populate

populate fills a target directory, /var/src by default, on the machine it runs on or on another one via ssh, with sources: directories and files, Git work trees, secrets from pass or passage, the output of commands, Nix derivations, flakes, and inputs of flakes, which can draw their own inputs from other sources. Running it again brings the target directory up to date with the sources. It doesn't change anything outside the target directory.

krops deploys NixOS systems with it.

Building

nix-build              # in this directory, which builds result/bin/populate
cargo build            # for development, with Rust 1.71 or later

default.nix takes nixpkgs' rustPlatform, from <nixpkgs> by default, so callPackage builds it as well. It needs nixpkgs 23.11 or later, and fetches the program's dependencies from crates.io.

Requirements

populate runs rsync, ssh, Git, Nix, pass, and passage as the sources need them, found in PATH, as well as a POSIX shell, coreutils, and findutils. The target needs a POSIX shell, coreutils, findutils, rsync, patch for sources with patches, Git for git sources, and Nix, and, if remote, ssh access as root, or, with --sudo, as a user who may use sudo. NixOS has all of them but Git, and rsync as long as environment.defaultPackages contains it. Features that say so need Nix with flake support (Nix 2.4 or later) on the machine that uses them.

Command line

populate [OPTION]... SOURCES [TARGET]

populate delivers the sources in the file SOURCES, or standard input if SOURCES is -, to TARGET, which is [USER@][HOST][:PORT][PATH]. The user ends at the first @, the host at a : or a /, and the port at a /. The user defaults to $LOGNAME, the host to the name of the machine populate runs on, from $HOSTNAME or /etc/hostname, and the path to /var/src. A target is local if its user is $LOGNAME, and its host is localhost or the name of the machine; populate reaches other targets via ssh, as their user.

Option	Meaning
`--backup`	Copy the target directory to a backup first.
`--force`	Create the target directory's sentinel if it's missing.
`--prune`	Remove what isn't a source from the target directory.
`--sudo`	Deliver as root, via sudo on the target.
`--ssh-arg ARG`	Pass `ARG` to ssh; repeatable.
`--user USER`	The target's user; empty for ssh's default.
`--host HOST`	The target's host, e.g. one `TARGET` can't express.
`--port PORT`	The port ssh connects to.
`--path PATH`	The target directory.
`--local`	The target is the machine populate runs on.
`--remote`	The target isn't, even if it looks so.
`-h`, `--help`	Print the usage and exit.
`-V`, `--version`	Print populate's version and exit.

--user, --host, --port, and --path take precedence over TARGET's parts. Options take their argument as the next argument or after =, and may come before, between, or after SOURCES and TARGET, up to --.

Without the sentinel, .populate in the target directory, populate refuses to touch the target directory, unless --force creates it; see The target directory.

Exit status:

0: populate delivered all sources.
1: populate refused the sources or the target, or delivering failed, with a message of its own.
The exit status of the command that failed, if delivering failed with that: e.g. 255 if ssh can't reach the target, or 128 if git fetch fails.
2: the command line is wrong, or the sources can't be read, aren't JSON, or aren't sources.
129, 130, 143: populate got SIGHUP, SIGINT, or SIGTERM, and stopped.

Sources

The sources are a JSON object, of sources by name. Each source is an object with the field type, the source's type, and a field named after the type, holding its settings, which Source types describes:

{
  "nixpkgs": {
    "type": "git",
    "git": { "url": "https://github.com/NixOS/nixpkgs", "ref": "nixos-26.05" }
  },
  "config": { "type": "file", "file": { "path": "/home/me/config" } },
  "nixos-config": {
    "type": "symlink",
    "symlink": { "target": "config/configuration.nix" }
  }
}

populate --force sources.json root@server

Settings with defaults may be left out. Fields and settings populate doesn't know are errors, so a misspelt one doesn't go unnoticed. The sources carry no version: this is version 0, and a later version of populate that reads them differently will mark them.

