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The simplest recurring alert is a bounded pipeline: select the newest lines, optionally keep only ERROR or CRITICAL records, add the host and selection rule, and pass the result to a mail-delivery command or authenticated SMTP. For a text log on a Unix-like host, the basic shape is:
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tail -n 50 /path/to/app.log | mailx -s 'Recent errors on HOSTNAME' [email protected]Treat that command as a template rather than a universal recipe. The available mailx flags, SMTP authentication, log location, permissions and scheduler all depend on the operating system and implementation. The sections below show safe shell, Python, journald and PowerShell patterns, then explain scheduling and failure diagnosis.
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An alert is useful only when its selection rule is explicit. Decide these values before writing code:
- Source: a plain text application log, the systemd journal, or a syslog file.
- Bound: a maximum number of lines, such as the newest 50, or a time window such as the previous 15 minutes.
- Filter: all records, or lines containing a known marker such as
ERROR,CRITICALor a service name. - Context: UTC time, host name, source path or unit, and the exact line or time-window rule.
- Empty-result policy: send a “no matches” notice, or suppress normal messages and alert only when a match exists.
Always cap the excerpt and the total message size. A recurring job that emails an ever-growing file can flood your mailbox and create a second outage.
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Identify the log source and its platform conventions
| Source | Typical command or location | Important qualification |
|---|---|---|
| Plain text application log | tail -n 50 /path/to/app.log |
The account running the scheduler must be able to read the file; quote paths containing spaces. |
| systemd journal | journalctl -u myservice -n 50 --no-pager |
Use a unit, time window or both. Journal access can require membership in a system log group or elevated privileges. |
| Debian-based syslog text | /var/log/syslog |
Rotation can rename or compress files; read the active file or query the journal when that is the real source. |
| RHEL-based syslog text | /var/log/messages |
The exact files and retention depend on the syslog daemon and distribution configuration. |
Microsoft documents that PowerShell on Linux writes to journald and that journalctl --grep powershell can query those entries. The same documentation identifies /var/log/syslog on Debian-based systems and /var/log/messages on RHEL-based systems as common text-syslog locations. These are examples, not guarantees for every image or container.
Shell method for a host with working mailx or sendmail delivery
Send the newest 50 lines
For a host that already has a configured local mail-delivery agent, feed the body on standard input and set the subject and recipient:
HOST=$(hostname -s)
LOG='/var/log/myapp/app.log'
tail -n 50 -- "$LOG" | mailx -s "Recent log lines on $HOST" [email protected]The -- prevents a path beginning with a hyphen from being interpreted as an option. POSIX mailx passes message input to the mail-delivery software, but option names and authentication behavior vary among implementations. Confirm the manual page on the target host instead of assuming that an SMTP username, password or TLS flag is portable.
Filter severity before mailing
HOST=$(hostname -s)
LOG='/var/log/myapp/app.log'
body=$(tail -n 200 -- "$LOG" | grep -E 'ERROR|CRITICAL' | tail -n 50)
printf 'UTC: %snHost: %snSource: %snRule: newest 200 lines, then ERROR or CRITICAL, final 50 matchesnn%sn'
"$(date -u '+%Y-%m-%dT%H:%M:%SZ')" "$HOST" "$LOG" "${body:-No matching lines found.}"
| mailx -s "Errors on $HOST" [email protected]The first tail bounds the scan, while the second bounds the email. This avoids silently searching an entire multi-gigabyte file and makes the selection rule visible to the recipient. If your logs use structured fields or case-sensitive levels, replace the regular expression with the exact pattern used by the application.
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Handle empty matches and oversized lines
Decide whether an empty match should produce mail. The example sends an explicit “No matching lines found” body, which confirms that the job ran. To alert only on errors, test the variable and exit without invoking mailx when it is empty. For very long stack traces, add a byte cap after filtering, for example with head -c 200000, and state in the message that truncation may have occurred. Do not put SMTP passwords in the command line: process listings, shell history and scheduler logs can expose them.
Use awk when you need a field-aware rule
tail -n 500 -- "$LOG" | awk '/ERROR|CRITICAL/ { print }' | tail -n 50Use awk when you need to inspect a timestamp or another field rather than match anywhere in the line. If a log contains multiline exceptions, line-based tools may separate the header from its stack trace; use the application’s structured export or journal fields when preserving records as units matters.
