commit f6911a020e7e7be42d9d4f71baf5eb856efd0757 · pyrox.dev/nixpkgs

+152

nixos/doc/manual/from_md/installation/upgrading.chapter.xml

···

       1
       +
       <chapter xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xml:id="sec-upgrading">

     

       2
       +
         <title>Upgrading NixOS</title>

     

       3
       +
         <para>

     

       4
       +
           The best way to keep your NixOS installation up to date is to use

     

       5
       +
           one of the NixOS <emphasis>channels</emphasis>. A channel is a Nix

     

       6
       +
           mechanism for distributing Nix expressions and associated binaries.

     

       7
       +
           The NixOS channels are updated automatically from NixOS’s Git

     

       8
       +
           repository after certain tests have passed and all packages have

     

       9
       +
           been built. These channels are:

     

       10
       +
         </para>

     

       11
       +
         <itemizedlist>

     

       12
       +
           <listitem>

     

       13
       +
             <para>

     

       14
       +
               <emphasis>Stable channels</emphasis>, such as

     

       15
       +
               <link xlink:href="https://nixos.org/channels/nixos-21.05"><literal>nixos-21.05</literal></link>.

     

       16
       +
               These only get conservative bug fixes and package upgrades. For

     

       17
       +
               instance, a channel update may cause the Linux kernel on your

     

       18
       +
               system to be upgraded from 4.19.34 to 4.19.38 (a minor bug fix),

     

       19
       +
               but not from 4.19.x to 4.20.x (a major change that has the

     

       20
       +
               potential to break things). Stable channels are generally

     

       21
       +
               maintained until the next stable branch is created.

     

       22
       +
             </para>

     

       23
       +
           </listitem>

     

       24
       +
           <listitem>

     

       25
       +
             <para>

     

       26
       +
               The <emphasis>unstable channel</emphasis>,

     

       27
       +
               <link xlink:href="https://nixos.org/channels/nixos-unstable"><literal>nixos-unstable</literal></link>.

     

       28
       +
               This corresponds to NixOS’s main development branch, and may

     

       29
       +
               thus see radical changes between channel updates. It’s not

     

       30
       +
               recommended for production systems.

     

       31
       +
             </para>

     

       32
       +
           </listitem>

     

       33
       +
           <listitem>

     

       34
       +
             <para>

     

       35
       +
               <emphasis>Small channels</emphasis>, such as

     

       36
       +
               <link xlink:href="https://nixos.org/channels/nixos-21.05-small"><literal>nixos-21.05-small</literal></link>

     

       37
       +
               or

     

       38
       +
               <link xlink:href="https://nixos.org/channels/nixos-unstable-small"><literal>nixos-unstable-small</literal></link>.

     

       39
       +
               These are identical to the stable and unstable channels

     

       40
       +
               described above, except that they contain fewer binary packages.

     

       41
       +
               This means they get updated faster than the regular channels

     

       42
       +
               (for instance, when a critical security patch is committed to

     

       43
       +
               NixOS’s source tree), but may require more packages to be built

     

       44
       +
               from source than usual. They’re mostly intended for server

     

       45
       +
               environments and as such contain few GUI applications.

     

       46
       +
             </para>

     

       47
       +
           </listitem>

     

       48
       +
         </itemizedlist>

     

       49
       +
         <para>

     

       50
       +
           To see what channels are available, go to

     

       51
       +
           <link xlink:href="https://nixos.org/channels">https://nixos.org/channels</link>.

     

       52
       +
           (Note that the URIs of the various channels redirect to a directory

     

       53
       +
           that contains the channel’s latest version and includes ISO images

     

       54
       +
           and VirtualBox appliances.) Please note that during the release

     

       55
       +
           process, channels that are not yet released will be present here as

     

       56
       +
           well. See the Getting NixOS page

     

       57
       +
           <link xlink:href="https://nixos.org/nixos/download.html">https://nixos.org/nixos/download.html</link>

     

       58
       +
           to find the newest supported stable release.

