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README.md+48−31 c+43−11 flake.nix+96−47

README.md+48−31

@@ -5,8 +5,10 @@ A Nix flake that builds an **OCI image** (via

55Apple's [`container`](https://github.com/apple/container) CLI on macOS.

77It's a plain, single-process image — no NixOS, no systemd — defined in

8−[`flake.nix`](./flake.nix). It runs `fish` by default; you choose what it ships

9−via a per-project `container.nix`.

8+[`flake.nix`](./flake.nix). `c start` drops you into a

9+[`herdr`](https://herdr.dev) session whose panes use your host login shell

10+(detected and bundled by `c init`). You choose what else it ships via a

11+per-project `container.nix`.

1012

1113## Prerequisites

1214

@@ -14,32 +16,43 @@ via a per-project `container.nix`.

1416- Apple's [`container`](https://github.com/apple/container) CLI installed, with

1517 services started: `container system start` (first run downloads a Linux

1618 kernel).

17−- A **Linux builder** — the image *contents* are Linux binaries, which can't be

18− built on macOS. Use a [remote builder][builders] or, on Apple Silicon, the

19− `nix-darwin` `linux-builder`. With one configured, `nix` dispatches Linux

20− builds to it automatically (most packages also come prebuilt from the binary

21− cache, so the builder is only needed for the uncached bits).

19+- A **Linux builder** *only if you use packages that aren't in the binary

20+ cache*. The image is assembled entirely on macOS — the root filesystem is just

21+ a host-built symlink tree over the **substituted** Linux store paths — so a

22+ fully-cached package set needs no builder. Compiling a Linux package that

23+ `cache.nixos.org` doesn't have does, though: use a [remote builder][builders]

24+ or, on Apple Silicon, the `nix-darwin` `linux-builder`.

2225

2326[builders]: https://nix.dev/manual/nix/latest/advanced-topics/distributed-builds

2427

2528## Packages

2629

27−`init` builds the image from a **`container.nix`** in the current directory — a

28−`{ pkgs, nur }` function returning the package list (`nur` is

29−[NUR](https://github.com/nix-community/NUR), for packages outside nixpkgs). It

30−is **required**; there is no default set.

30+`init` builds the image from a **`container.nix`** in the current directory — an

31+attrset with a `packages` function (`{ pkgs, nur }` returning the package list;

32+`nur` is [NUR](https://github.com/nix-community/NUR), for packages outside

33+nixpkgs) and an optional `mounts` list. It is **required**; there is no default.

3134

3235```nix

3336# ./container.nix

34−{ pkgs, nur }: with pkgs; [

35− fish # the default cmd is /bin/fish, so include it (or change config.cmd)

36− coreutils

37− git

38− go

39− nur.repos.charmbracelet.crush

40−]

37+{

38+ packages = { pkgs, nur }: with pkgs; [

39+ git

40+ go

41+ nur.repos.charmbracelet.crush

42+ ];

43+ # Extra "host:container" bind mounts, on top of $PWD → /workspace. A leading

44+ # ~/ in the host path expands to $HOME. Optional.

45+ mounts = [

46+ "~/data:/data"

47+ ];

48+}

4149```

4250

51+Your **host login shell** (detected by `c init`, bash fallback), **coreutils**,

52+[**herdr**](https://herdr.dev) (the session `c start` launches), **Nix** itself

53+(with CA certs and `nix-command`/`flakes` enabled, single-user), and the ncurses

54+**terminfo** database are always included — no need to list them.

55+

4356Point at a different file with `-c`/`--config`:

4457

4558```sh

@@ -55,17 +68,17 @@ that holds a `container.nix`; the container is named after that directory.

5568# build the image, load it, and create the container (named after $PWD):

5669nix run github:ilyakooo0/nix-container -- init

5770

58−# start it and attach — drops you into fish:

71+# start it and attach — drops you into a herdr session:

5972nix run github:ilyakooo0/nix-container -- start

6073```

6174

6275- `init` builds the OCI archive in a temp dir, loads it into `container`,

6376 deletes the temp file, then (re)creates the container. It mounts the current

64− directory at `/workspace` (the container's working directory) and your

65− `~/.config` at `/root/.config`.

