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doc: update docs for portableService, follow doc conventions
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# pkgs.portableService {#sec-pkgs-portableService}
`pkgs.portableService` is a function to create _portable service images_,
as read-only, immutable, `squashfs` archives.
systemd supports a concept of [Portable Services](https://systemd.io/PORTABLE_SERVICES/).
Portable Services are a delivery method for system services that uses two specific features of container management:
* Applications are bundled. I.e. multiple services, their binaries and
all their dependencies are packaged in an image, and are run directly from it.
* Stricter default security policies, i.e. sandboxing of applications.
This allows using Nix to build images which can be run on many recent Linux distributions.
The primary tool for interacting with Portable Services is `portablectl`,
and they are managed by the `systemd-portabled` system service.
`pkgs.portableService` is a function to create [Portable Services](https://systemd.io/PORTABLE_SERVICES/) in a read-only, immutable, `squashfs` raw disk image.
This lets you use Nix to build images which can be run on many recent Linux distributions.
::: {.note}
Portable services are supported starting with systemd 239 (released on 2018-06-22).
:::
A very simple example of using `portableService` is described below:
The generated image will contain the file system structure as required by the Portable Services specification, along with the packages given to `portableService` and all of their dependencies.
When generated, the image will exist in the Nix store with the `.raw` file extension, as required by the specification.
See [](#ex-portableService-hello) to understand how to use the output of `portableService`.
## Inputs {#ssec-pkgs-portableService-inputs}
`portableService` expects one argument with the following attributes:
`pname` (String)
: The name of the portable service.
The generated image will be named according to the template `$pname_$version.raw`, which is supported by the Portable Services specification.
`version` (String)
: The version of the portable service.
The generated image will be named according to the template `$pname_$version.raw`, which is supported by the Portable Services specification.
`units` (List of Attribute Set)
: A list of derivations for systemd unit files.
Each derivation must produce a single file, and must have a name that starts with the value of `pname` and ends with the suffix of the unit type (e.g. ".service", ".socket", ".timer", and so on).
See [](#ex-portableService-hello) to better understand this naming constraint.
`description` (String or Null; _optional_)
: If specified, the value is added as `PORTABLE_PRETTY_NAME` to the `/etc/os-release` file in the generated image.
This could be used to provide more information to anyone inspecting the image.
_Default value:_ `null`.
`homepage` (String or Null; _optional_)
: If specified, the value is added as `HOME_URL` to the `/etc/os-release` file in the generated image.
This could be used to provide more information to anyone inspecting the image.
_Default value:_ `null`.
`symlinks` (List of Attribute Set; _optional_)
: A list of attribute sets in the format `{object, symlink}`.
For each item in the list, `portableService` will create a symlink in the path specified by `symlink` (relative to the root of the image) that points to `object`.
All packages that `object` depends on and their dependencies are automatically copied into the image.
This can be used to create symlinks for applications that assume some files to exist globally (`/etc/ssl` or `/bin/bash`, for example).
See [](#ex-portableService-symlinks) to understand how to do that.
_Default value:_ `[]`.
`contents` (List of Attribute Set; _optional_)
: A list of additional derivations to be included as-is in the image.
These derivations will be included directly in a `/nix/store` directory inside the image.
_Default value:_ `[]`.
`squashfsTools` (Attribute Set; _optional_)
: Allows you to override the package that provides {manpage}`mksquashfs(1)`, which is used internally by `portableService`.
_Default value:_ `pkgs.squashfsTools`.
`squash-compression` (String; _optional_)
: Passed as the compression option to {manpage}`mksquashfs(1)`, which is used internally by `portableService`.
_Default value:_ `"xz -Xdict-size 100%"`.
`squash-block-size` (String; _optional_)
: Passed as the block size option to {manpage}`mksquashfs(1)`, which is used internally by `portableService`.
_Default value:_ `"1M"`.
## Examples {#ssec-pkgs-portableService-examples}
[]{#ex-pkgs-portableService}
:::{.example #ex-portableService-hello}
# Building a Portable Service image
The following example builds a Portable Service image with the `hello` package, along with a service unit that runs it.
```nix
pkgs.portableService {
pname = "demo";
version = "1.0";
units = [ demo-service demo-socket ];
{ lib, writeText, portableService, hello }:
let
hello-service = writeText "hello.service" ''
[Unit]
Description=Hello world service
[Service]
Type=oneshot
ExecStart=${lib.getExe hello}
'';
in
portableService {
pname = "hello";
inherit (hello) version;
units = [ hello-service ];
}
```
The above example will build an squashfs archive image in `result/$pname_$version.raw`. The image will contain the
file system structure as required by the portable service specification, and a subset of the Nix store with all the
dependencies of the two derivations in the `units` list.
`units` must be a list of derivations, and their names must be prefixed with the service name (`"demo"` in this case).
Otherwise `systemd-portabled` will ignore them.