     

       59
       +
         </para>

     

       60
       +
         <para>

     

       61
       +
           When you first install NixOS, you’re automatically subscribed to the

     

       62
       +
           NixOS channel that corresponds to your installation source. For

     

       63
       +
           instance, if you installed from a 21.05 ISO, you will be subscribed

     

       64
       +
           to the <literal>nixos-21.05</literal> channel. To see which NixOS

     

       65
       +
           channel you’re subscribed to, run the following as root:

     

       66
       +
         </para>

     

       67
       +
         <programlisting>

     

       68
       +
       # nix-channel --list | grep nixos

     

       69
       +
       nixos https://nixos.org/channels/nixos-unstable

     

       70
       +
       </programlisting>

     

       71
       +
         <para>

     

       72
       +
           To switch to a different NixOS channel, do

     

       73
       +
         </para>

     

       74
       +
         <programlisting>

     

       75
       +
       # nix-channel --add https://nixos.org/channels/channel-name nixos

     

       76
       +
       </programlisting>

     

       77
       +
         <para>

     

       78
       +
           (Be sure to include the <literal>nixos</literal> parameter at the

     

       79
       +
           end.) For instance, to use the NixOS 21.05 stable channel:

     

       80
       +
         </para>

     

       81
       +
         <programlisting>

     

       82
       +
       # nix-channel --add https://nixos.org/channels/nixos-21.05 nixos

     

       83
       +
       </programlisting>

     

       84
       +
         <para>

     

       85
       +
           If you have a server, you may want to use the <quote>small</quote>

     

       86
       +
           channel instead:

     

       87
       +
         </para>

     

       88
       +
         <programlisting>

     

       89
       +
       # nix-channel --add https://nixos.org/channels/nixos-21.05-small nixos

     

       90
       +
       </programlisting>

     

       91
       +
         <para>

     

       92
       +
           And if you want to live on the bleeding edge:

     

       93
       +
         </para>

     

       94
       +
         <programlisting>

     

       95
       +
       # nix-channel --add https://nixos.org/channels/nixos-unstable nixos

     

       96
       +
       </programlisting>

     

       97
       +
         <para>

     

       98
       +
           You can then upgrade NixOS to the latest version in your chosen

     

       99
       +
           channel by running

     

       100
       +
         </para>

     

       101
       +
         <programlisting>

     

       102
       +
       # nixos-rebuild switch --upgrade

     

       103
       +
       </programlisting>

     

       104
       +
         <para>

     

       105
       +
           which is equivalent to the more verbose

     

       106
       +
           <literal>nix-channel --update nixos; nixos-rebuild switch</literal>.

     

       107
       +
         </para>

     

       108
       +
         <note>

     

       109
       +
           <para>

     

       110
       +
             Channels are set per user. This means that running

     

       111
       +
             <literal>nix-channel --add</literal> as a non root user (or

     

       112
       +
             without sudo) will not affect configuration in

     

       113
       +
             <literal>/etc/nixos/configuration.nix</literal>

     

       114
       +
           </para>

     

       115
       +
         </note>

     

       116
       +
         <warning>

     

       117
       +
           <para>

     

       118
       +
             It is generally safe to switch back and forth between channels.

     

       119
       +
             The only exception is that a newer NixOS may also have a newer Nix

     

       120
       +
             version, which may involve an upgrade of Nix’s database schema.

     

       121
       +
             This cannot be undone easily, so in that case you will not be able

     

       122
       +
             to go back to your original channel.

     

       123
       +
           </para>

     

       124
       +
         </warning>

     

       125
       +
         <section xml:id="sec-upgrading-automatic">

     

       126
       +
           <title>Automatic Upgrades</title>

     

       127
       +
           <para>

     

       128
       +
             You can keep a NixOS system up-to-date automatically by adding the

     

       129
       +
             following to <literal>configuration.nix</literal>:

     

       130
       +
           </para>

     

       131
       +
           <programlisting language="bash">

     

       132
       +
       system.autoUpgrade.enable = true;

     

       133
       +
       system.autoUpgrade.allowReboot = true;

     

       134
       +
       </programlisting>

     

       135
       +
           <para>

     

       136
       +
             This enables a periodically executed systemd service named

     

       137
       +
             <literal>nixos-upgrade.service</literal>. If the

     

       138
       +
             <literal>allowReboot</literal> option is <literal>false</literal>,

     

       139
       +
             it runs <literal>nixos-rebuild switch --upgrade</literal> to

     

       140
       +
             upgrade NixOS to the latest version in the current channel. (To

     

       141
       +
             see when the service runs, see

     

       142
       +
             <literal>systemctl list-timers</literal>.) If

     

       143
       +
             <literal>allowReboot</literal> is <literal>true</literal>, then

     

       144
       +
             the system will automatically reboot if the new generation

     

       145
       +
             contains a different kernel, initrd or kernel modules. You can

     

       146
       +
             also specify a channel explicitly, e.g.

