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Include documentation for template manager design
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WillyPillow
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developer/system/template-manager.md
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developer/system/template-manager.md
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Template Manager Design
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=======================
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Motivation
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----------
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Previously, template VMs were distributed by RPM packages and managed by `yum`/`dnf`.
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However, tracking inherently dynamic VM images with a package manager suited for static files creates some challenges.
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For example, users may accidentally update the images, overriding local changes ([#996], [#1647]).
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(Or in the case of [#2061], want to specifically override the changes.)
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Other operations that work well on normal VMs are also somewhat inconsistent on RPM-managed templates.
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This includes actions such as renaming ([#839]), removal ([#5509]) and backup/restore ([#1385], [#1453], [discussion thread 1], [discussion thread 2]).
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In turn, this creates inconveniences and confusion for users ([#1403], [#4518]).
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Also, the usage of RPM packages meant that installing a template results in arbitrary code execution, which is not ideal.
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Besides distribution, users may also wish to have an integrated template management application ([#2062], [#2064], [#2534], [#3040]), as opposed to the situation where multiple programs are required for different purposes, e.g., `qubes-dom0-update`, `dnf`, `qvm-remove`, `qubes-manager`.
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To tackle these issues, `qvm-template` is created.
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It strives to provide not only a better mechanism for handling template installation but also a consistent user-facing interface to deal with template management.
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[#996]: https://github.com/QubesOS/qubes-issues/issues/996
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[#1647]: https://github.com/QubesOS/qubes-issues/issues/1647
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[#2061]: https://github.com/QubesOS/qubes-issues/issues/2061
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[#839]: https://github.com/QubesOS/qubes-issues/issues/839
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[#5509]: https://github.com/QubesOS/qubes-issues/issues/5509
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[#1385]: https://github.com/QubesOS/qubes-issues/issues/1385
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[#1453]: https://github.com/QubesOS/qubes-issues/issues/1453
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[#1403]: https://github.com/QubesOS/qubes-issues/issues/1403
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[#4518]: https://github.com/QubesOS/qubes-issues/issues/4518
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[#2062]: https://github.com/QubesOS/qubes-issues/issues/2062
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[#2064]: https://github.com/QubesOS/qubes-issues/issues/2064
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[#2534]: https://github.com/QubesOS/qubes-issues/issues/2534
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[#3040]: https://github.com/QubesOS/qubes-issues/issues/3040
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[discussion thread 1]: https://groups.google.com/forum/#!topic/qubes-devel/rwc2_miCNNE/discussion
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[discussion thread 2]: https://groups.google.com/forum/#!topic/qubes-users/uQEUpv4THsY/discussion
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Features
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--------
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- Install/reinstall/downgrade/upgrade templates, either from local packages or remote repositories
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- Ability to install templates in alternative pools
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- Possibility for the template package to specify options such as the kernel or `virt_mode` used
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- List and show information about local and available templates
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- Machine-readable output for easy extension
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- Search for templates
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- Remove templates
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- Ability to disassociate or remove VMs based on the template to be removed
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- Show available repositories
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- Works in both dom0 and management VMs by utilizing the Admin API
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- Works well with existing tools
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- Command-line interface with DNF-like usage
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- A graphical interface also available
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Package Format
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--------------
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The RPM package format is still used.
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However, the contents are manually extracted instead of installing the whole package.
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This allows us to take advantage of existing tools for things like repository management.
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We can also avoid the burden of dealing with verification, reducing the risk of issues like [QSB-28].
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The package name should be in the form `qubes-template-${template_name}`.
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For the package metadata (e.g., summary, description), one needs to ensure that the character `|` does not appear.
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This is because we use the character as a separator internally.
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Failing to do so may result in `qvm-template` to fail with cryptic error messages.
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Note that as we already consider the repository metadata untrusted, this should not result in security issues.
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The file structure should be quite similar to previous template RPMs.
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Namely, there should be the following files in the package:
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- `var/lib/qubes/vm-templates/${template_name}/root.img.part.{00,01,...}`
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- Split tarball of template `root.img`
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- Note that the file is still split due to tools such as `rpm2cpio` not supporting large files.
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(Notably, the cpio format does not support files over 4GiB.)
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- `var/lib/qubes/vm-templates/${template_name}/template.conf`
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- Stores custom package metadata (as RPM does not support custom attributes).
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- Uses `KEY=VALUE` format.
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- Fields (corresponding to [qvm-prefs]/[qvm-features] tags with the same names)
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- `virt_mode`
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- Setting this to `pv` requires user confirmation.
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- `kernel`
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- Only allowed to be set to "", i.e., "none", for PVGrub.
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- `no-monitor-layout`
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- `pci-e820-host`
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- `linux-stubdom`
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- `gui`
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- `gui-emulated`
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- `qrexec`
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- These six flags are only allowed to be set to "1" or "0", denoting "true" and "false" respectively.
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- `net.fake-ip`
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- `net.fake-gateway`
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- `net.fake-netmask`
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- `var/lib/qubes/vm-templates/${template_name}/whitelisted-appmenus.list`
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- Contains default app menu entries of the template itself.
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- `var/lib/qubes/vm-templates/${template_name}/vm-whitelisted-appmenus.list`
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- Contains default app menu entries of VMs based on the template.
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- `var/lib/qubes/vm-templates/${template_name}/netvm-whitelisted-appmenus.list`
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- Contains default app menu entries of NetVMs based on the template.
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- These three files are the same as the current format.
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- Note that the contents of these files are stored in `qvm-features` upon installation.
