fsmon 0.3.0

Lightweight High-Performance File System Change Tracking Tool
Documentation

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Crates.io

fsmon is a real-time Linux filesystem change monitor powered by fanotify. It watches files and directories, captures every event (create, modify, delete, move, attribute change, etc.), and attributes each change back to the process that caused it — including the PID, command name, user, parent PID, thread group ID, and optional full process ancestry chain.

Features

  • Real-time Monitoring: Captures 14 fanotify event types (default: 8 core events; use --types all for all 14)
  • Process Attribution: Tracks PID, command name, user, PPID, and TGID for every file change — even short-lived processes like touch, rm, mv
  • Process Tree Tracking (<CMD> positional arg): Pinpoint a specific process (e.g., openclaw) and fsmon will track it plus all its descendants (fork/exec children), building a complete ancestry chain per event.
  • Recursive Monitoring: Watch entire directory trees with automatic tracking of newly created subdirectories
  • Complete Deletion Capture: Captures every file deleted during rm -rf via persistent directory handle cache
  • High Performance: Rust + Tokio, <5MB memory footprint, zero-copy FID event parsing, binary-search log querying
  • Capture-time Filtering: Filter by event type and file size — in-process, nanosecond-fast, no fork.
  • Live Updates: Add/remove paths while daemon runs — no restart needed.

Quick Start

Prerequisites

  • OS: Linux 5.9+ (requires fanotify FID mode)
  • Tested Filesystems: ext4, XFS, btrfs
  • Build: Rust toolchain (cargo)
# Verify kernel version
uname -r  # requires â‰Ĩ 5.9

# Install Rust if needed
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
source $HOME/.cargo/env

Installation

# Build from source
git clone https://github.com/lenitain/fsmon.git
cd fsmon
cargo install --path .

# Or install from crates.io
cargo install fsmon

Fanotify requires root privileges for the daemon:

sudo cp ~/.cargo/bin/fsmon /usr/local/bin/

A Complete Walkthrough

Monitor a web project directory, see what gets logged, then use standard Unix tools to filter and clean.

# Terminal 1: start the daemon (sudo for fanotify)
sudo fsmon daemon &

# Terminal 1 (or another): add paths to monitor
# Monitor /var/www/myapp recursively, MODIFY + CREATE events only,
# tracking the nginx and vim processes.
fsmon add nginx --path /var/www/myapp -r --types MODIFY --types CREATE
fsmon add vim --path /var/www/myapp -r --types MODIFY --types CREATE

# List what's being monitored
fsmon monitored
# {"cmd":"nginx","paths":{"/var/www/myapp":{"recursive":true,"types":["MODIFY","CREATE"]}}}
# {"cmd":"vim","paths":{"/var/www/myapp":{"recursive":true,"types":["MODIFY","CREATE"]}}}

Now trigger some real file changes:

# Terminal 2: simulate real usage
echo "<h1>Hello</h1>" > /var/www/myapp/index.html      # nginx writes a file
sleep 2
rm /var/www/myapp/index.html                              # file gets deleted
sleep 2
vim /var/www/myapp/config.json                            # vim edits config

Look at what fsmon captured:

# The raw log — one JSONL line per event
cat ~/.local/state/fsmon/*_log.jsonl
# → {"time":"2026-05-07T10:00:01+00:00","event_type":"CREATE","path":"/var/www/myapp/index.html","pid":1234,"cmd":"nginx","user":"www-data","file_size":0,"ppid":1,"tgid":1234}
# → {"time":"2026-05-07T10:00:01+00:00","event_type":"CLOSE_WRITE","path":"/var/www/myapp/index.html","pid":1234,"cmd":"nginx","user":"www-data","file_size":21,"ppid":1,"tgid":1234}
# → {"time":"2026-05-07T10:00:03+00:00","event_type":"DELETE","path":"/var/www/myapp/index.html","pid":5678,"cmd":"rm","user":"deploy","file_size":0,"ppid":1234,"tgid":5678}
# → {"time":"2026-05-07T10:00:05+00:00","event_type":"CREATE","path":"/var/www/myapp/.config.json.swp","pid":9012,"cmd":"vim","user":"dev","file_size":4096,"ppid":5678,"tgid":9012,"chain":"9012|vim|dev;5678|sh|deploy;1234|openclaw|root;1|systemd|root"}

Every event includes ppid (parent PID) and tgid (thread group ID). When a <CMD> is specified on add, matching events also include chain — a compact process ancestry string tracing back to PID 1.

Query with pipe

# What did nginx do in the last hour?
fsmon query -t '>1h' | jq 'select(.cmd == "nginx")'

# What files were deleted?
fsmon query | jq 'select(.event_type == "DELETE")'

# Who made the biggest changes?
fsmon query | jq -s 'sort_by(.file_size)[] | {cmd, user, file_size, path}'

# Real-time tail with filter (watch for deployments)
tail -f ~/.local/state/fsmon/*_log.jsonl | jq 'select(.user == "deploy")'

No built-in --pid, --cmd, --user, --sort flags needed — jq does it all.