     

       147
       +
           </para>

     

       148
       +
           <programlisting language="bash">

     

       149
       +
       system.autoUpgrade.channel = https://nixos.org/channels/nixos-21.05;

     

       150
       +
       </programlisting>

     

       151
       +
         </section>

     

       152
       +
       </chapter>

+1 -1

nixos/doc/manual/installation/installation.xml

···

       13
        
        <xi:include href="../from_md/installation/obtaining.chapter.xml" />

     

       14
        
        <xi:include href="installing.xml" />

     

       15
        
        <xi:include href="../from_md/installation/changing-config.chapter.xml" />

     

       16
       -
        <xi:include href="upgrading.xml" />

     

       17
        
       </part>

···

       13
        
        <xi:include href="../from_md/installation/obtaining.chapter.xml" />

     

       14
        
        <xi:include href="installing.xml" />

     

       15
        
        <xi:include href="../from_md/installation/changing-config.chapter.xml" />

     

       16
       +
        <xi:include href="../from_md/installation/upgrading.chapter.xml" />

     

       17
        
       </part>

+118

nixos/doc/manual/installation/upgrading.chapter.md

···

       1
       +
       # Upgrading NixOS {#sec-upgrading}

     

       2
       +
       

     

       3
       +
       The best way to keep your NixOS installation up to date is to use one of

     

       4
       +
       the NixOS *channels*. A channel is a Nix mechanism for distributing Nix

     

       5
       +
       expressions and associated binaries. The NixOS channels are updated

     

       6
       +
       automatically from NixOS's Git repository after certain tests have

     

       7
       +
       passed and all packages have been built. These channels are:

     

       8
       +
       

     

       9
       +
       -   *Stable channels*, such as [`nixos-21.05`](https://nixos.org/channels/nixos-21.05).

     

       10
       +
           These only get conservative bug fixes and package upgrades. For

     

       11
       +
           instance, a channel update may cause the Linux kernel on your system

     

       12
       +
           to be upgraded from 4.19.34 to 4.19.38 (a minor bug fix), but not

     

       13
       +
           from 4.19.x to 4.20.x (a major change that has the potential to break things).

     

       14
       +
           Stable channels are generally maintained until the next stable

     

       15
       +
           branch is created.

     

       16
       +
       

     

       17
       +
       -   The *unstable channel*, [`nixos-unstable`](https://nixos.org/channels/nixos-unstable).

     

       18
       +
           This corresponds to NixOS's main development branch, and may thus see

     

       19
       +
           radical changes between channel updates. It's not recommended for

     

       20
       +
           production systems.

     

       21
       +
       

     

       22
       +
       -   *Small channels*, such as [`nixos-21.05-small`](https://nixos.org/channels/nixos-21.05-small)

     

       23
       +
           or [`nixos-unstable-small`](https://nixos.org/channels/nixos-unstable-small).

     

       24
       +
           These are identical to the stable and unstable channels described above,

     

       25
       +
           except that they contain fewer binary packages. This means they get updated

     

       26
       +
           faster than the regular channels (for instance, when a critical security patch

     

       27
       +
           is committed to NixOS's source tree), but may require more packages to be

     

       28
       +
           built from source than usual. They're mostly intended for server environments

     

       29
       +
           and as such contain few GUI applications.

     

       30
       +
       

     

       31
       +
       To see what channels are available, go to <https://nixos.org/channels>.

     

       32
       +
       (Note that the URIs of the various channels redirect to a directory that

     

       33
       +
       contains the channel's latest version and includes ISO images and

     

       34
       +
       VirtualBox appliances.) Please note that during the release process,

     

       35
       +
       channels that are not yet released will be present here as well. See the

     

       36
       +
       Getting NixOS page <https://nixos.org/nixos/download.html> to find the

     

       37
       +
       newest supported stable release.

