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.
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:
Type=oneshot with KillMode=none, but only do this if you know what you are doing).