77+ directory at `/workspace` (the container's working directory).

6678- `--ssh` forwards your host SSH agent socket into the container (so `git` over

6779 SSH works with your keys).

68−- Add more mounts by passing them through (mounts can only be set at creation):

80+- Add more mounts persistently via `mounts` in `container.nix` (above), or

81+ ad-hoc by passing them through to `create` (mounts can only be set at creation):

6982

7083 ```sh

7184 nix run github:ilyakooo0/nix-container -- init -v $HOME/data:/data

@@ -86,18 +99,19 @@ nix run github:ilyakooo0/nix-container -- init

8699## Customize further

87100

88101Fork or clone to change what isn't per-project — `config.cmd`/`env` or the image

89−name — in [`flake.nix`](./flake.nix). The flake also exposes build helpers, both

90−taking a `{ pkgs, nur }: [ ... ]` function:

102+name — in [`flake.nix`](./flake.nix). The flake also exposes build helpers:

91103

92−- `lib.<system>.mkImage` → an OCI image.

93−- `lib.<system>.copyWith` → a skopeo copy app, for building/pushing the image

94− anywhere skopeo can write (e.g. a registry):

104+- `lib.<system>.copyWith` → a skopeo copy app for a `{ pkgs, nur }: [ ... ]`

105+ function, for building/pushing the image anywhere skopeo can write (e.g. a

106+ registry):

95107

96108 ```sh

97109 drv=$(nix build --no-link --print-out-paths --impure --expr \

98− '(builtins.getFlake "github:ilyakooo0/nix-container").lib.${builtins.currentSystem}.copyWith (import ./container.nix)')

110+ '(builtins.getFlake "github:ilyakooo0/nix-container").lib.${builtins.currentSystem}.copyWith (import ./container.nix).packages')

99111 $drv/bin/copy-to docker://ghcr.io/me/img:latest

100112 ```

113+- `lib.<system>.mkImage <name> <shell>` → the underlying OCI image (`copyWith`

114+ defaults these to `nix-container` / `bash`).

101115

102116The flake uses `flake-utils` over `{aarch64,x86_64}-{darwin,linux}`; the host

103117system maps to the matching Linux target automatically (an Intel Mac builds an

@@ -105,9 +119,12 @@ system maps to the matching Linux target automatically (an Intel Mac builds an

105119

106120## How it works

107121

108−- **Contents** are built from `pkgsLinux` → the Linux builder.

109−- **Assembly** (`nix2container` + the `skopeo` copy) runs on your Mac, so a

110− single `init` produces a loadable archive.

122+- **Contents** are Linux store paths — downloaded prebuilt from the binary cache

123+ (only built on a Linux builder if not cached).

124+- **Assembly** runs entirely on your Mac: `copyToRoot` is a *host-arch* `buildEnv`

125+ symlink tree over those Linux paths (symlinks are arch-neutral), and

126+ `nix2container` + `skopeo` package it — so for a cached set, `init` produces a

127+ loadable Linux image with no Linux build at all.

111128- Apple's `container` runs each container in its own lightweight Linux VM and

112129 consumes standard OCI images — no `--privileged` or systemd needed.

113130

c+43−11

@@ -1,7 +1,7 @@

11#!/usr/bin/env fish

22# Manage a container named after the current directory.

33# c init [-c FILE] [extra create args] build + load the image, then (re)create it

4−# (packages from FILE, default ./container.nix)

4+# (config from FILE, default ./container.nix)

55# c start start it and attach

77set -l name (path basename $PWD)

@@ -22,17 +22,23 @@ switch $argv[1]

2222 end

2323

2424 if not test -e $config

25− echo "c: package file not found: $config" >&2

26− echo " create one ('{ pkgs, nur }: [ ... ]') or pass -c FILE" >&2

25+ echo "c: config file not found: $config" >&2

26+ echo " create one ('{ packages = { pkgs, nur }: [ ... ]; mounts = [ ]; }') or pass -c FILE" >&2

2727 exit 1

2828 end

2929

30− # Build an image with exactly the packages from $config, into a temp

31− # dir, load it, then clean up.