     

       38
       +
       

     

       39
       +
       When you first install NixOS, you're automatically subscribed to the

     

       40
       +
       NixOS channel that corresponds to your installation source. For

     

       41
       +
       instance, if you installed from a 21.05 ISO, you will be subscribed to

     

       42
       +
       the `nixos-21.05` channel. To see which NixOS channel you're subscribed

     

       43
       +
       to, run the following as root:

     

       44
       +
       

     

       45
       +
       ```ShellSession

     

       46
       +
       # nix-channel --list | grep nixos

     

       47
       +
       nixos https://nixos.org/channels/nixos-unstable

     

       48
       +
       ```

     

       49
       +
       

     

       50
       +
       To switch to a different NixOS channel, do

     

       51
       +
       

     

       52
       +
       ```ShellSession

     

       53
       +
       # nix-channel --add https://nixos.org/channels/channel-name nixos

     

       54
       +
       ```

     

       55
       +
       

     

       56
       +
       (Be sure to include the `nixos` parameter at the end.) For instance, to

     

       57
       +
       use the NixOS 21.05 stable channel:

     

       58
       +
       

     

       59
       +
       ```ShellSession

     

       60
       +
       # nix-channel --add https://nixos.org/channels/nixos-21.05 nixos

     

       61
       +
       ```

     

       62
       +
       

     

       63
       +
       If you have a server, you may want to use the "small" channel instead:

     

       64
       +
       

     

       65
       +
       ```ShellSession

     

       66
       +
       # nix-channel --add https://nixos.org/channels/nixos-21.05-small nixos

     

       67
       +
       ```

     

       68
       +
       

     

       69
       +
       And if you want to live on the bleeding edge:

     

       70
       +
       

     

       71
       +
       ```ShellSession

     

       72
       +
       # nix-channel --add https://nixos.org/channels/nixos-unstable nixos

     

       73
       +
       ```

     

       74
       +
       

     

       75
       +
       You can then upgrade NixOS to the latest version in your chosen channel

     

       76
       +
       by running

     

       77
       +
       

     

       78
       +
       ```ShellSession

     

       79
       +
       # nixos-rebuild switch --upgrade

     

       80
       +
       ```

     

       81
       +
       

     

       82
       +
       which is equivalent to the more verbose `nix-channel --update nixos; nixos-rebuild switch`.

     

       83
       +
       

     

       84
       +
       ::: {.note}

     

       85
       +
       Channels are set per user. This means that running `nix-channel --add`

     

       86
       +
       as a non root user (or without sudo) will not affect

     

       87
       +
       configuration in `/etc/nixos/configuration.nix`

     

       88
       +
       :::

     

       89
       +
       

     

       90
       +
       ::: {.warning}

     

       91
       +
       It is generally safe to switch back and forth between channels. The only

     

       92
       +
       exception is that a newer NixOS may also have a newer Nix version, which

     

       93
       +
       may involve an upgrade of Nix's database schema. This cannot be undone

     

       94
       +
       easily, so in that case you will not be able to go back to your original

     

       95
       +
       channel.

     

       96
       +
       :::

     

       97
       +
       

     

       98
       +
       ## Automatic Upgrades {#sec-upgrading-automatic}

     

       99
       +
       

     

       100
       +
       You can keep a NixOS system up-to-date automatically by adding the

     

       101
       +
       following to `configuration.nix`:

     

       102
       +
       

     

       103
       +
       ```nix

     

       104
       +
       system.autoUpgrade.enable = true;

     

       105
       +
       system.autoUpgrade.allowReboot = true;

     

       106
       +
       ```

     

       107
       +
       

     

       108
       +
       This enables a periodically executed systemd service named

     

       109
       +
       `nixos-upgrade.service`. If the `allowReboot` option is `false`, it runs

     

       110
       +
       `nixos-rebuild switch --upgrade` to upgrade NixOS to the latest version

     

       111
       +
       in the current channel. (To see when the service runs, see `systemctl list-timers`.)

     

       112
       +
       If `allowReboot` is `true`, then the system will automatically reboot if

     

       113
       +
       the new generation contains a different kernel, initrd or kernel

     

       114
       +
       modules. You can also specify a channel explicitly, e.g.

