore any children of the first process, so do not use these modes with forking processes (note: you may use Type=oneshot with KillMode=none, but only do this if you know what you are doing).
intelfx
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    intelfx says that the `Type=oneshot` service unit "can have no `ExecStart=`". This is not true. In fact, services of type `oneshot` can have multiple `ExecStart=` directives. See any recent man page for `systemd.service` for more information. – rlandster Oct 22 '17 at 01:09
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    @rlandster: Why is this "not true"? [systemd.service(5)](https://www.freedesktop.org/software/systemd/man/systemd.service.html#ExecStart=) says under `ExecStart=`: "Unless Type= is oneshot, exactly one command must be given. When Type=oneshot is used, zero or more commands may be specified." – intelfx Oct 22 '17 at 01:22
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    agree that "can have no" is confusing. that is not true. but i think you can't have `ExecReload` – Tony Apr 02 '18 at 01:43
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    By "can have no" I assume that @intelfx meant "can have zero or more," or "does not require a," and not "can't have any." However, rewording it for clarity would help future readers. – seh May 01 '18 at 18:17
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    "Can even have no" would be better wording perhaps – Manchineel Jul 15 '20 at 09:07
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From systemd's point of view, Type=simple is kind of fire and forget. Systemd just forks a process defined in ExecStart= and goes on its way, even if the process fails to start.

Alexis Wilke
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Umut
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    `Type=simple` processes are still monitored by systemd, and will be restarted depending on the value of the `Restart` setting. – ldx.a.ldy.c Feb 01 '18 at 16:51