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chezmoi architecture guide

Introduction

This document gives a high-level overview of chezmoi's source code for anyone interested in contributing to chezmoi.

You can generate Go documentation for chezmoi's source code with go doc, for example:

$ go doc -all -u github.com/twpayne/chezmoi/v2/internal/chezmoi

You can also browse chezmoi's generated documentation online but this only includes exported symbols.

Directory structure

The important directories in chezmoi are:

Directory Contents
docs/ The documentation single source of truth. Help text, examples, and the chezmoi.io website are generated from the files in this directory, particularly docs/REFERENCE.md.
internal/chezmoi/ chezmoi's core functionality.
internal/cmd/ Code for the chezmoi command.
internal/cmd/testdata/scripts/ High-level tests of chezmoi's commands using testscript.

Key concepts

As described in the reference manual, chezmoi evaluates the source state to compute a target state for the destination directory (typically your home directory). It then compares the target state to the actual state of the destination directory and performs any changes necessary to update the destination directory to match the target state. The concepts are represented directly in chezmoi's code.

chezmoi uses the generic term entry to describe something that it manages. Entries can be files, directories, symlinks, scripts, amongst other things.

internal/chezmoi/

All of chezmoi's interaction with the operating system is abstracted through the System interface. A System includes functionality to read and write files and directories and execute commands. chezmoi makes a distinction between idempotent commands that can be run multiple times without modifying the underlying system and arbitrary commands that may modify the underlying system.

The real underlying system is implemented via a RealSystem struct. Other Systems are composed on top of this to provide further functionality. For example, the --debug flag is implemented by wrapping the RealSystem with a DebugSystem that logs all calls to the underlying RealSystem. --dry-run is implemented by wrapping the RealSystem with a DryRunSystem that allows reads to pass through but silently discards all writes.

The SourceState struct represents a source state, including reading a source state from the source directory, executing templates, applying the source state (i.e. updating a System to match the desired source state), and adding more entries to the source state.

Entries in the source state are abstracted by the SourceStateEntry interface implemented by the SourceStateFile and SourceStateDir structs, as the source state only consists of regular files and directories.

A SourceStateFile includes a FileAttr struct describing the attributes parsed from its file name. Similarly, a SourceStateDir includes a DirAttr struct describing the directory attributes parsed from a directory name.

SourceStateEntrys can compute their target state entries, i.e. what the equivalent entry should be in the target state, abstracted by the TargetStateEntry interface.

Actual target state entries include TargetStateFile structs, representing a file with contents and permissions, TargetStateDir structs, representing a directory, TargetStateSymlink for symlinks, TargetStateRemove for entries that should be removed, and TargetStateScript for scripts that should be run.

The actual state of an entry in the target state is abstracted via the ActualStateEntry interface, with ActualStateAbsent, ActualStateDir, ActualStateFile, ActualStateSymlink structs implementing this interface.

Finally, an EntryState struct represents a serialization of an ActualEntryState for storage in and retrieval from chezmoi's persistent state. It stores a SHA256 of the entry's contents, rather than the full contents, to avoid storing secrets in the persistent state.

With these concepts, chezmoi's apply command is effectively:

  1. Read the source state from the source directory.
  2. For each entry in the source state (SourceStateEntry), compute its TargetStateEntry and read its actual state in the destination state (ActualStateEntry).
  3. If the ActualStateEntry is not equivalent to the TargetStateEntry then apply the minimal set of changes to the ActualStateEntry so that they are equivalent.

Furthermore, chezmoi stores the EntryState of each entry that it writes in its persistent state. chezmoi can then detect if a third party has updated a target since chezmoi last wrote it by comparing the actual state entry in the target state with the entry state in the persistent state.

internal/cmd/

internal/cmd/*cmd.go contains the code for each individual command and internal/cmd/*templatefuncs.go contain the template functions.

Commands are defined as methods on the Config struct. The Config struct is large, containing all configuration values read from the config file, command line arguments, and computed and cached values.

The Config.persistentPreRunRootE and Config.persistentPostRunRootE methods set up and tear down state for individual commands based on the command's Annotations field.

Path handling

chezmoi uses separate types for absolute paths (AbsPath) and relative paths (RelPath) to avoid errors where paths are combined (e.g. joining two absolute paths). A further type SourceRelPath is a relative path within the source directory and handles file and directory attributes.

Internally, chezmoi normalizes all paths to use forward slashes with an optional upper-cased Windows volume so they can be compared with string comparisons. Paths read from the user may include tilde (~) to represent the user's home directory, use forward or backward slashes, and are treated as external paths (ExtPath). These are normalized to absolute paths. chezmoi is case-sensitive internally and makes no attempt to handle case-insensitive or case-preserving filesystems.

Persistent state

Persistent state is treated as a two-level key-value store with the pseudo-structure map[Bucket]map[Key]Value, where Bucket, Key, and Value are all []bytes. The PersistentState interface defines interaction with them. Sometimes temporary persistent states are used. For example, in dry run mode (--dry-run) the actual persistent state is copied into a temporary persistent state in memory which remembers writes but does not persist them to disk.

Encryption

Encryption tools are abstracted by the Encryption interface that contains methods of encrypting and decrypting files and []bytes. Implementations are the AGEEncryption and GPGEncryption structs. A DebugEncryption struct wraps an Encryption interface and logs the methods called.

run_once_ scripts

The execution of run_once_ scripts is recorded by storing the SHA256 of their contents in the persistent state. run_once_ scripts are only run if they are new or if their contents have not changed.

Testing

chezmoi has a mix of, unit, integration, and end-to-end tests. Unit and integration tests use the github.com/stretchr/testify framework. End-to-end tests use github.com/rogpeppe/go-internal/testscript with the test scripts themselves in testdata/scripts. You can run individual end-to-end tests with

$ go test ./internal/cmd -run=TestScript/<name>

where <name> is the basename of file in testdata/scripts.

Tests should, if at all possible, run unmodified on all operating systems tested in CI (Linux, macOS, Windows, and FreeBSD). Windows will sometimes need special handling due to its path separator and lack of POSIX-style file permissions.