After building the package, the generated image can be loaded into a system through {manpage}`portablectl(1)`:
::: {.note}
The `.raw` file extension of the image is required by the portable services specification.
```shell
$ nix-build
(some output removed for clarity)
/nix/store/8c20z1vh7z8w8dwagl8w87b45dn5k6iq-hello-img-2.12.1
$ portablectl attach /nix/store/8c20z1vh7z8w8dwagl8w87b45dn5k6iq-hello-img-2.12.1/hello_2.12.1.raw
Created directory /etc/systemd/system.attached.
Created directory /etc/systemd/system.attached/hello.service.d.
Written /etc/systemd/system.attached/hello.service.d/20-portable.conf.
Created symlink /etc/systemd/system.attached/hello.service.d/10-profile.conf → /usr/lib/systemd/portable/profile/default/service.conf.
Copied /etc/systemd/system.attached/hello.service.
Created symlink /etc/portables/hello_2.12.1.raw → /nix/store/8c20z1vh7z8w8dwagl8w87b45dn5k6iq-hello-img-2.12.1/hello_2.12.1.raw.
$ systemctl start hello
$ journalctl -u hello
Feb 28 22:39:16 hostname systemd[1]: Starting Hello world service...
Feb 28 22:39:16 hostname hello[102887]: Hello, world!
Feb 28 22:39:16 hostname systemd[1]: hello.service: Deactivated successfully.
Feb 28 22:39:16 hostname systemd[1]: Finished Hello world service.
$ portablectl detach hello_2.12.1
Removed /etc/systemd/system.attached/hello.service.
Removed /etc/systemd/system.attached/hello.service.d/10-profile.conf.
Removed /etc/systemd/system.attached/hello.service.d/20-portable.conf.
Removed /etc/systemd/system.attached/hello.service.d.
Removed /etc/portables/hello_2.12.1.raw.
Removed /etc/systemd/system.attached.
```
:::
Some other options available are:
- `description`, `homepage`
:::{.example #ex-portableService-symlinks}
# Specifying symlinks when building a Portable Service image
Are added to the `/etc/os-release` in the image and are shown by the portable services tooling.
Default to empty values, not added to os-release.
- `symlinks`
Some services may expect files or directories to be available globally.
An example is a service which expects all trusted SSL certificates to exist in a specific location by default.
A list of attribute sets {object, symlink}. Symlinks will be created in the root filesystem of the image to
objects in the Nix store. Defaults to an empty list.
- `contents`
To make things available globally, you must specify the `symlinks` attribute when using `portableService`.
The following package builds on the package from [](#ex-portableService-hello) to make `/etc/ssl` available globally (this is only for illustrative purposes, because `hello` doesn't use `/etc/ssl`).
A list of additional derivations to be included in the image Nix store, as-is. Defaults to an empty list.
- `squashfsTools`
Defaults to `pkgs.squashfsTools`, allows you to override the package that provides `mksquashfs`.
- `squash-compression`, `squash-block-size`
Options to `mksquashfs`. Default to `"xz -Xdict-size 100%"` and `"1M"` respectively.
A typical usage of `symlinks` would be:
```nix
symlinks = [
{ object = "${pkgs.cacert}/etc/ssl"; symlink = "/etc/ssl"; }
{ object = "${pkgs.bash}/bin/bash"; symlink = "/bin/sh"; }
{ object = "${pkgs.php}/bin/php"; symlink = "/usr/bin/php"; }
];
```
to create these symlinks for legacy applications that assume them existing globally.
{ lib, writeText, portableService, hello, cacert }:
let
hello-service = writeText "hello.service" ''
[Unit]
Description=Hello world service
Once the image is created, and deployed on a host in `/var/lib/portables/`, you can attach the image and run the service. As root run:
```console
portablectl attach demo_1.0.raw
systemctl enable --now demo.socket
systemctl enable --now demo.service
[Service]
Type=oneshot
ExecStart=${lib.getExe hello}
'';
in
portableService {
pname = "hello";
inherit (hello) version;
units = [ hello-service ];
symlinks = [
{ object = "${cacert}/etc/ssl"; symlink = "/etc/ssl"; }
];
}
```
::: {.note}
See the [man page](https://www.freedesktop.org/software/systemd/man/portablectl.html) of `portablectl` for more info on its usage.
:::

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"group(5)": "https://man.archlinux.org/man/group.5",
"login.defs(5)": "https://man.archlinux.org/man/login.defs.5",
"unshare(1)": "https://man.archlinux.org/man/unshare.1.en",
"nix-shell(1)": "https://nixos.org/manual/nix/stable/command-ref/nix-shell.html"
"nix-shell(1)": "https://nixos.org/manual/nix/stable/command-ref/nix-shell.html",
"mksquashfs(1)": "https://man.archlinux.org/man/extra/squashfs-tools/mksquashfs.1.en"
}