Sources in Nix

lib is populate's Nix library: the types of sources, which describe how to write them in Nix, with shorthands, e.g. config.file = "/home/me/config", and evalSource, which turns sources written so, an attribute set or a list of them, into the sources populate reads. It takes nixpkgs' lib, from <nixpkgs/lib> by default.

nix-instantiate --eval --strict --json -E '
  (import ./lib {}).evalSource {
    nixpkgs.git = {
      url = "https://github.com/NixOS/nixpkgs";
      ref = "nixos-26.05";
    };
    config.file = "/home/me/config";
    nixos-config.symlink = "config/configuration.nix";
  }
' | populate --force - root@server

Store paths in the sources, e.g. of a file source that is a derivation, must exist when populate runs. nix-instantiate --eval doesn't build them; building a file that contains the sources, e.g. with pkgs.writeText "sources.json" (builtins.toJSON sources), builds them too.

The target directory

Populate owns the target directory, /var/src by default: it replaces whatever is at the target path of a source, and with prune it removes everything else. It doesn't change or remove anything outside:

Target paths must be absolute, must not be the root directory, and must not contain ... Source names must consist of letters, digits, and the characters ._-, not start with -, and be neither ., .., nor .populate. populate refuses anything else, and so does krops' pkgs.populate during evaluation.
A symlink at the target path of a source gets replaced by what the source delivers, not followed, and populate says so. To keep a source elsewhere, e.g. on another disk or in memory, mount that place at the target path, or make the target directory itself a symlink. A Git work tree whose .git is a symlink, or a file naming a repository elsewhere, gets replaced by a git source as well.
A git source only uses a work tree whose .git directory contains no symlinks, no commondir file and no linked work trees, and only the configuration that git clone and populate write, and in which each .git of a submodule is a file naming a repository in .git/modules. Otherwise Git's commands could change what's elsewhere, or run commands, so populating fails, telling what's wrong, unless the source's reclone is set. Git runs no hooks.
A file in the target directory that has other hard links, e.g. one in /etc/nixos, gets replaced by a copy of its own before rsync syncs it, so the other ones keep their mode and time. Populate writes its own files, and replaces lock files it rewrites, by renaming new files into place, and only makes directories writable that it removes.
Populate keeps its state in the sentinel, .populate, which becomes a directory once there is state to keep. Populating fails if the sentinel, or a directory populate needs therein, is a symlink.

Source Types

`derivation`

Nix expression to be built at the target machine.

Supported attributes:

text - Nix expression to be built.

`file`

The file source type transfers local files (and folders) to the target using rsync.

Supported attributes:

path - absolute path to files that should by transferred.
useChecksum (optional) - boolean that controls whether file contents should be checked to decide whether a file has changed. This is useful when path points at files with mangled timestamps, e.g. the Nix store.

The default value is true if path is a derivation, and false otherwise.
filters (optional) List of filters that should be passed to rsync. Filters are specified as attribute sets with the attributes type and pattern. Supported filter types are include and exclude. Checkout the filter rules section in the rsync manual for further information.
deleteExcluded (optional) boolean that controls whether the excluded directories should be deleted if they exist on the target. This is passed to the --delete-excluded option of rsync. Defaults to true.
insecureLinks (optional) - what to do if rsync can't read .rsync-filter files that apply to path: the ones therein, and the ones in the directories above it. rsync 3.5.0 and later open them more strictly than the files they transfer, and ignore the ones they can't open, e.g. because a symlink on the way belongs to neither root nor the user deploying, or, with rsync 3.5.0, because a directory on the way may be entered but not read, like a home directory with mode 711. By default, with null, populating fails with an error naming the file. With true, rsync reads such files anyway (--insecure-links), which also lets it pass such symlinks on the way to the target path of a local target; with false, rsync syncs without them. Both print a warning. Older versions of rsync aren't affected.
inputs (optional) - inputs of the flake in this source to draw from other sources, see Flake inputs from sources.
rewriteLock (optional) - boolean that controls whether inputs get drawn from other sources by rewriting the lock file of the flake in this source, or else through a generated flake, see Flake inputs from sources. Defaults to true.