     

       115
       +
       

     

       116
       +
       ```nix

     

       117
       +
       system.autoUpgrade.channel = https://nixos.org/channels/nixos-21.05;

     

       118
       +
       ```

-139

nixos/doc/manual/installation/upgrading.xml

···

       1
       -
       <chapter xmlns="http://docbook.org/ns/docbook"

     

       2
       -
                xmlns:xlink="http://www.w3.org/1999/xlink"

     

       3
       -
                version="5.0"

     

       4
       -
                xml:id="sec-upgrading">

     

       5
       -
        <title>Upgrading NixOS</title>

     

       6
       -
        <para>

     

       7
       -
         The best way to keep your NixOS installation up to date is to use one of the

     

       8
       -
         NixOS <emphasis>channels</emphasis>. A channel is a Nix mechanism for

     

       9
       -
         distributing Nix expressions and associated binaries. The NixOS channels are

     

       10
       -
         updated automatically from NixOS’s Git repository after certain tests have

     

       11
       -
         passed and all packages have been built. These channels are:

     

       12
       -
         <itemizedlist>

     

       13
       -
          <listitem>

     

       14
       -
           <para>

     

       15
       -
            <emphasis>Stable channels</emphasis>, such as

     

       16
       -
            <literal

     

       17
       -
           xlink:href="https://nixos.org/channels/nixos-21.05">nixos-21.05</literal>.

     

       18
       -
            These only get conservative bug fixes and package upgrades. For instance,

     

       19
       -
            a channel update may cause the Linux kernel on your system to be upgraded

     

       20
       -
            from 4.19.34 to 4.19.38 (a minor bug fix), but not from

     

       21
       -
            4.19.<replaceable>x</replaceable> to 4.20.<replaceable>x</replaceable> (a

     

       22
       -
            major change that has the potential to break things). Stable channels are

     

       23
       -
            generally maintained until the next stable branch is created.

     

       24
       -
           </para>

     

       25
       -
           <para></para>

     

       26
       -
          </listitem>

     

       27
       -
          <listitem>

     

       28
       -
           <para>

     

       29
       -
            The <emphasis>unstable channel</emphasis>,

     

       30
       -
            <literal

     

       31
       -
           xlink:href="https://nixos.org/channels/nixos-unstable">nixos-unstable</literal>.

     

       32
       -
            This corresponds to NixOS’s main development branch, and may thus see

     

       33
       -
            radical changes between channel updates. It’s not recommended for

     

       34
       -
            production systems.

     

       35
       -
           </para>

     

       36
       -
          </listitem>

     

       37
       -
          <listitem>

     

       38
       -
           <para>

     

       39
       -
            <emphasis>Small channels</emphasis>, such as

     

       40
       -
            <literal

     

       41
       -
           xlink:href="https://nixos.org/channels/nixos-21.05-small">nixos-21.05-small</literal>

     

       42
       -
            or

     

       43
       -
            <literal

     

       44
       -
           xlink:href="https://nixos.org/channels/nixos-unstable-small">nixos-unstable-small</literal>.

     

       45
       -
            These are identical to the stable and unstable channels described above,

     

       46
       -
            except that they contain fewer binary packages. This means they get

     

       47
       -
            updated faster than the regular channels (for instance, when a critical

     

       48
       -
            security patch is committed to NixOS’s source tree), but may require

     

       49
       -
            more packages to be built from source than usual. They’re mostly

     

       50
       -
            intended for server environments and as such contain few GUI applications.

     

       51
       -
           </para>

     

       52
       -
          </listitem>

     

       53
       -
         </itemizedlist>

     

       54
       -
         To see what channels are available, go to

     

       55
       -
         <link xlink:href="https://nixos.org/channels"/>. (Note that the URIs of the

     

       56
       -
         various channels redirect to a directory that contains the channel’s latest

     

       57
       -
         version and includes ISO images and VirtualBox appliances.) Please note that

     

       58
       -
         during the release process, channels that are not yet released will be

     

       59
       -
         present here as well. See the Getting NixOS page

     

       60
       -
         <link xlink:href="https://nixos.org/nixos/download.html"/> to find the newest

     

       61
       -
         supported stable release.