30+ # Host login shell (by name): added to the image automatically and baked

31+ # in as the cmd + default $SHELL (what herdr panes etc. spawn), so it

32+ # needn't be listed in container.nix. Falls back to bash.

33+ set -l shell bash

34+ test -n "$SHELL"; and set shell (path basename $SHELL)

35+

36+ # Build an image with the config's `packages` (plus the shell) into a

37+ # temp dir, load it, then clean up.

3238 set -l tmp (mktemp -d)

3339 set -l archive $tmp/image.tar.gz

34− set -lx NIX_CONTAINER_PKGS (path resolve $config)

35− set -l expr "(builtins.getFlake \"$flake\").lib.\${builtins.currentSystem}.copyWith (import (/. + builtins.getEnv \"NIX_CONTAINER_PKGS\"))"

40+ set -lx NIX_CONTAINER_CONFIG (path resolve $config)

41+ set -l expr "(builtins.getFlake \"$flake\").lib.\${builtins.currentSystem}.copyWithShell \"nix-container-$name\" \"$shell\" (import (/. + builtins.getEnv \"NIX_CONTAINER_CONFIG\")).packages"

3642 set -l copyer (nix build --impure --no-link --print-out-paths --expr "$expr")

3743 set -l rc $status

3844 if test $rc -eq 0

@@ -47,14 +53,40 @@ switch $argv[1]

4753 rm -rf $tmp

4854 test $rc -eq 0; or exit $rc

4955

50− # Mount the project at /workspace (the default cwd) and host ~/.config.

56+ # Mount the project at /workspace (the default cwd), plus any extra

57+ # mounts from the config (`mounts = [ "host:container" … ]`). A leading

58+ # ~/ in the host path expands to $HOME (nix leaves it literal).

5159 set -l mounts -v "$PWD:/workspace"

52− test -d $HOME/.config; and set -a mounts -v "$HOME/.config:/root/.config"

60+ for m in (nix eval --impure --raw --expr "builtins.concatStringsSep \"\n\" ((import (/. + builtins.getEnv \"NIX_CONTAINER_CONFIG\")).mounts or [])")

61+ set -a mounts -v (string replace -r '^~/' "$HOME/" -- $m)

62+ end

63+

64+ # TERM to bake into the container env (`-e TERM`): the runtime hands the

65+ # `start -ai` session a bare "xterm", so set the host's real terminal,

66+ # mapped to an entry the bundled ncurses ships — Ghostty reports

67+ # "xterm-ghostty", which ncurses ships as "ghostty" — falling back to

68+ # xterm-256color for an empty or bare-"xterm" value.

69+ set -l term xterm-256color

70+ if test "$TERM" = xterm-ghostty

71+ set term ghostty

72+ else if test -n "$TERM"; and test "$TERM" != xterm

73+ set term $TERM

74+ end

5375

54− # Replace any existing container of the same name.

76+ # Replace any existing container of the same name, then create it running

77+ # herdr in a session named after the project (`<dir>-container`). This

78+ # overrides the image's default plain-herdr cmd; `c start` attaches to it.

5579 container rm --force $name 2>/dev/null

56− container create --name $name --ssh -it --cwd /workspace $mounts $argv $image

80+ # The image only lays down /bin and /etc, so /tmp and /run don't exist;

81+ # without them anything that writes temp files (herdr, build tools, …)

82+ # fails with ENOENT. Mount writable tmpfs at both.

83+ container create --name $name --ssh -it -e "TERM=$term" \

84+ --tmpfs /tmp --tmpfs /run --memory 8g --cwd /workspace $mounts $argv \

85+ $image /bin/herdr session attach "$name-container"

5786 case start

87+ # Start the container and attach to its cmd (herdr). TERM was baked into

88+ # the container env at create time (`-e TERM`), which survives this path.

89+ # Exiting herdr (PID1) stops the container; `c start` boots it again.

5890 container start -ai $name

5991 case '*'

6092 echo "usage: c init [-c FILE] [create args] | start" >&2

flake.nix+96−47

@@ -72,57 +72,99 @@

7272 # Host package set, for the wrapper app below.

