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Redirecting Output and Logs

Automation without logs is guessing. Cron jobs often fail silently unless you redirect output. This page shows the essential operators and safe logging patterns for production scripts.

Quick Summary
  • > overwrites; >> appends.
  • 2> captures stderr; 2>&1 merges stderr into stdout.
  • Always log to a directory with correct permissions.
  • Avoid logging secrets.

stdout vs stderr​

  • stdout: normal output
  • stderr: errors and diagnostics

Cron only emails output if a mail system is configured. If you want reliable logs, redirect to files.

Core operators​

OperatorMeaning
>write stdout, overwrite file
>>write stdout, append to file
2>write stderr, overwrite file
2>>write stderr, append to file
2>&1merge stderr into stdout

Examples​

Redirect stdout (overwrite)​

redirect-stdout-overwrite.txt
* * * * * /path/script.sh > /var/log/script.log

Redirect stdout (append)​

redirect-stdout-append.txt
* * * * * /path/script.sh >> /var/log/script.log

Redirect stderr only​

redirect-stderr-only.txt
* * * * * /path/script.sh 2>> /var/log/script.err

Redirect both stdout and stderr to one file​

redirect-both-to-one-file.txt
* * * * * /path/script.sh >> /var/log/script.log 2>&1

Separate stdout and stderr​

separate-stdout-and-stderr.txt
* * * * * /path/script.sh >>/var/log/script.out 2>>/var/log/script.err

Create a log directory safely​

create-log-directory.sh
sudo mkdir -p /var/log/automation
sudo chown root:root /var/log/automation
sudo chmod 750 /var/log/automation

Logging from scripts​

If you control the script, you can redirect inside the script.

script-level-logging.sh
#!/usr/bin/env bash
set -euo pipefail

LOG="/var/log/automation/job.log"
exec >>"$LOG" 2>&1

echo "[$(date -Is)] start"
echo "doing work"
echo "[$(date -Is)] done"
warning

Do not write secrets to logs. If your script prints environment variables or command lines containing passwords, fix the script before enabling logging.

systemd timers and journald​

systemd services capture stdout/stderr into journald by default. This is one reason timers are easier to debug.

view-systemd-service-logs.sh
journalctl -u wp-backup.service --since '24 hours ago' --no-pager

Send output to syslog/journald with logger​

If you prefer central logs, you can send messages via logger.

pipe-output-to-logger.sh
/usr/local/bin/job 2>&1 | logger -t myjob

Cron entry example:

cron-pipe-to-logger.txt
*/10 * * * * /usr/local/bin/job 2>&1 | /usr/bin/logger -t myjob

This avoids log files growing unbounded on disk (but you still need journald retention configured).

Use tee when you want both console and file​

tee writes to a file and stdout. It is useful when you run a command interactively and want to capture output.

use-tee-to-capture-output.sh
/usr/local/bin/job 2>&1 | tee -a /var/log/automation/job.log

Debug mode (temporarily)​

If you need to see each command as it executes:

bash-debug-mode-example.sh
set -x
command_one
command_two
set +x
warning

Do not leave set -x enabled in scripts that handle secrets. It can print sensitive values into logs.

Log rotation​

If you log to files under /var/log, use logrotate.

logrotate-example.txt
/var/log/automation/*.log {
  daily
  rotate 14
  compress
  missingok
  notifempty
  create 0640 root adm
}

Permissions for log files​

Logs should not be world-readable if they may contain sensitive paths or identifiers.

fix-log-permissions.sh
sudo chown root:adm /var/log/automation/job.log || true
sudo chmod 640 /var/log/automation/job.log || true

Next steps​

  • Cron basics: opt/docker-data/apps/docusaurus/site/docs/server/linux-server/11-automation-task-execution/cron.mdx
  • Cron vs systemd: opt/docker-data/apps/docusaurus/site/docs/server/linux-server/11-automation-task-execution/cron-vs-systemd.mdx