go-exiftool: ExifTool in Go (zeroperl + wasm2go)

Go library that runs ExifTool in-process: embedded Perl via zeroperl (Perl compiled to WebAssembly), executed as native Go via a wasm2go-generated module with the wasm2go-wasi-host WASI host. You do not need a system exiftool binary or a separate Perl install.

Upstream ExifTool (Phil Harvey) and metadata: exiftool.org, exiftool/exiftool.

Why this shape

• Self-contained - Perl stdlib, ExifTool, and the interpreter ship inside the module (embed/). The stdlib is stored LZ4-compressed; perlFS() transparently decompresses files on first read using a 2000-entry LRU cache (lbe/cfsread).
• WASI host - github.com/lbe/wasm2go-wasi-host implements WASI snapshot-preview1 for the wasm2go module. Mounts and syscalls are configured through wasihost.NewModuleConfig() in buildModuleConfig: writable host directory preopen at /host for the caller's working directory (guest cwd /host via Perl preamble chdir); read-only FS mounts at /lib and /bin; host cwd path aliases; os.TempDir(); and operand parent directories outside cwd/temp for [Command] (see operand_preopen.go).
• Same behavior everywhere - one toolchain-based interpreter implementation on Linux, macOS, Windows, etc.
• Trade-off - cold start is heavy (on the order of ~7-10 seconds to initialize the wasm2go-backed Perl runtime). For many operations, use NewServer and Server.Command so ExifTool stays open (-stay_open) and work is amortized.

Requirements

• Go 1.26+
• Dependencies are listed in go.mod.

Usage

// One-shot: paths are relative to the process working directory; guest cwd is /host.
out, err := exiftool.Command(nil, "-json", "photo.jpg")

// Batch: one Perl startup, many commands.
e, err := exiftool.NewServer("-fast")
if err != nil { /* ... */ }
defer e.Shutdown() // graceful; safe after NewServer returns. Use Close if stopping during init.
out, err = e.Command("-Artist", "photo.jpg")

Package docs, API details, and filesystem semantics (mount layout, temp dir, Arg1 / Config): run go doc -all or open pkg.go.dev.

Development

Tests

See TESTING.md and ARCHITECTURE.md. Fast local check (unit only):

go test ./... -count=1

Full suite matches CI:

go test -tags=integration,e2e -race ./... -count=1

Or: make test-race ARGS="-count=1". Lint: make lint.

Refreshing embed/ from zeroperl

The generated zeroperl Go source and Perl install prefix come from zeroperl. Follow that repository's Build section (Docker or Apple Container): build the image, run the container, and copy /artifacts into a host directory (as shown there, e.g. ./output/).

From the build output directory, install these into this repo under embed/:

Artifact from zeroperl output	Path in go-exiftool
zeroperl.go	internal/zeroperl/zeroperl.go
perl-wasi-prefix/ (entire tree)	embed/perl-wasi-prefix/

Use the generated zeroperl.go artifact unless you intentionally switch runtimes. The ExifTool script is loaded from embed/perl-wasi-prefix/bin/exiftool in the embedded prefix.

After copying perl-wasi-prefix/ into embed/, run go generate ./... to re-compress the tree with cfsread-lz4. This updates the LZ4-compressed files in place so the embedded binary reflects the new Perl build.

zeroperl's README also documents build arguments (PERL_VERSION, EXIFTOOL_VERSION, BUILD_EXIFTOOL, memory/stack, etc.). The embedded prefix layout (for example lib/perl5 and lib/perl5/wasm32-wasi) is fixed for a given embed refresh; exiftoolEvalWrapper prepends those paths to @INC at eval time.

Building ExifTool prefix with dist.pl

Use dist.pl to download, build, minify, test, and install ExifTool into embed/perl-wasi-prefix/.

perl dist.pl --exiftool-version 13.56

Version resolution precedence is:

1. --exiftool-version
2. EXIFTOOL_VERSION environment variable
3. Latest version parsed from https://exiftool.org/history.html

Writing Files

Command and CommandContext run ExifTool with file paths relative to the process working directory (or host-absolute paths). The host cwd is preopened at /host; guest cwd is /host. os.TempDir() and operand parent directories outside cwd/temp are preopened for [Command]. File operands are passed to ExifTool as host-absolute paths. ExifTool can write (or overwrite) files in the working directory using flags like -overwrite_original or -o <outfile>.

License

See LICENSE. ExifTool and embedded components carry their own licenses under embed/.