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docs/podman-systemd: Try to clarify `Exec=` more 

In podman-systemd we are intersecting the worlds of containers
and systemd, and I had to stop and think to understand what
`Exec=` does.

I tried to clarify things more here.

I found it especially confusing because the example at the
very top of the file does:

```
Image=quay.io/fedora/fedora
Exec=sleep 10
```

But that only makes sense because the fedora base image
(being generic) doesn't define an `ENTRYPOINT`, just a `CMD`.

But IMO by far the most common usage for podman-systemd
is "app images" which conventionally should use `ENTRYPOINT`
in general. Maybe we should change the default example,
but I'm leaving that for a later followup.

(It perhaps would have been less confusing if this field
 had been called `Args=` to make clear it's quite different
 in practice from systemd `ExecStart=`)

Signed-off-by: Colin Walters <walters@verbum.org>
This commit is contained in:
Colin Walters 2024-08-02 09:41:24 -04:00
parent 40df14012b
commit d26341332c
1 changed files with 13 additions and 3 deletions
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16
docs/source/markdown/podman-systemd.unit.5.md
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@ -435,9 +435,19 @@ Use the host environment inside of the container.
### `Exec=` ### `Exec=`
If this is set then it defines what command line to run in the container. If it is not set the Additional arguments for the container; this has exactly the same effect as passing
default entry point of the container image is used. The format is the same as for more arguments after a `podman run <image> <arguments>` invocation.
[systemd command lines](https://www.freedesktop.org/software/systemd/man/systemd.service.html#Command%20lines).
The format is the same as for [systemd command lines](https://www.freedesktop.org/software/systemd/man/systemd.service.html#Command%20lines),
However, unlike the usage scenario for similarly-named systemd `ExecStart=` verb
which operates on the ambient root filesystem, it is very common for container
images to have their own `ENTRYPOINT` or `CMD` metadata which this
which this interacts with.
The default expectation for many images is that the image will include an `ENTRYPOINT`
with a default binary, and this field will add arguments to that entrypoint.
Another way to describe this is that it works the same way as the [args field in a Kubernetes pod](https://kubernetes.io/docs/tasks/inject-data-application/define-command-argument-container/#running-a-command-in-a-shell).
### `ExposeHostPort=` ### `ExposeHostPort=`