7373 pkgsHost = nixpkgs.legacyPackages.${system};

7474

75− # Name/tag baked into the image's OCI config.

75+ # Default image name (for `copyWith`); `c init` passes a per-directory

76+ # name (`nix-container-<dir>`) to `copyWithShell` instead.

7677 imageName = "nix-container";

7778 imageTag = "latest";

7879

79− # Build the OCI image from a `{ pkgs, nur }: [ ... ]` package function.

80− # The default `cmd` is /bin/fish, so the package set should include

81− # `fish` (warned about below if missing) or override `config.cmd`.

80+ # Build the OCI image (named `name`) from a `{ pkgs, nur }: [ ... ]`

81+ # package function, for the named shell. The image's `cmd` is herdr

82+ # (the session `c start` attaches to); the shell is the default `$SHELL`

83+ # (what herdr panes spawn) and is added automatically, so `container.nix`

84+ # lists only your extras. Nix (+ CA certs) and the ncurses terminfo

85+ # database are always shipped too. Nothing here is fish-specific: the

86+ # TERM fixup happens host-side in `c` (passed via `-e TERM` at create).

8287 mkImage =

83− packages:

84− let

85− pkgList = packages {

86− pkgs = pkgsLinux;

87− nur = nurPkgs;

88− };

89− in

90− pkgsLinux.lib.warnIf (!builtins.any (p: (p.pname or "") == "fish") pkgList)

91− "nix-container: config.cmd is /bin/fish but the package set has no `fish` — the container will fail to start unless something provides /bin/fish (or change config.cmd)."

92− (

93− n2c.buildImage {

94− name = imageName;

95− tag = imageTag;

96− inherit arch;

88+ name: shell: packages:

89+ n2c.buildImage {

90+ inherit name;

91+ tag = imageTag;

92+ inherit arch;

9793

98− # A plain, single-process root filesystem. No NixOS, no systemd.

99− copyToRoot = pkgsLinux.buildEnv {

100− name = "root";

101− paths = pkgList;

102− pathsToLink = [ "/bin" ];

103− };

94+ # A plain, single-process root filesystem. No NixOS, no systemd.

95+ # buildEnv only *symlinks* paths (never runs them), so build it on

96+ # the host arch — the tree is arch-neutral and links the

97+ # (substituted) Linux store paths, sparing the Linux builder.

98+ copyToRoot = pkgsHost.buildEnv {

99+ name = "root";

100+ paths =

101+ packages {

102+ pkgs = pkgsLinux;

103+ nur = nurPkgs;

104+ }

105+ # The shell (cmd + $SHELL), when it exists in nixpkgs.

106+ ++ pkgsLinux.lib.optional (pkgsLinux ? ${shell}) pkgsLinux.${shell}

107+ ++ [

108+ pkgsLinux.nix

109+ pkgsLinux.cacert

110+ # Basic userland (ls, cat, cp, sleep, …) so the container is

111+ # usable out of the box and tools that shell out don't fail.

112+ pkgsLinux.coreutils-full

113+ # Networking glue glibc needs: /etc/nsswitch.conf (+passwd,

114+ # group) so DNS/host lookups resolve, and /etc/protocols,

115+ # /etc/services. Without these, hostname resolution fails.

116+ pkgsLinux.fakeNss

117+ pkgsLinux.iana-etc

118+ # The terminfo database (+ tput/clear/reset) so TUIs find the

119+ # entry for whatever TERM `c` passes in and render correctly.

120+ pkgsLinux.ncurses

121+ # The session `c start` always launches.

122+ pkgsLinux.herdr

123+ ];

124+ # Link all of /etc so the above files (nsswitch.conf, protocols,

125+ # services, passwd, group, ssl certs) are present.

126+ pathsToLink = [

127+ "/bin"

128+ "/etc"

129+ ];

130+ };

104131

105− initializeNixDatabase = true;

132+ initializeNixDatabase = true;

106133

107− config = {

108− # `cmd` (not `entrypoint`): it's the default command and is

109− # *replaced* by `container run … -- <cmd>`. An entrypoint would

110− # be prepended instead, so `run … -- /bin/sh` would become

111− # `/bin/bash /bin/sh` and fail with "cannot execute binary file".

