# fstool
[](https://github.com/KarpelesLab/fstool/actions/workflows/ci.yml)
[](https://crates.io/crates/fstool)
[](https://docs.rs/fstool)
Build, inspect, modify, and repack disk images and filesystem images.
In the spirit of `genext2fs`, but covering whole disks, multiple filesystems,
and round-tripping between formats — all from a TOML spec or directly from
the command line.
fstool ships as a Rust library (`fstool`) plus a thin CLI binary (`fstool`).
Public API is **unstable** until v0.5.
```sh
cargo install fstool
fstool ext-build --kind ext4 ./src -o out.img # build an ext4 image from a dir
fstool info out.img # what's inside
fstool ls out.img / # walk it
fstool repack out.img out.tar # convert ext4 → tar (and back)
```
## Filesystem support
| ext2 | ✅ | ✅ | byte-exact with `genext2fs` on the same input |
| ext3 | ✅ | ✅ | + JBD2 journal |
| ext4 | ✅ | ✅ | extents, FILETYPE, `metadata_csum`, xattrs |
| FAT32 | ✅ | ✅ | VFAT LFN entries, 8.3 short-name aliases |
| exFAT | ✅ | ✅ | format + create + remove + flush |
| tar | ✅ | ✅ | ustar + PAX, `SCHILY.xattr.*` for xattrs |
| XFS | ✅ (shortform + block / leaf / node dirs + BMBT) | — | + inline + remote symlinks; B-tree dirs deferred |
| HFS+ / HFSX| ✅ | — | inline + extents-overflow, symlinks, hard links |
| APFS | ✅ | — | multi-level omap + fs-tree; no snapshots / crypto |
| NTFS | ✅ | — | MFT, attributes, $DATA + ADS, indexes; xattr map |
| F2FS | ✅ | — | CP / NAT / dnodes / inline data + dentries |
| SquashFS | ✅ (uncompressed) | — | gzip/xz/lz4/zstd refused until compression lands |
| qcow2 | ✅ | ✅ | v2 + v3, allocate-on-write writer |
The reader for each FS streams: file contents are never fully resident in
memory regardless of size. The writers do the same, two-pass: scan to size
the geometry, then stream bytes from each source file into the image.
NTFS metadata that has no POSIX analogue (DOS attributes, ADS, security
descriptors, NT-FILETIME timestamps, short names, reparse data) round-trips
through xattrs under `user.ntfs.*` and `system.ntfs_security`.
## CLI commands
| `build` | Build from a TOML spec — bare FS or a partitioned disk image. |
| `ext-build` | Bare ext2 / ext3 / ext4 image from a host directory tree. |
| `fat-build` | Bare FAT32 image from a host directory tree. |
| `info` | Print partition table (whole-disk) or FS summary + root listing. |
| `ls` | List a directory inside an image. |
| `cat` | Stream a file's bytes out of an image to stdout. |
| `add` | Copy a host file / tree into an existing image (ext or FAT). |
| `rm` | Unlink a file, symlink, device, or empty directory. |
| `shell` | SFTP-style REPL — `ls cd pwd cat put rm mkdir info`. |
| `convert` | Byte-level raw ↔ qcow2 conversion with optional grow. |
| `repack` | Walk source FS, rebuild into a fresh image, optionally a different FS. |
| `fstool …` | Plus `ext-build`, `fat-build`, partition-aware `disk.img:N` targets. |
All inspection / modification commands accept a `disk.img:N` (1-indexed)
target to walk into a partition of a GPT or MBR disk image. `fstool info
disk.img` without the suffix prints the partition table itself.
## Partitions, block devices, qcow2
- **Partition tables** — MBR (4 primaries) and GPT (128-entry, CRC32 on
header + entry array, primary + backup, protective MBR). Cross-checked
against `sgdisk -v` and `fdisk -l`.
- **Block devices** — on Unix, fstool can format and mutate real block
devices (`/dev/sdX`, `/dev/nvme0n1`, loop devices). Capacity is queried via
the kernel ioctl (`BLKGETSIZE64` on Linux, `DKIOCGETBLOCK*` on macOS) and
open uses `O_EXCL` so the kernel refuses if any partition is mounted.
Build commands require `--force` when the output is a block device.
- **qcow2** — `Qcow2Backend` reads QEMU v2 / v3 images and writes fresh v3
ones with allocate-on-write. Path-based factories (`block::open_image`,
`block::create_image`) auto-dispatch by qcow2 magic or file extension, so
`fstool ext-build src -o out.qcow2` Just Works.