`flake`

The flake source type delivers a flake to the target. The flake gets archived locally using nix flake archive, which includes only files tracked by Git, so use a file source for work trees with untracked files. The flake and its inputs get copied into the target's Nix store using nix copy, and the target path becomes a symlink to the flake's store path, registered as garbage collector root. For flakes in a subdirectory, e.g. git+file:/src/mono?dir=sub, the symlink points at that subdirectory of the store path. The inputs get garbage collector roots in .populate/flake-inputs/<name>, which turns the sentinel file into a directory.

Supported attributes:

fetchOnTarget (optional) - boolean that controls whether the target fetches the flake's inputs itself instead of receiving them from the deploying machine. The target then uses the arguments recorded in the lock file, so inputs that refer to local paths can only be fetched if the target has these paths as well; populating fails otherwise, naming the input. If the flake reference names a scheme that doesn't refer to the deploying machine, e.g. github:, the target handles the flake entirely on its own, and the deploying machine doesn't fetch it. Otherwise the target fetches the flake itself as well if its locked reference doesn't refer to the deploying machine, e.g. for indirect references resolving to github: flakes, or else the flake gets transferred using rsync, unless the target's Nix store has it already, and added to the target's Nix store, where it gets the same store path as on the deploying machine. This requires Nix with flake support (Nix 2.4 or later) on the target, and works for targets using sudo. Defaults to false.
url - flake reference, e.g. the absolute path of a directory containing a flake.nix. Its lock file is used as is; populating fails if the lock file needs to be updated.
inputs (optional) - inputs of the flake in this source to draw from other sources, see Flake inputs from sources.
materialize (optional) - boolean that controls whether the target path becomes a writable copy instead of a link into the store. The copy gets made again only if what it derives from changes: the delivered store path, the patches, or which inputs get drawn from which sources. Until then, changes made to it on the target stay. Nix copies it into the store whenever evaluating a flake changed it, which is noticeable for big flakes like nixpkgs. For a flake in a subdirectory, only that subdirectory gets copied. Defaults to false.
patches (optional) - list of paths of patches on the deploying machine, which get applied in order with patch -p1 on the target, in the directory the target path shows. Populating fails if a patch doesn't apply. Defaults to [].
rewriteLock (optional) - boolean that controls whether inputs get drawn from other sources by rewriting the lock file of the flake in this source, or else through a generated flake, see Flake inputs from sources. Defaults to true.

Instead of an attribute set, the flake reference can be given as a string.

The flake commands on the deploying machine enable flakes themselves, so flakes don't need to be enabled there. Without fetchOnTarget, the target only needs nix-store. Because the copied store paths aren't signed, the target user must be allowed to import them, i.e. be root or a trusted user, and targets using sudo aren't supported.

`flake-input`

The flake-input source type delivers an input of a flake, as recorded in the flake's lock file, to the target. This makes the lock file of a flake the single place that pins, e.g., nixpkgs, for flakes as well as for NIX_PATH. It works like the flake source type, but delivers the selected input instead of the flake, so the target path becomes a symlink to the input's store path. The inputs that the input's own lock file pins, which evaluating the input needs, get delivered as well, with garbage collector roots in .populate/flake-inputs/<name>. An input without a lock file comes without its inputs, because locking it would change it.

Because the target path points at the store path named by the lock file, it can be used in NIX_PATH as well as by flakes evaluated on the target, without network access.

Supported attributes:

url - flake reference of the flake whose input gets delivered, e.g. the absolute path of a directory containing a flake.nix. Its lock file is used as is; populating fails if the lock file needs to be updated.
name - name of the input to deliver. Inputs of inputs are separated by /, e.g. stockholm/nixpkgs, and resolved as the flake's lock file resolves them, so an input that follows another one names the other one.
requireInputs (optional) - boolean that controls whether populating fails if the inputs that the input's own lock file pins can't be delivered, e.g. because the lock file pins something unreachable, or lacks an input its flake.nix has. With false, populate prints a warning and delivers the input with the inputs Nix locks when it adds what the lock file lacks, or else without its inputs. Evaluating the input as a flake then needs network access. Defaults to true.
fetchOnTarget (optional) - boolean that controls whether the target fetches the input itself instead of receiving it from the deploying machine. The target then uses the arguments recorded in the lock file, so inputs that refer to local paths can only be fetched if the target has these paths as well; populating fails otherwise, naming the input. This requires Nix with flake support (Nix 2.4 or later) on the target, and works for targets using sudo. Defaults to false.
inputs (optional) - inputs of the flake in this source to draw from other sources, see Flake inputs from sources.
materialize (optional) - boolean that controls whether the target path becomes a writable copy instead of a link into the store. The copy gets made again only if what it derives from changes: the delivered store path, the patches, or which inputs get drawn from which sources. Until then, changes made to it on the target stay. Nix copies it into the store whenever evaluating a flake changed it, which is noticeable for big flakes like nixpkgs. For a flake in a subdirectory, only that subdirectory gets copied. Defaults to false.
patches (optional) - list of paths of patches on the deploying machine, which get applied in order with patch -p1 on the target, in the directory the target path shows. Populating fails if a patch doesn't apply. Defaults to [].
rewriteLock (optional) - boolean that controls whether inputs get drawn from other sources by rewriting the lock file of the flake in this source, or else through a generated flake, see Flake inputs from sources. Defaults to true.

Without fetchOnTarget, the same requirements as for the flake source type apply.

Example:

{
  nixpkgs.flake-input = {
    url = toString ./.;
    name = "nixpkgs";
  };
}

`git`

Git sources that will be fetched on the target machine.

Supported attributes:

url - URL of the Git repository that should be fetched.
ref - Branch / tag / commit that should be fetched. A branch or tag gets fetched on every run, so the work tree follows it. A commit, given by its hash or a prefix thereof of at least four hex digits, only gets fetched if the work tree lacks it, unless fetchAlways is set. So a branch or tag whose name consists of hex digits only counts as a commit.
clean.exclude - List of patterns that should be excluded from Git cleaning.
shallow (optional) boolean that controls whether only the requested commit ref. should be fetched instead of the whole history, to save disk space and bandwith. Defaults to false.
reclone (optional) - boolean that controls whether a work tree populate can't use, see The target directory, gets replaced by a fresh clone, instead of failing. Defaults to false.
inputs (optional) - inputs of the flake in this source to draw from other sources, see Flake inputs from sources.
rewriteLock (optional) - boolean that controls whether inputs get drawn from other sources by rewriting the lock file of the flake in this source, or else through a generated flake, see Flake inputs from sources. Defaults to true.

`pass`

The pass source type transfers contents from a local password store to the target machine.

Supported attributes:

dir - absolute path to the password store.
name - sub-directory in the password store.

`passage`

The passage source type decrypts files from a local passage store and transfers them to the target using rsync.

Supported attributes:

dir - Path to the passage store. For a partial transfer, this may point to a subdirectory. Example: ~/.passage/store/hosts/MYHOSTNAME
identities_file (optional) - Path to the identities file. Defaults to ~/.passage/identities.
age (optional) - Path of the age binary. Defaults to age (absolute path gets resolved using passage's search path.)

`pipe`

Executes a local command, capture its stdout, and send that as a file to the target machine.

Supported attributes:

command - The (shell) command to run.

`symlink`

Symlink to create at the target, relative to the target directory. This can be used to reference files in other sources.

Supported attributes:

target - Content of the symlink. This is typically a relative path.
inputs (optional) - inputs of the flake in this source to draw from other sources, see Flake inputs from sources.

Flake inputs from sources

Sources of type file, flake, flake-input, git, and symlink can declare which inputs of the flake they contain get drawn from other sources, in the style of flake.nix, where follows names a source instead of an input. This makes flakes on the target use what populate delivers there, much like NIX_PATH does for Nix expressions.

