pylogsentinel 0.2.0

Lightweight cron-friendly log monitoring utility with regex rules and per-rule shell actions

pylogsentinel

A lightweight, zero-dependency (standard library only) log monitoring sentinel intended to be invoked periodically (e.g. once per minute via cron). It incrementally scans configured log files for user-defined regular expression rules and triggers shell actions with rich contextual environment variables.

pylogsentinel is intentionally simple: no daemons, no background threads, no long-running processes. Each run processes just the new bytes appended since the previous run (tracked per-file, per-inode) and then exits.

Key Features

• INI-style configuration (pylogsentinel.conf).
• Two log discovery modes: static paths or shell command
• Per-rule regular expressions with optional flags (/pattern/i style).
• Per-rule or actions executed as shell commands with rich contextual environment variables.
• Reliable state tracking via inode-based files (<inode> in state_dir).
• Safe handling of truncation & rotation (offset reset if file shrinks).
• Strict lock file (LOCK) prevents concurrent overlapping runs.
• Skip-actions mode (--skip-actions) processes new data but skips executing actions (use on the first run to avoid firing on historical log lines).
• Human-friendly size option for max_block_size (e.g. 512K, 10M).
• Automatic configuration file discovery when -c/--config is not supplied.
• Directory path monitoring uses the system 'file' command; only paths whose description contains 'text' are monitored (explicit file paths are always processed).
• Python 3.9+ (only standard library imports).

Installation

pip install pylogsentinel

Quick Start

1. Write a configuration in a standard location:

   touch /etc/pylogsentinel.conf
   chmod 600 /etc/pylogsentinel.conf
   

2. Edit paths, rules, and actions to your needs.

3. Add a cron entry (run every minute):

   * * * * * /usr/bin/env python3 -m pylogsentinel -c /etc/pylogsentinel.conf
   

   If you omit the -c option (or the specified file does not exist), pylogsentinel will look for a configuration file in this order:

   • ~/.pylogsentinel.conf
   • ~/.config/pylogsentinel.conf
   • /etc/pylogsentinel.conf
   • /usr/local/etc/pylogsentinel.conf.

4. First build state without executing actions (initial run with --skip-actions):

   python -m pylogsentinel -c /etc/pylogsentinel.conf --skip-actions
   

Configuration File Reference

The configuration is an INI-style file. An example:

state_dir = /var/run/pylogsentinel
max_block_size = 10M

[logs]
# Choose exactly one, `paths` or `cmd`:
paths = /var/log /custom/app/logs/app.log
cmd = find /var/log -type f -name '*.log'

[action.default]
cmd = echo -e "Matched $RULE_ID in $FILE at line $LINE, context:\n\n$CONTEXT" | mail -s "Sentinel alert" root

[action.another]
cmd = echo "Another action"

[rule.error]
description = Error-like conditions
pattern = /(error|fatal|exception|kill|crash)/i
action = another # if omitted, use default action

[system] Section

Option	Required	Description	Default
state_dir	Yes	Directory storing lock + per-inode state files. Must be writable. (Defined under [system].)	(none)
max_block_size	No	Maximum newly appended bytes to read per file per run. Supports suffixes K, M, G (in [system]).	10M

[logs] Section

Supply either:

• paths: Space-separated list of file and/or directory paths. Directories are traversed recursively; only files whose file -b output contains the substring "text" are monitored (others are skipped). Explicit file paths are always processed.
• cmd: A shell command producing one path per line on stdout.

Exactly one must be present. If using cmd, ensure it returns relatively quickly (recommended under a second) as it executes on every run.

[rule.<rule_id>] Sections

Field	Required	Description
pattern	Yes	Regular expression in the mandatory form /pattern/flags (flags optional). Supported flags: i (IGNORECASE).
description	No	Human-readable description for environment variable RULE_DESCRIPTION.
action	No	The action_id to invoke. Defaults to default if omitted.

At least one rule is required.

Pattern Forms Examples

Pattern	Meaning
/error/i	Case-insensitive match of "error"
/^panic:.+/	Match lines starting with "panic:" (case-sensitive)
/^panic:.+/i	Same, case-insensitive
/\b(?:CRIT|FATAL)\b/	Word boundary critical terms

If you supply an invalid regex or an unsupported flag, startup will fail with a configuration error.

[action.<action_id>] Sections

Each action defines a shell cmd executed when a matching rule triggers. The special action id default must exist; it is used as a fallback when a rule references it explicitly or omits action.

Environment variables available to the command:

Variable	Description
RULE_ID	The rule identifier (e.g. error).
RULE_PATTERN	The normalized pattern string (e.g. /error/i).
RULE_DESCRIPTION	Description text or placeholder string if not provided.
FILE	Absolute path of the log file where the match occurred.
LINE	1-based absolute line number within the file at time of scan.
CONTEXT	Concatenated lines around the match (default radius 2 before & after).

Your cmd can reference these with typical shell expansion, for example:

[action.notify]
cmd = printf "%s\n%s\n" "$RULE_DESCRIPTION" "$CONTEXT" | mail -s "Alert $RULE_ID: $FILE:$LINE" [e-mail]

State Tracking

For each processed file:

1. The file's inode (st_ino) is determined.
2. A state file named <inode> inside state_dir stores:

   <byte_offset> <line_number>
   

3. On the next run only new bytes (up to max_block_size) beyond <byte_offset> are read.
4. If the file shrinks (truncation), the stored offset is treated as invalid and scanning restarts from the beginning with line numbers reset.

This scheme tolerates typical log rotation patterns.

Lock File

A lock file named LOCK in state_dir prevents overlapping runs. If it exists the process exits immediately. Remove a stale lock only after verifying no instance is active.

Dry Run

--skip-actions processes new data but does not run the actions (recommended for the very first run to prevent alert storms from historical data).

Exit Codes

Code	Meaning
0	Success
2	Configuration error
130	Interrupted (Ctrl+C)
1	Other runtime error

License

MIT License (see LICENSE).