112− cmd = [ "/bin/fish" ];

113− env = [

114− "PATH=/bin"

115− "HOME=/root"

116− # Marker so shells/scripts can detect they're in here.

117− "NIX_CONTAINER=1"

118− ];

119− };

120− }

121− );

134+ config = {

135+ # `cmd` (not `entrypoint`): the default command, *replaced* by any

136+ # command passed to `container run/create`. Plain herdr here;

137+ # `c init` overrides it with `herdr session attach <dir>-container` so

138+ # the session is named per project. `c start` attaches via

139+ # `container start -ai`; exiting herdr (PID1) stops the container.

140+ cmd = [ "/bin/herdr" ];

141+ env = [

142+ "PATH=/bin"

143+ "HOME=/root"

144+ # Default shell for tools that spawn one (herdr panes, etc.).

145+ "SHELL=/bin/${shell}"

146+ # Marker so shells/scripts can detect they're in here.

147+ "NIX_CONTAINER=1"

148+ # Advertise truecolor for 24-bit-aware tools; `c` passes the

149+ # resolved TERM via -e (the runtime would otherwise force xterm).

150+ "COLORTERM=truecolor"

151+ # A UTF-8 locale so tools handle multibyte/wide characters

152+ # correctly. glibc ships C.UTF-8 built in, so this needs no

153+ # locale-archive.

154+ "LANG=C.UTF-8"

155+ # CA trust for every TLS client (curl/git/…) and Nix.

156+ "SSL_CERT_FILE=/etc/ssl/certs/ca-bundle.crt"

157+ "NIX_SSL_CERT_FILE=/etc/ssl/certs/ca-bundle.crt"

158+ # Make the bundled Nix usable: flake commands on, and

159+ # single-user (no daemon/nixbld).

160+ "NIX_CONFIG=experimental-features = nix-command flakes\nbuild-users-group =\nsandbox = false"

161+ ];

162+ };

163+ };

122164

123165 # `nix run . -- init|start` runs the bundled `c` manager script. Its

124166 # `init` builds the image from the project's `container.nix` (via

125− # `lib.copyWith`); the script finds the flake from its own (store)

167+ # `lib.copyWithShell`); the script finds the flake from its own (store)

126168 # location, so this works from a checkout or straight from

127169 # `nix run github:…/nix-container`.

128170 cApp = pkgsHost.writeShellScriptBin "c" ''

@@ -135,15 +177,22 @@

135177 default = flake-utils.lib.mkApp { drv = cApp; };

136178 };

137179

138− # Build helpers. `c init` uses `copyWith` to honour a per-project

139− # `container.nix`:

180+ # Build helpers take a `{ pkgs, nur }: [ ... ]` packages function (the

181+ # `packages` attr of a project's `container.nix`). `c init` uses

182+ # `copyWithShell` (host shell baked in); `copyWith` is the generic form:

140183 # (builtins.getFlake "github:…/nix-container")

141− # .lib.<system>.copyWith (import ./container.nix)

184+ # .lib.<system>.copyWith (import ./container.nix).packages

142185 lib = {

143186 inherit mkImage;

144− # skopeo copy app (`skopeo copy nix:<image> "$@"`) for an image built

145− # from a `{ pkgs, nur }: [ ... ]` function.

146− copyWith = packages: (mkImage packages).copyTo;

187+ # Build the image's `copyTo` script (a skopeo-copy wrapper) from a

188+ # `{ pkgs, nur }: [ ... ]` function, named `imageName`. Defaults the

189+ # shell (cmd + $SHELL) to bash; use `copyWithShell` to pick another.

190+ copyWith = packages: (mkImage imageName "bash" packages).copyTo;

191+ # Like `copyWith`, but for the given image `name` and shell — `c init`

192+ # passes `nix-container-<dir>` and the host login shell, which is added

193+ # to the image and used as cmd + $SHELL without being listed in

194+ # `container.nix`.

195+ copyWithShell = name: shell: packages: (mkImage name shell packages).copyTo;

147196 };

148197 }

149198 );