Python: selection and authenticated SMTP in one script
Python is a good choice when the notifier needs authenticated SMTP, deterministic formatting, retries or a scheduler-independent local failure log. The standard library supplies file handling, email construction and SMTP transport. Its logging package also provides handlers, formatters and an SMTP-capable handler. As the Python Software Foundation’s Logging HOWTO puts it: “An application may want to send all log messages to a log file, all log messages of error or higher to stdout, and all messages of critical to an email address.”
Complete bounded-file example
#!/usr/bin/env python3
import os
import re
import socket
import smtplib
from collections import deque
from datetime import datetime, timezone
from email.message import EmailMessage
from pathlib import Path
LOG_PATH = Path(os.environ.get('LOG_PATH', '/var/log/myapp/app.log'))
RECIPIENT = os.environ['ALERT_TO']
SMTP_HOST = os.environ['SMTP_HOST']
SMTP_PORT = int(os.environ.get('SMTP_PORT', '587'))
SMTP_USER = os.environ.get('SMTP_USER')
SMTP_PASSWORD = os.environ.get('SMTP_PASSWORD')
LINE_COUNT = int(os.environ.get('LINE_COUNT', '50'))
SCAN_COUNT = int(os.environ.get('SCAN_COUNT', '500'))
PATTERN = os.environ.get('ERROR_PATTERN', r'ERROR|CRITICAL')
MAX_CHARS = int(os.environ.get('MAX_CHARS', '200000'))
def newest_lines(path: Path, count: int):
lines = deque(maxlen=count)
with path.open('r', encoding='utf-8', errors='replace') as handle:
for line in handle:
lines.append(line.rstrip('n'))
return list(lines)
def main():
host = socket.getfqdn()
now = datetime.now(timezone.utc).isoformat(timespec='seconds')
scanned = newest_lines(LOG_PATH, SCAN_COUNT)
matches = [line for line in scanned if re.search(PATTERN, line)]
selected = matches[-LINE_COUNT:]
body_lines = [
f'UTC: {now}',
f'Host: {host}',
f'Source: {LOG_PATH}',
f'Rule: newest {SCAN_COUNT} lines, pattern /{PATTERN}/, final {LINE_COUNT} matches',
'',
*(selected or ['No matching lines found.']),
]
body = 'n'.join(body_lines)
truncated = len(body) > MAX_CHARS
if truncated:
body = body[:MAX_CHARS] + 'nn[message truncated by notifier]'
message = EmailMessage()
message['From'] = SMTP_USER or f'log-alert@{host}'
message['To'] = RECIPIENT
message['Subject'] = f'Errors on {host}: {len(selected)} matching lines'
message.set_content(body)
try:
with smtplib.SMTP(SMTP_HOST, SMTP_PORT, timeout=20) as smtp:
smtp.ehlo()
smtp.starttls()
smtp.ehlo()
if SMTP_USER and SMTP_PASSWORD:
smtp.login(SMTP_USER, SMTP_PASSWORD)
smtp.send_message(message)
except Exception:
# Keep notifier failures separate from the application log.
with open('/var/log/myapp-log-notifier.log', 'a', encoding='utf-8') as failure_log:
failure_log.write(f'{now} SMTP delivery failed for {LOG_PATH}n')
raise
if __name__ == '__main__':
main()This example reads only the newest 500 lines, retains the final 50 matches, records UTC time and the rule, and caps the body at 200,000 characters. It uses STARTTLS on the configured port; your relay may require a different TLS mode or a certificate policy. Store SMTP_PASSWORD in the scheduler’s protected environment, a secret manager or an equivalent facility, and restrict the script and failure-log permissions. The account needs read access to the application log and write access to the separate notifier log.
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Use Python’s logging handlers for application-owned records
If the same Python application produces the errors, attach a file handler for normal retention and an SMTP-capable handler at ERROR or CRITICAL. A handler dispatches records to a destination, so severity routing can remain in the application instead of repeatedly scanning a file. Keep a bounded digest job as a separate control when you specifically need the latest N physical lines, because a logging handler normally sends individual records rather than a periodic excerpt.
