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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=`
If this is set then it defines what command line to run in the container. If it is not set the
default entry point of the container image is used. The format is the same as for
[systemd command lines](https://www.freedesktop.org/software/systemd/man/systemd.service.html#Command%20lines).
Additional arguments for the container; this has exactly the same effect as passing
more arguments after a `podman run <image> <arguments>` invocation.
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=`