     

       62
       -
        </para>

     

       63
       -
        <para>

     

       64
       -
         When you first install NixOS, you’re automatically subscribed to the NixOS

     

       65
       -
         channel that corresponds to your installation source. For instance, if you

     

       66
       -
         installed from a 21.05 ISO, you will be subscribed to the

     

       67
       -
         <literal>nixos-21.05</literal> channel. To see which NixOS channel you’re

     

       68
       -
         subscribed to, run the following as root:

     

       69
       -
       <screen>

     

       70
       -
       <prompt># </prompt>nix-channel --list | grep nixos

     

       71
       -
       nixos https://nixos.org/channels/nixos-unstable

     

       72
       -
       </screen>

     

       73
       -
         To switch to a different NixOS channel, do

     

       74
       -
       <screen>

     

       75
       -
       <prompt># </prompt>nix-channel --add https://nixos.org/channels/<replaceable>channel-name</replaceable> nixos

     

       76
       -
       </screen>

     

       77
       -
         (Be sure to include the <literal>nixos</literal> parameter at the end.) For

     

       78
       -
         instance, to use the NixOS 21.05 stable channel:

     

       79
       -
       <screen>

     

       80
       -
       <prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-21.05 nixos

     

       81
       -
       </screen>

     

       82
       -
         If you have a server, you may want to use the “small” channel instead:

     

       83
       -
       <screen>

     

       84
       -
       <prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-21.05-small nixos

     

       85
       -
       </screen>

     

       86
       -
         And if you want to live on the bleeding edge:

     

       87
       -
       <screen>

     

       88
       -
       <prompt># </prompt>nix-channel --add https://nixos.org/channels/nixos-unstable nixos

     

       89
       -
       </screen>

     

       90
       -
        </para>

     

       91
       -
        <para>

     

       92
       -
         You can then upgrade NixOS to the latest version in your chosen channel by

     

       93
       -
         running

     

       94
       -
       <screen>

     

       95
       -
       <prompt># </prompt>nixos-rebuild switch --upgrade

     

       96
       -
       </screen>

     

       97
       -
         which is equivalent to the more verbose <literal>nix-channel --update nixos;

     

       98
       -
         nixos-rebuild switch</literal>.

     

       99
       -
        </para>

     

       100
       -
        <note>

     

       101
       -
         <para>

     

       102
       -
          Channels are set per user. This means that running <literal> nix-channel

     

       103
       -
          --add</literal> as a non root user (or without sudo) will not affect

     

       104
       -
          configuration in <literal>/etc/nixos/configuration.nix</literal>

     

       105
       -
         </para>

     

       106
       -
        </note>

     

       107
       -
        <warning>

     

       108
       -
         <para>

     

       109
       -
          It is generally safe to switch back and forth between channels. The only

     

       110
       -
          exception is that a newer NixOS may also have a newer Nix version, which may

     

       111
       -
          involve an upgrade of Nix’s database schema. This cannot be undone easily,

     

       112
       -
          so in that case you will not be able to go back to your original channel.

     

       113
       -
         </para>

     

       114
       -
        </warning>

     

       115
       -
        <section xml:id="sec-upgrading-automatic">

     

       116
       -
         <title>Automatic Upgrades</title>

     

       117
       -
       

     

       118
       -
         <para>

     

       119
       -
          You can keep a NixOS system up-to-date automatically by adding the following

     

       120
       -
          to <filename>configuration.nix</filename>:

     

       121
       -
       <programlisting>

     

       122
       -
       <xref linkend="opt-system.autoUpgrade.enable"/> = true;

     

       123
       -
       <xref linkend="opt-system.autoUpgrade.allowReboot"/> = true;

     

       124
       -
       </programlisting>

     

       125
       -
          This enables a periodically executed systemd service named

     

       126
       -
          <literal>nixos-upgrade.service</literal>. If the <literal>allowReboot</literal>

     

       127
       -
          option is <literal>false</literal>, it runs <command>nixos-rebuild switch

     

       128
       -
          --upgrade</command> to upgrade NixOS to the latest version in the current

     

       129
       -
          channel. (To see when the service runs, see <command>systemctl list-timers</command>.)

     

       130
       -
          If <literal>allowReboot</literal> is <literal>true</literal>, then the

     

       131
       -
          system will automatically reboot if the new generation contains a different

     

       132
       -
          kernel, initrd or kernel modules.

     

       133
       -
          You can also specify a channel explicitly, e.g.

     

       134
       -
       <programlisting>

     

       135
       -
       <xref linkend="opt-system.autoUpgrade.channel"/> = https://nixos.org/channels/nixos-21.05;

     

       136
       -
       </programlisting>

     

       137
       -
         </para>

     

       138
       -
        </section>

     

       139
       -
       </chapter>