Reading journald instead of a text file
Bound by unit and time
UNIT='myservice.service'
HOST=$(hostname -s)
body=$(journalctl -u "$UNIT" --since '15 minutes ago' -p err..emerg -n 50 --no-pager)
printf 'UTC: %snHost: %snUnit: %snRule: previous 15 minutes, priority err through emerg, newest 50 entriesnn%sn'
"$(date -u '+%Y-%m-%dT%H:%M:%SZ')" "$HOST" "$UNIT" "${body:-No matching journal entries.}"
| mailx -s "Journal errors on $HOST" [email protected]A time window is usually more meaningful for journald than blindly taking lines, because journal entries can contain multiline fields and unrelated units. Add --grep 'ERROR|CRITICAL' when the message text carries those markers, or use journal priority filtering when the service sets priorities correctly. --no-pager is essential for unattended execution. Test the exact unit name with systemctl list-units and run the command as the same account the scheduler will use.
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Keep source collection separate from delivery: have a small wrapper run journalctl with a unit and time window, write its bounded output to standard input or a temporary file with restrictive permissions, then let the Python sender format and transmit it. This allows one SMTP implementation to serve text files and journals while preserving the source name in the subject. Remove temporary files after a successful or failed attempt; logs can contain passwords, tokens or personal data.
PowerShell and mixed Linux/Windows deployments
On Linux, PowerShell can query journald through the native command:
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$unit = 'myservice.service'
$lines = & journalctl -u $unit --since '15 minutes ago' -n 50 --no-pager
$body = "UTC: $([DateTime]::UtcNow.ToString('o'))`nUnit: $unit`nRule: previous 15 minutes, newest 50 entries`n`n" + ($lines -join "`n")
$body | Send-MailMessage -To '[email protected]' -From '[email protected]' -Subject "Journal errors on $env:COMPUTERNAME" -SmtpServer $env:SMTP_HOST -UseSslSend-MailMessage is present on some Windows PowerShell installations but is obsolete in newer environments; use your organization’s approved SMTP library or relay client when it is unavailable. Never assume that a Linux journal command exists on native Windows. For Windows event logs, query the relevant event log with PowerShell’s event-log cmdlets, select a bounded time or record count, then pass the formatted result to the same delivery layer.
Schedule the notifier without hiding its failures
Cron
Use absolute paths, an explicit environment and a separate stderr destination. For a script at /usr/local/sbin/email-log-errors that runs every 15 minutes:
*/15 * * * * /usr/bin/env -i PATH=/usr/local/bin:/usr/bin:/bin LOG_PATH=/var/log/myapp/app.log LINE_COUNT=50 SCAN_COUNT=500 [email protected] SMTP_HOST=smtp.example.net SMTP_PORT=587 [email protected] SMTP_PASSWORD_FILE=/etc/myapp/smtp-password /usr/local/sbin/email-log-errors >>/var/log/myapp/notifier-cron.log 2>&1Do not put a real password directly in a crontab. The illustrative SMTP_PASSWORD_FILE variable should be read by the script from a root-owned file or replaced with a protected secret-injection mechanism. Ensure only the intended account can read the crontab, script, environment file and local logs. Cron jobs start with a sparse environment and a different working directory, so test using the exact command line and account.
systemd timer
A service unit gives you dependency ordering and journaled execution. The service’s ExecStart should call the notifier with absolute paths; the timer can use OnCalendar=*:0/15 for a fifteen-minute cadence and Persistent=true if missed runs should be triggered after a host outage. Use systemctl status and journalctl -u your-notifier.service to inspect exit status and stderr. Keep the application log and notifier service separate so one failure cannot conceal the other.
Windows Task Scheduler
Create a task that runs the full path to pwsh.exe or powershell.exe, sets the working directory explicitly, and runs under an account with log-read permission. Store SMTP credentials in Windows Credential Manager, a managed secret provider or an encrypted task configuration rather than in the task’s argument string. Configure a nonzero program exit code as a task failure and retain task history; an email that never arrives is not proof that the script never ran.
Security and reliability controls
- Least privilege: grant read access only to the required log and write access only to the notifier’s own failure log.
- Transport security: use authenticated SMTP with TLS when relaying off-host; validate certificates and set a finite connection timeout.
