[systemd-devel] ExecStart vs ExecStartPre

[Christian Seiler]

On 05/26/2015 11:12 PM, Steven Noonan wrote:
> Hi there,
> 
> I'm wondering what the functional difference is between doing:
> 
> ExecStartPre=/bin/foo
> ExecStart=/bin/bar
> 
> and
> 
> ExecStart=/bin/foo
> ExecStart=/bin/bar
> 
> From my read of the systemd.service man page, they appear to have the
> same behavior in the common use case.

Three differences come directly to mind:

 - If you have unit of type that is NOT oneshot (simple, forking,
   etc.), you can have only a single ExecStart= line, not multiple
   ones. The main service process must be started in ExecStart=
   and not ExecStartPre=.

 - Even in Type=oneshot units you must have at least one ExecStart=
   line (but can in any case have an arbitrary amount of ExecStartPre=
   lines, even zero).

 - If you set PermissionsStartOnly= or RootDirectoryStartOnly=, then
   certain settings will be applied to ExecStart= but not to
   ExecStartPre= (see manpage for details).

(There are probably more.)

Generally speaking, I follow the following guidelines when writing
units:

 - Type=oneshot: I typically use only ExecStart= and only use
   ExecStartPre= if I have to use *StartOnly=true (see above).

 - other types: ExecStart= to start the service proper, ExecStartPre=
   for preparatory things (like generating a default config if none
   is already present or something along those lines)

But it really depends on your use case and as always YMMV.

Hope that helps!

Christian