`fsmon`

Real-time Linux filesystem change monitoring with process attribution.

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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 ≥ 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 _global -t '>1h' | jq 'select(.cmd == "nginx")'

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

# Who made the biggest changes?
fsmon query _global | 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 _global --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                        # or Ctrl+C (foreground)
# If managed via systemd:
sudo systemctl stop fsmon       # Stop
sudo systemctl status fsmon     # Status
journalctl -u fsmon -f          # Logs

File Locations

Purpose	Path	Format
Infrastructure config	`~/.config/fsmon/fsmon.toml`	TOML (created by `fsmon init`, all-commented — defaults apply)
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)

Auto-start on boot (optional — systemd recommended)

Recommended (systemd):

sudo fsmon init --service        # Create service file + systemctl daemon-reload
sudo systemctl enable --now fsmon # Enable auto-start + start now
sudo systemctl status fsmon       # Check status
journalctl -u fsmon -f            # View logs

Fallback (crontab, for non-systemd environments):

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
sudo fsmon daemon --debug                     # Enable debug output (event matching + cache stats)
sudo fsmon daemon --channel-capacity N        # Event channel capacity (default: unbounded)
sudo fsmon daemon --disk-min-free 10%         # Warn when disk space drops below threshold
sudo fsmon daemon --cache-dir-cap N           # Directory handle cache capacity (default: 100000)
sudo fsmon daemon --cache-dir-ttl N           # Directory handle cache TTL in seconds (default: 3600)
sudo fsmon daemon --cache-file-size N         # File size cache capacity (default: 10000)
sudo fsmon daemon --cache-proc-ttl N          # Process cache TTL in seconds (default: 600)
sudo fsmon daemon --cache-stats-interval N    # Cache stats log interval in debug mode (default: 60, 0=off)
sudo fsmon daemon --buffer-size N             # Fanotify read buffer in bytes (default: 32768)

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 _global --path /home -r              # Monitor all events on /home (global)
fsmon add _global --path /home --types MODIFY  # Filter by event types
fsmon add _global --path /home --types all     # All 14 event types
fsmon add _global --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`.
Global (_global)	`fsmon add _global --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 global monitoring (all processes).

remove

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

fsmon remove _global               # Remove entire global cmd group
fsmon remove nginx                 # Remove entire nginx cmd group
fsmon remove nginx --path /home    # Remove /home from nginx group
fsmon remove _global --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 _global                     # Query global log
fsmon query nginx                       # Query nginx log only
fsmon query _global -t '>1h'            # Events from last hour
fsmon query _global -t '>=2026-05-01'   # From absolute time
fsmon query _global -t '<30m'           # Events until 30 minutes ago
fsmon query _global -t '>1h' -t '<now'  # Time range (since + until)
fsmon query _global --path /tmp         # Filter events by path prefix

Examples with jq:

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

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

# Traditional cmd/user filtering
fsmon query _global -t '>1h' | jq 'select(.cmd == "nginx")'
fsmon query _global | jq 'select(.event_type == "DELETE")'
fsmon query _global | 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 — fsmon init creates a fully-commented reference config; defaults apply without modifications.

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 — fsmon init creates a fully-commented reference config; defaults apply without modifications.

# 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"

[cache]
# Directory handle cache capacity (default: 100000, ~15-20MB).
# Each entry maps a kernel file handle to a directory path.
# Lower on memory-constrained systems; raise when monitoring
# large directory trees (>100k dirs) to reduce handle re-resolution.
dir_capacity = 100000

# Directory handle cache TTL in seconds (default: 3600 = 1 hour).
# Shorter TTL frees memory faster for volatile directory structures;
# longer TTL reduces handle re-resolution for stable directories.
dir_ttl_secs = 3600

# File size cache capacity (default: 10000, ~1MB).
# Avoids stat() calls for files with known sizes.
# Raise for high-file-volume workloads (git checkout, npm install).
file_size_capacity = 10000

# Process cache TTL in seconds (default: 600 = 10 minutes).
# Applies to both process info cache (PID→cmd/user/ppid/tgid) and
# process tree cache (PID→parent for ancestor chain tracking).
# Shorter TTL cleans up zombie process entries faster;
# longer TTL reduces /proc reads for long-lived processes.
proc_ttl_secs = 600

# Cache stats log interval in seconds in debug mode (default: 60).
# Set to 0 to disable periodic cache stats output.
stats_interval_secs = 60

Override priority

CLI arguments (--channel-capacity, --disk-min-free, --cache-dir-cap, --cache-dir-ttl, --cache-file-size, --cache-proc-ttl, --cache-stats-interval, --buffer-size)
    > fsmon.toml [cache] section
        > code defaults

CLI flags override both config file and defaults:

# Override dir_cache capacity and fanotify buffer size at startup
sudo fsmon daemon --cache-dir-cap 200000 --buffer-size 65536 &

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 the event process up to PID 1 (root).

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       # hNetlink 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

Name		Name	Last commit message	Last commit date
Latest commit History 579 Commits
.github/workflows		.github/workflows
images		images
src		src
.cargoignore		.cargoignore
.gitignore		.gitignore
CHANGELOG.md		CHANGELOG.md
Cargo.lock		Cargo.lock
Cargo.toml		Cargo.toml
LICENSE		LICENSE
README.md		README.md
README.zh-CN.md		README.zh-CN.md
plan.md		plan.md

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`fsmon`

Real-time Linux filesystem change monitoring with process attribution.

Features

Quick Start

Prerequisites

Installation

A Complete Walkthrough

Query with pipe

Clean with safety

File Locations

Auto-start on Boot (Optional)

Auto-start on boot (optional — systemd recommended)

Complete Commands

daemon

add

remove

monitored

query

clean

init

cd

Configuration

Override priority

Event Types

Log Format

Architecture

Source Tree

License

About

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Repository files navigation

fsmon

Real-time Linux filesystem change monitoring with process attribution.

Features

Quick Start

Prerequisites

Installation

A Complete Walkthrough

Query with pipe

Clean with safety

File Locations

Auto-start on Boot (Optional)

Auto-start on boot (optional — systemd recommended)

Complete Commands

daemon

add

remove

monitored

query

clean

init

cd

Configuration

Override priority

Event Types

Log Format

Architecture

Source Tree

License

About

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`fsmon`

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