Clean with safety

# Preview what would be deleted (config default: keep 30 days)
fsmon clean --dry-run

# Actually clean with custom retention
fsmon clean _global --time '>7d'

# Or just use Unix tools directly on the files
for f in ~/.local/state/fsmon/*_log.jsonl; do
  tail -500 "$f" > "${f}.tmp" && mv "${f}.tmp" "$f"
done

# Stop the daemon
kill %1

File Locations

Purpose Path Format
Infrastructure config ~/.config/fsmon/fsmon.toml TOML (optional — defaults without it)
Monitored paths database ~/.local/share/fsmon/monitored.jsonl JSONL (grouped by cmd, paths as map keys)
Event logs ~/.local/state/fsmon/*_log.jsonl JSONL (one event per line)
Unix socket /tmp/fsmon-<UID>.sock TOML over stream

Both the store path and log directory are configurable in ~/.config/fsmon/fsmon.toml (see [monitored].path and [logging].path).

The daemon runs as root (via sudo) but resolves your original user's home directory via SUDO_UID + getpwuid_r, so it writes to /home/<you>/... not /root/....

Note for vfat/exfat/NFS users: The daemon tries to chown log files back to your user. Filesystems without standard Unix ownership (vfat, exfat, NFS with no_root_squash off) don't support this. Logs remain owned by root. If fsmon clean fails as a normal user, run sudo fsmon clean or use the Unix tools directly on the .jsonl files.

Auto-start on Boot (Optional)

fsmon does not install a systemd service. The daemon requires sudo (root) for fanotify. To start automatically on login:

sudo crontab -e
@reboot /usr/local/bin/fsmon daemon &

Note: Use sudo crontab -e (root's crontab) — the daemon needs root privileges. Add the fsmon command to sudoers with NOPASSWD if using a user crontab instead.

Complete Commands

daemon

Start the fsmon daemon — requires sudo for fanotify.

sudo fsmon daemon          Start daemon in foreground
sudo fsmon daemon &        Start daemon in background

add

Add a path (optionally with process tracking) to the monitoring list. No sudo needed.

fsmon add nginx --path /var/www/myapp -r          Track nginx on /myapp recursively
fsmon add nginx --path /var/www/myapp             Track nginx on /myapp (non-recursive)
fsmon add --path /home -r                         Monitor all events on /home (global)
fsmon add --path /home --types MODIFY --types CREATE  Filter by event types
fsmon add --path /home --types all                     All 14 event types
fsmon add --path /home --size '>=1MB'                  Minimum file size filter

Modes:

Mode Example Behavior
CMD + --path fsmon add openclaw --path /home Track openclaw (and descendants) on /home. Matching events include chain.
--path only fsmon add --path /home All events on /home captured. Each event has ppid/tgid.
  • <CMD> (positional arg) enables process tree tracking: fork/exec children are automatically included. Matching events get a chain field (e.g., "102|touch|root;101|sh|root;100|openclaw|root;1|systemd|root").
  • Multiple entries with different <CMD> values can be added (OR logic per entry).
  • --path is required. Use _global as CMD for process-only global monitoring (uncommon).

remove

Remove one or more paths from the monitoring list. No sudo needed.

fsmon remove                    Remove entire global (null) cmd group
fsmon remove nginx              Remove entire nginx cmd group
fsmon remove nginx --path /home  Remove /home from nginx group
fsmon remove --path /home        Remove /home from global group

monitored

List all monitored paths with their filtering configuration (JSONL).

fsmon monitored                 Show all monitored path groups

Each line is a JSON object with cmd and paths fields. Pipe to jq for filtering.

query

Query historical events from log files. Output is JSONL — pipe to jq for filtering.

fsmon query                     Query all log files
fsmon query nginx               Query nginx log only
fsmon query -t '>1h'            Events from last hour
fsmon query -t '>=2026-05-01'   From absolute time
fsmon query -t '<30m'           Events until 30 minutes ago
fsmon query -t '>1h' -t '<now'  Time range (since + until)
fsmon query --path /tmp         Filter events by path prefix

Examples with jq:

# Search by process (ppid/tgid always present)
fsmon query | jq 'select(.ppid == 100)'

# Search by ancestry chain (only when --cmd was used on add)
fsmon query | jq 'select(.chain != "") | .chain'

# Traditional cmd/user filtering
fsmon query -t '>1h' | jq 'select(.cmd == "nginx")'
fsmon query | jq 'select(.event_type == "DELETE")'
fsmon query | jq -s 'sort_by(.file_size)[] | {cmd, user, file_size, path}'

clean

Clean log files for a specific cmd group. Defaults from fsmon.toml: keep_days=30, size==>=1GB.

fsmon clean _global              Clean global log (defaults)
fsmon clean nginx --time '>7d'  Keep last 7 days of nginx events
fsmon clean nginx --size '>=500MB'  Size limit for nginx log
fsmon clean _global --dry-run    Preview without deleting

Priority: CLI arg > fsmon.toml > code default (keep_days=30, size=>=1GB)

You can also clean the raw log files directly without fsmon clean:

# Keep only last 500 lines per log file
for f in ~/.local/state/fsmon/*_log.jsonl; do
  tail -500 "$f" > "${f}.tmp" && mv "${f}.tmp" "$f"
done

# Delete logs older than 30 days by mtime
find ~/.local/state/fsmon/ -name '*.jsonl' -mtime +30 -delete

Note: Native fsmon clean parses JSONL accurately (won't cut mid-line) and handles both time and size constraints. Raw Unix tools are simpler but may produce partial lines.

init

Initialize fsmon data directories. Creates log dir and monitored data dir. Does NOT write a config file — config is optional, defaults apply without it.

fsmon init

cd

Open a subshell in the log directory. Type exit to return:

fsmon cd
ls _global_log.jsonl

Configuration

Config file is optional — defaults apply without it.