## TOML spec
Declarative image descriptions — either a bare filesystem (`[filesystem]`)
or a partitioned disk (`[image]` + `[[partitions]]`):
```toml
[image]
size = "64MiB"
partition_table = "gpt"
[[partitions]]
name = "EFI"
type = "esp"
size = "16MiB"
[[partitions]]
name = "root"
type = "linux"
size = "remaining"
[partitions.filesystem]
type = "ext4"
source = "./rootfs"
```
```sh
fstool build disk.toml -o disk.img
sgdisk -v disk.img # "No problems found."
```
## Architecture
```
┌────────────────────────────────────────────┐
│ CLI (clap) — bin/fstool │
└────────────────────────────────────────────┘
│
┌────────────────────────────────────────────┐
│ Spec layer (TOML → ImageSpec / FsSpec) │
└────────────────────────────────────────────┘
│
┌────────────────────────────────────────────┐
│ Filesystem trait → ext, fat, xfs, ntfs, … │
└────────────────────────────────────────────┘
│
┌────────────────────────────────────────────┐
│ PartitionTable trait → Mbr, Gpt │
└────────────────────────────────────────────┘
│
┌────────────────────────────────────────────┐
│ BlockDevice trait → File, Mem, Sliced, │
│ Qcow2 │
└────────────────────────────────────────────┘
```
Each layer is substitutable. A filesystem implementation talks only to a
`BlockDevice`; it doesn't know or care whether the device is a real file,
an in-memory buffer in a test, a slice carved out of a larger disk by a
partition table, or a qcow2-backed sparse container.
## ext-specific niceties
- `BuildPlan` auto-sizes a filesystem to fit a source tree exactly
(genext2fs-style "size to fit").
- `Ext::populate_rootdevs` drops a `Minimal` or `Standard` `/dev/*` tree
(console, null, zero, ptmx, tty, fuse, random, urandom — plus tty0..15,
ttyS0..3, kmsg, mem, port, hda..hdd, sda..sdd + partitions for
`Standard`), so a non-root user can build a Linux root FS without
`CAP_MKNOD`.
- xattrs round-trip through repack: both inline (extended-inode-body) and
external `file_acl`-block sources are read; the destination writes to an
external block with a correctly-computed CRC32C when `metadata_csum` is on.
`debugfs ea_get` confirms identical values after repack.
## Cross-FS repack
`fstool repack` walks the source filesystem and rebuilds the tree into a
fresh image. With `--fs-type` it changes filesystem on the fly; `--shrink`
auto-sizes the output to the minimum that fits the content. The ext → ext
path uses a direct FS-to-FS copier (no host-filesystem intermediation),
preserving symlinks, device nodes, mode, uid/gid, and xattrs. tar in either
direction round-trips content + mode + uid/gid + mtime + symlinks + device
nodes + xattrs.
For the read-only filesystems (XFS, HFS+, APFS, NTFS, F2FS, SquashFS),
repack works **from** them. Repacking **to** them isn't supported until
their writers land.
## Limitations
Things explicitly out of scope today, in rough order of likely-to-change:
- SquashFS compression decode (needs flate2 / zstd / xz / lz4 dependencies).
Compressed blocks return a clean `Unsupported` naming the algorithm.
- NTFS / F2FS / XFS / APFS / HFS+ writers.
- NTFS compressed and encrypted `$DATA`, `$ATTRIBUTE_LIST` spill, `$Secure`
security-descriptor indirection — all return `Unsupported`.
- APFS snapshots, encryption, sealed-volume integrity.
- XFS B-tree-format directories (block / leaf / node formats are covered).
- ext4 `flex_bg` on the *write* path (the reader handles it).
- Partial-file rewrites — in-place modification is whole-file granularity.
## Try it
```sh
cargo install fstool # or: cargo install --path .
mkdir -p /tmp/src/etc && echo hi > /tmp/src/greeting.txt
fstool ext-build --kind ext4 /tmp/src -o /tmp/out.img
fstool info /tmp/out.img
fstool ls /tmp/out.img /
fstool cat /tmp/out.img /greeting.txt
e2fsck -fn /tmp/out.img # must report clean
```
Run the test suite:
```sh
cargo test # unit tests + external cross-checks if tools present
```
CI runs the full suite on Linux (with `apt`-installed `e2fsprogs`,
`dosfstools`, `mtools`, `gdisk`, `qemu-utils` for cross-validation) plus a
build + test pass on macOS (Homebrew `qemu`) and Windows.
## Licence
MIT. Copyright © 2026 Karpelès Lab Inc. See [LICENSE](LICENSE).