Add a "nixos-enter" command

This factors out the functionality in nixos-install for running a
command inside a NixOS installation (nixos-install --chroot).

nixos/doc/manual/man-nixos-enter.xml

@@ -0,0 +1,119 @@
+<refentry xmlns="http://docbook.org/ns/docbook"
+          xmlns:xlink="http://www.w3.org/1999/xlink"
+          xmlns:xi="http://www.w3.org/2001/XInclude">
+
+<refmeta>
+  <refentrytitle><command>nixos-enter</command></refentrytitle>
+  <manvolnum>8</manvolnum>
+  <refmiscinfo class="source">NixOS</refmiscinfo>
+  <!-- <refmiscinfo class="version"><xi:include href="version.txt" parse="text"/></refmiscinfo> -->
+</refmeta>
+
+<refnamediv>
+  <refname><command>nixos-enter</command></refname>
+  <refpurpose>run a command in a NixOS chroot environment</refpurpose>
+</refnamediv>
+
+<refsynopsisdiv>
+  <cmdsynopsis>
+    <command>nixos-enter</command>
+    <arg>
+      <arg choice='plain'><option>--root</option></arg>
+      <replaceable>root</replaceable>
+    </arg>
+    <arg>
+      <arg choice='plain'><option>--system</option></arg>
+      <replaceable>system</replaceable>
+    </arg>
+    <arg>
+      <arg choice='plain'><option>-c</option></arg>
+      <replaceable>shell-command</replaceable>
+    </arg>
+    <arg>
+      <arg choice='plain'><option>--help</option></arg>
+    </arg>
+    <arg>
+      <arg choice='plain'><option>--</option></arg>
+      <replaceable>arguments</replaceable>
+    </arg>
+  </cmdsynopsis>
+</refsynopsisdiv>
+
+
+<refsection><title>Description</title>
+
+<para>This command runs a command in a NixOS chroot environment, that
+is, in a filesystem hierarchy previously prepared using
+<command>nixos-install</command>.</para>
+
+</refsection>
+
+<refsection><title>Options</title>
+
+<para>This command accepts the following options:</para>
+
+<variablelist>
+
+  <varlistentry>
+    <term><option>--root</option></term>
+    <listitem>
+      <para>The path to the NixOS system you want to enter. It defaults to <filename>/mnt</filename>.</para>
+    </listitem>
+  </varlistentry>
+
+  <varlistentry>
+    <term><option>--system</option></term>
+    <listitem>
+      <para>The NixOS system configuration to use. It defaults to
+      <filename>/nix/var/nix/profiles/system</filename>. You can enter
+      a previous NixOS configuration by specifying a path such as
+      <filename>/nix/var/nix/profiles/system-106-link</filename>.</para>
+    </listitem>
+  </varlistentry>
+
+  <varlistentry>
+    <term><option>--command</option></term>
+    <term><option>-c</option></term>
+    <listitem>
+      <para>The bash command to execute.</para>
+    </listitem>
+  </varlistentry>
+
+  <varlistentry>
+    <term><option>--</option></term>
+
+    <listitem><para>Interpret the remaining arguments as the program
+    name and arguments to be invoked. The program is not executed in a
+    shell.</para></listitem>
+
+  </varlistentry>
+
+</variablelist>
+
+</refsection>
+
+
+<refsection><title>Examples</title>
+
+<para>Start an interactive shell in the NixOS installation in
+<filename>/mnt</filename>:</para>
+
+<screen>
+# nixos-enter /mnt
+</screen>
+
+<para>Run a shell command:</para>
+
+<screen>
+# nixos-enter -c 'ls -l /; cat /proc/mounts'
+</screen>
+
+<para>Run a non-shell command:</para>
+
+<screen>
+# nixos-enter -- cat /proc/mounts
+</screen>
+
+</refsection>
+
+</refentry>

nixos/doc/manual/man-pages.xml

@@ -15,7 +15,7 @@
     </author>
 
     <copyright>
-      <year>2007-2015</year>
+      <year>2007-2018</year>
       <holder>Eelco Dolstra</holder>
     </copyright>
 
@@ -25,6 +25,7 @@
   <xi:include href="man-nixos-build-vms.xml" />
   <xi:include href="man-nixos-generate-config.xml" />
   <xi:include href="man-nixos-install.xml" />
+  <xi:include href="man-nixos-enter.xml" />
   <xi:include href="man-nixos-option.xml" />
   <xi:include href="man-nixos-rebuild.xml" />
   <xi:include href="man-nixos-version.xml" />

nixos/modules/installer/tools/nixos-enter.sh

@@ -0,0 +1,58 @@
+#! @shell@
+
+set -e
+
+# Re-exec ourselves in a private mount namespace so that our bind
+# mounts get cleaned up automatically.
+if [ "$(id -u)" = 0 ]; then
+    if [ -z "$NIXOS_ENTER_REEXEC" ]; then
+        export NIXOS_ENTER_REEXEC=1
+        exec unshare --mount --uts -- "$0" "$@"
+    else
+        mount --make-rprivate /
+    fi
+fi
+
+mountPoint=/mnt
+command=("bash" "--login")
+system=/nix/var/nix/profiles/system
+
+while [ "$#" -gt 0 ]; do
+    i="$1"; shift 1
+    case "$i" in
+        --root)
+            mountPoint="$1"; shift 1
+            ;;
+        --system)
+            system="$1"; shift 1
+            ;;
+        --help)
+            exec man nixos-enter
+            exit 1
+            ;;
+        --command|-c)
+            command=("bash" "-c" "$1")
+            shift 1
+            ;;
+        --)
+            command=("$@")
+            break
+            ;;
+        *)
+            echo "$0: unknown option \`$i'"
+            exit 1
+            ;;
+    esac
+done
+
+# Set up some bind mounts we'll want regardless of chroot or not
+mkdir -m 0755 -p "$mountPoint/dev" "$mountPoint/proc" "$mountPoint/sys" "$mountPoint/run"
+mount --rbind /dev "$mountPoint/dev"
+mount -t proc none "$mountPoint/proc"
+mount -t sysfs none "$mountPoint/sys"
+mount -t tmpfs none "$mountPoint/run"
+
+# Run the activation script. Set $LOCALE_ARCHIVE to supress some Perl locale warnings.
+LOCALE_ARCHIVE=$system/sw/lib/locale/locale-archive chroot "$mountPoint" "$system/activate" >&2
+
+exec chroot "$mountPoint" "${command[@]}"

nixos/modules/installer/tools/tools.nix

@@ -1,7 +1,9 @@
 # This module generates nixos-install, nixos-rebuild,
 # nixos-generate-config, etc.
 
-{ config, pkgs, modulesPath, ... }:
+{ config, lib, pkgs, modulesPath, ... }:
+
+with lib;
 
 let
   cfg = config.installer;
@@ -69,6 +71,11 @@ let
     inherit (config.system) nixosVersion nixosCodeName nixosRevision;
   };
 
+  nixos-enter = makeProg {
+    name = "nixos-enter";
+    src = ./nixos-enter.sh;
+  };
+
 in
 
 {
@@ -83,10 +90,11 @@ in
         nixos-generate-config
         nixos-option
         nixos-version
+        nixos-enter
       ];
 
     system.build = {
-      inherit nixos-install nixos-prepare-root nixos-generate-config nixos-option nixos-rebuild;
+      inherit nixos-install nixos-prepare-root nixos-generate-config nixos-option nixos-rebuild nixos-enter;
     };
 
   };