- Secret handling: avoid command-line arguments and world-readable environment files. Rotate credentials and use a dedicated relay identity where possible.
- Data minimization: error logs may include access tokens, email addresses or request bodies. Redact known secrets before mailing and restrict recipients.
- Bounded work: cap scanned lines, selected lines, bytes and execution time. Prevent overlapping runs with a lock if a slow relay could outlast the schedule interval.
- Rotation awareness: handle rename-and-recreate rotation, compressed archives and container-specific paths. A file descriptor held by a long-running process may point to an old inode while the visible path is new.
- Independent observability: write delivery errors to a separate local log or the scheduler’s journal, and consider a second health check for repeated notifier failures.
Diagnose “the script ran but no email arrived”
- Prove selection first. Run the source command as the scheduler account and print the exact bounded body. Confirm the path, unit, time zone, pattern and permissions.
- Prove the scheduler. Check cron output, systemd service status and journal entries, or Task Scheduler history. Capture stderr and the process exit code.
- Test delivery independently. Send a short fixed message through the same
mailx, SMTP host, port, TLS mode and credentials. This separates source problems from relay problems. - Inspect the mail-delivery agent. If using local
mailxorsendmail, inspect the MTA queue and its mail-facility logs. Sendmail records mail activity through syslog with theLOG_MAILfacility, so those records are distinct from the application log you are reporting. - Check recipient-side rejection. Look for authentication, certificate, relay-permission, DNS, rate-limit, spam-policy and message-size errors. A successful SMTP handoff means the relay accepted the message, not that the recipient placed it in the inbox.
- Check time and rotation. UTC timestamps in the body reveal clock skew. Verify that the scheduled process is reading the current file or the intended journal rather than a rotated or empty source.
Shell, Python and PowerShell: which approach fits?
| Decision factor | Shell plus mailx/sendmail | Python SMTP script | PowerShell |
|---|---|---|---|
| Best source fit | Plain files and simple journal output | Plain files, journals collected by a wrapper, or application-owned records | Windows event logs; journald when PowerShell runs on Linux |
| Filtering and windows | Strong with grep, awk, tail and journalctl |
Explicit regex, line, byte and time logic in code | Rich object filtering; native journal access depends on platform |
| SMTP authentication | Implementation-specific; often delegated to a local MTA | Explicit TLS, timeout and login handling in the standard library | Depends on the installed cmdlet or approved mail library |
| Secret management | Use protected MTA configuration or secret injection; never expose passwords in arguments | Environment or a secret store read by the process | Credential Manager, managed secrets or encrypted task settings |
| Scheduler integration | Excellent with cron and systemd timers | Works with cron, systemd timers and Task Scheduler | Native Task Scheduler integration; also usable from cron on Linux |
| Failure visibility | Inspect command stderr, MTA queue and LOG_MAIL records |
Can log structured delivery failures separately from the application | Use task history, exit codes and a separate event or file log |
Production checklist
- Can the scheduled account read the selected source?
- Is the excerpt bounded by both records and bytes?
- Does the message state UTC time, host, source and selection rule?
- Is the empty-match behavior intentional?
- Are credentials absent from command lines, crontabs and world-readable files?
- Is TLS, authentication and a finite SMTP timeout configured for the chosen relay?
- Are notifier failures written somewhere other than the application log?
- Have you tested rotation, permission denial, an unavailable relay, an empty match and a message larger than the cap?
- Is the schedule protected against overlapping executions and alert storms?
The reliable pattern is therefore not merely “pipe tail to mail.” It is a bounded, documented selection step followed by a delivery method appropriate to the host, with credentials protected and failures observable outside the log being reported.
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Should I send the newest lines or use a time window?
Use a time window for journald and high-volume services when you need events since the previous run; use newest lines for a simple text-file snapshot. You can combine both, such as the newest 50 entries from the previous 15 minutes.
Why does a local mailx command work manually but fail from cron?
Cron supplies a minimal environment and may run under a different account. Use absolute command paths, define required variables explicitly, verify file permissions and capture both stdout and stderr in a separate notifier log.
How can I avoid emailing secrets that appear in stack traces?
Redact tokens, cookies, authorization headers and personal fields before constructing the message, cap the body, restrict recipients and retain the unredacted log only under the access controls required for incident response.
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