# fsmon configuration file
#
# Infrastructure paths for fsmon. Monitored paths are added via
# 'fsmon add' / 'fsmon remove' and persisted in [monitored].path.
# All paths support ~ expansion. <UID> is replaced with the numeric UID at runtime.

[monitored]
# Path to the auto-monitored monitored paths database.
path = "~/.local/share/fsmon/monitored.jsonl"

[logging]
# Path to the event log directory (per-cmd *_log.jsonl files).
path = "~/.local/state/fsmon"
# Defaults for 'fsmon clean' (no auto-clean; use cron/timer).
keep_days = 30
size = ">=1GB"

[socket]
# Unix socket path for daemon-CLI live communication.
path = "/tmp/fsmon-<UID>.sock"

Event Types

Default captures 8 core events. Use --types all for all 14.

Default (8): CLOSE_WRITE, ATTRIB, CREATE, DELETE, DELETE_SELF, MOVED_FROM, MOVED_TO, MOVE_SELF

All 14 (via --types all): + ACCESS, MODIFY, OPEN, OPEN_EXEC, CLOSE_NOWRITE, FS_ERROR

FS_ERROR only works with filesystem-level marks (requires a filesystem that supports it).

Log Format

Every event is a single JSON line. All fields are always present.

{
  "time": "2026-05-07T10:00:01+00:00",
  "event_type": "CREATE",
  "path": "/var/www/myapp/index.html",
  "pid": 1234,
  "cmd": "nginx",
  "user": "www-data",
  "file_size": 0,
  "ppid": 1,
  "tgid": 1234
}

When <CMD> was specified on add and the event matches: chain is also included.

{
  ...
  "ppid": 101,
  "tgid": 102,
  "chain": "102|touch|root;101|sh|root;100|openclaw|root;1|systemd|root"
}

The chain format: pid|cmd|user per entry, ;-separated from root (PID 1) down to the event process.

Old logs without ppid/tgid/chain are backward compatible — missing fields default to 0 or "".

Architecture

Linux Kernel (fanotify FID mode)
    → Raw FID events pushed to kernel queue
    → tokio reads events asynchronously
    → fid_parser: resolves paths (two-pass + DashMap dir handle cache)
    → filters: event type, size, recursive/non-recursive scope
    → (if <CMD> was specified) process tree check:
      → not in tracked tree → drop immediately (zero /proc reads)
      → in tracked tree → build ancestry chain → append to event
    → write JSONL → per-cmd log file (<cmd>_log.jsonl)

Process tree (proc connector):
    Fork/Exec/Exit events from netlink connector socket
    → DashMap: pid → {cmd, ppid, user, tgid, start_time}
    On daemon start: /proc/*/stat snapshot seeds existing processes
    is_descendant(pid, "openclaw") → O(depth) DashMap lookups

User pipe:
    tail -f *.jsonl | jq 'select(...)'

Clean:
    fsmon clean → parse JSONL, apply time/size filters, truncate

Source Tree

src/
├── bin/
│   ├── fsmon.rs               CLI entry: main(), argument structs, arg tests
│   └── commands/
│       ├── mod.rs              run() dispatch, parse_path_entries helper
│       ├── daemon.rs           Daemon: load store, Monitor::new(), run()
│       ├── add.rs              CLI add: path normalization, store + socket
│       ├── remove.rs           CLI remove: store + socket
│       ├── monitored.rs        CLI monitored: JSONL output
│       ├── query.rs            CLI query: time filter, execute query
│       ├── clean.rs            CLI clean: parser delegation
│       ├── init_cd.rs          CLI init, cd

├── lib.rs             FileEvent, EventType, DaemonLock (flock singleton)
├── clean.rs           Log cleanup engine: time/size trim, tail-offset
├── config.rs          TOML config, SUDO_UID home resolution
├── monitored.rs         Monitored paths database (JSONL store)
├── monitor.rs         Fanotify loop, socket handler, add/remove/events
├── fid_parser.rs      FID event parsing, two-pass path recovery
├── filters.rs         PathOptions, event/size filters, path matching
├── dir_cache.rs       Directory handle cache (DashMap + HandleKey)
├── proc_cache.rs      Netlink proc connector: Fork/Exec/Exit, build_chain
├── query.rs           Binary-search log query on sorted JSONL
├── socket.rs          Unix socket protocol (TOML req/resp)
├── utils.rs           Size/time parsing, process info lookup, chown
└── help.rs            Help text constants

License

MIT License