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See the section below for details.
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[QSB-28]: https://github.com/QubesOS/qubes-secpack/blob/master/QSBs/qsb-028-2016.txt
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[qvm-prefs]: https://dev.qubes-os.org/projects/core-admin-client/en/stable/manpages/qvm-prefs.html#common-properties
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[qvm-features]: https://dev.qubes-os.org/projects/core-admin-client/en/stable/manpages/qvm-features.html#list-of-known-features
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Metadata Storage
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----------------
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The template manager needs to keep metadata of installed templates such as versions and origin.
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This data can be stored via `qvm-features` to keep things consistent when, e.g., `qvm-remove` is used.
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Besides, backups are also more easily handled this way.
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Also, the fields can serve as an indicator of whether a template is installed by `qvm-template`.
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### Fields ###
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Most of the fields should be fairly self-explanatory.
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- `template-name`
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- Note that this field needs to be consistent with the template name to be considered valid.
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- `template-epoch`
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- `template-version`
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- `template-release`
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- `template-reponame`
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- `template-buildtime`
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- `template-install-date`
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- The times are in ISO 8601 format, and can be parsed in Python with `datetime.datetime.fromisoformat()`.
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- `template-licence`
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- `template-url`
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- `template-summary`
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- `template-description`
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- Note that the newlines in this field are converted to `|` to work better with existing tools like `qvm-features`.
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- `menu-items`
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- `default-menu-items`
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- `netvm-menu-items`
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- The `*menu-items` entries store the contents of `var/lib/qubes/vm-templates/${template_name}/whitelisted-appmenus.list`, `var/lib/qubes/vm-templates/${template_name}/vm-whitelisted-appmenus.list`, `var/lib/qubes/vm-templates/${template_name}/netvm-whitelisted-appmenus.list` respectively.
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- Note that newlines are converted to spaces, again for it to work better with existing tools.
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This should not cause ambiguity as [the FreeDesktop specifications](freedesktop-spec) forbid spaces from the names of desktop files.
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[freedesktop-spec]: https://specifications.freedesktop.org/desktop-entry-spec/desktop-entry-spec-latest.html
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Repository Management
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---------------------
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For UpdateVMs to access repository configuration, the package [qubes-repo-templates] is created with the following contents:
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- `/usr/share/qubes/repo-templates/*.repo`: repository configuration
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- `/usr/share/qubes/repo-templates/keys`: PGP keys
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As template keys may be less trusted, they are *not* added to the system RPM keychain but instead managed separately.
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[qubes-repo-templates]: https://github.com/WillyPillow/qubes-repo-templates
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Qrexec Protocol
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---------------
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Dom0 and management VMs without network access also need to interact with template repositories.
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The following qrexec calls that list and download templates are thus proposed.
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- `qubes.TemplateSearch`: wraps `dnf repoquery`
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- `qubes.TemplateDownload`: wraps `dnf download`
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### Input ###
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Both calls accept the following format from standard input:
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```
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arg1
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arg2
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...
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argN
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package-file-spec
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---
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repo config
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```
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In other words, the input consists of two parts separated by the line `---`.
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The first part contains some arguments and `package-file-spec` that indicates the pattern to be queried or downloaded.
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The following arguments are allowed:
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- `--enablerepo=<repoid>`
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- `--disablerepo=<repoid>`
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- `--repoid=<repoid>`
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- `--releasever=<release>`
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- `--refresh`
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where the usage is identical to that of DNF.
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For the exact definition of `package-file-spec`, refer to the DNF documentation.
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The second part contains the repository configurations in `yum.repos.d` format.
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### Output ###
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`qubes.TemplateSearch` prints each package in `%{name}|%{epoch}|%{version}|%{release}|%{reponame}|%{downloadsize}|%{buildtime}|%{license}|%{url}|%{summary}|%{description}|` format to standard output, separated by newlines.
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Note that there is a `|` at the end of the line.
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This is because `%{description}` may contain newlines, and doing so allows us to split the entries by `|\n`.
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(As we are using `dnf repoquery --qf`, we are unable to escape the newlines in advance.)
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`qubes.TemplateDownload`, on the other hand, directly outputs the downloaded content to standard output.
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Machine-readable Output
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-----------------------
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The commands `qvm-template list` and `qvm-template info` provide machine-readable output in both pipe(`|`)-separated and JSON format.
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See the `qvm-template` man page for details.
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Interactions with Existing Tools
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--------------------------------
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### `qvm-remove` ###
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Should work straightforwardly.
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### Renaming / Cloning ###
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A template is treated as non-manager-installed once renamed or cloned.
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However, relevant metadata in the VM features is still retained for future extension and to serve as a hint for the user.
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Further Reading
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---------------
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Initial Google Summer of Code (2020) project proposal:
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- <https://hackmd.io/aYauztkGR0iOIoh8fJLecw>
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Previous design document:
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- <https://gist.github.com/WillyPillow/b8a643ddbd9235a97bc187e6e44b16e4>
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Discussion threads:
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- <https://groups.google.com/forum/#!topic/qubes-devel/6Zb_WLy3GY4>
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- <https://groups.google.com/forum/#!topic/qubes-devel/PyJogqT1TUg>
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- <https://groups.google.com/forum/#!topic/qubes-devel/2XaMP4Us3kg>
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- <https://groups.google.com/forum/#!topic/qubes-devel/wF_84b1BR0A>
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- <https://groups.google.com/g/qubes-devel/c/pYHnihVCBM0>
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