: 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
{
  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 ];
}
```

After building the package, the generated image can be loaded into a system through {manpage}`portablectl(1)`:

```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.
```
:::

:::{.example #ex-portableService-symlinks}
# Specifying symlinks when building a Portable Service image

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.

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`).

```nix
{
  lib,
  writeText,
  portableService,
  hello,
  cacert,
}:
let
  hello-service = writeText "hello.service" ''
    [Unit]
    Description=Hello world service

    [Service]
    Type=oneshot
    ExecStart=${lib.getExe hello}
  '';
in
portableService {
  pname = "hello";
  inherit (hello) version;
  units = [ hello-service ];
  symlinks = [
    {
      object = "${cacert}/etc/ssl";
      symlink = "/etc/ssl";
    }
  ];
}
```
:::