{
  nixpkgs.flake-input = {
    url = toString ./.;
    name = "nixpkgs";
  };
  crate2nix.flake = {
    url = "github:nix-community/crate2nix";
    inputs.nixpkgs.follows = "nixpkgs";
  };
  myconf.file = {
    path = toString ./.;
    inputs = {
      crate2nix.follows = "crate2nix";
      stockholm.inputs.nixpkgs.follows = "nixpkgs";
    };
  };
}

Populate rewrites the lock file of the flake in each source that draws inputs from other sources, using nix flake lock --override-input, so the flake in the source itself uses the other sources. file and git sources get their lock file rewritten in place. flake and flake-input sources get delivered as a copy with the rewritten lock file, which gets added to the store, unless materialized, so the target path links to that instead of the delivered store path, as it does for patched ones. A source drawing an input from a source that draws inputs itself uses that source as rewritten, so in the example above, myconf's crate2nix uses nixpkgs from the nixpkgs source as well.

With rewriteLock = false, and for symlink sources, which point at other sources, the source stays as it is, and populate generates a flake in .populate/flakes/<name> instead, which re-exports the outputs of the flake in the source, with those inputs drawn from the other sources. A source drawing an input from a source with a generated flake uses that flake. krops' writeDeploy uses a source's generated flake automatically.

Sources in a Git work tree are referred to as git+file: (with submodules if there are any), all other sources as path:, with symlinks resolved. Nix runs Git in a git+file: flake, with its configuration and hooks, so that's only for work trees whose .git directory a git source would use, see The target directory, and has no hooks. Lock files get rewritten or generated on every run, so they match the sources. Copies of flake and flake-input sources get made again only if the delivered store path, the patches, or the sources they draw inputs from change. Inputs that get drawn from other sources don't get delivered with a flake or flake-input source, and neither does what they pin. Generated flakes and copies get removed once their sources no longer need them. This requires Nix with flake support (Nix 2.4 or later) on the target. Populating fails if a source that draws inputs from other sources contains no flake.nix, or if its flake has no such input, before anything changes. Sources that inputs get drawn from don't need to be flakes, as long as the inputs drawing from them are declared with flake = false.

Output

What populate prints on standard error is part of its interface: people read it, and tests check it. For example:

populating 'host:/var/src/nixpkgs'...
pruning 'host:/var/src/old'...
replacing symlink 'host:/var/src/x', which points to '/elsewhere'...
replacing the Git work tree 'host:/var/src/x', because ...

A target path in a message starts with user@host: for remote targets, without user@ if the user is root or not set, and is a plain path for the local target. Errors start with error: or krops:, warnings with warning:. rsync's itemized changes appear as %i %n%L %'''b/%'''l, with sizes in units of 1024.

Tests

tests/ contains NixOS VM tests: one test per source type, one for features spanning sources, such as flake inputs from sources, one for edge cases of flakes, such as unusual lock files, and one for staying inside the target directory, whatever links it contains. After every run, all tests check that nothing outside the target directory changed. The tests deliver combinations of a source type's settings to the machine populate runs on, to a remote target as root, and to a remote target using sudo, and check the result after delivering and after delivering again. They cover every combination that a code path tells apart; the exhaustive tests add the remaining ones, e.g. drawing inputs combined with each of rsync's options. The scenarios whose results depend on the version of rsync also run with rsync 3.4.1 and 3.5.0, which tests/rsync.nix builds, whatever version nixpkgs has. The tests require KVM and nixpkgs with pkgs.testers.runNixOSTest (23.05 or later), and use <nixpkgs> by default.

nix-build tests                   # all tests, except the exhaustive ones
nix-build tests -A flake          # the test for one source type
nix-build tests -A exhaustive     # all exhaustive tests
nix-build tests -A exhaustive.git # the remaining combinations of one test
nix-build tests -A rsync-3-5-0    # the scenarios depending on rsync's version
nix-build tests -A smoke          # a few scenarios of each kind, in one VM
nix-build tests -A only --argstr only 'lock-hooks-.*'  # scenarios by name
cargo test                        # the unit tests, without VMs

docs/design.md describes how populate works.