44r0n7
0265afa054
Import the runnable game code, content, docs, scripts, and repo guidance while leaving local agent state, dependency installs, build output, and backup copies out of the published tree.
41 lines
2.9 KiB
JSON
41 lines
2.9 KiB
JSON
{
|
||
"id": "cron-jobs",
|
||
"title": "Cron Jobs & Scheduled Tasks",
|
||
"category": "sysadmin",
|
||
"tags": ["cron", "crontab", "schedule", "backup", "automation"],
|
||
"updated": "2025-12-01",
|
||
"summary": "Cron syntax, user vs system crons, and common failure modes.",
|
||
"sections": [
|
||
{
|
||
"heading": "Cron Syntax",
|
||
"body": "<p>A crontab entry has five time fields followed by the command:</p>",
|
||
"code": "# ┌─── minute (0–59)\n# │ ┌─── hour (0–23)\n# │ │ ┌─── day of month (1–31)\n# │ │ │ ┌─── month (1–12)\n# │ │ │ │ ┌─── day of week (0–7, 0 and 7 are Sunday)\n# │ │ │ │ │\n * * * * * /path/to/command\n\n# Examples:\n0 2 * * * /usr/local/bin/backup.sh # 2am every day\n*/15 * * * * /usr/local/bin/check.sh # every 15 minutes\n0 0 1 * * /usr/local/bin/monthly.sh # midnight on the 1st"
|
||
},
|
||
{
|
||
"heading": "User Crontabs",
|
||
"body": "<p>Each user can have their own crontab. Commands run as that user.</p>",
|
||
"code": "crontab -e # edit your crontab\ncrontab -l # list your crontab\ncrontab -l -u alice # list alice's crontab (root only)\ncrontab -r # delete your crontab (dangerous—no confirmation)"
|
||
},
|
||
{
|
||
"heading": "System Cron Directories",
|
||
"body": "<p>Scripts dropped into these directories run at the corresponding interval without needing a crontab entry:</p>",
|
||
"code": "/etc/cron.daily/\n/etc/cron.weekly/\n/etc/cron.monthly/\n/etc/cron.hourly/\n\n# Scripts here must be executable and owned by root.\n# They must NOT have a file extension—run-parts ignores files with dots in the name."
|
||
},
|
||
{
|
||
"heading": "Ownership and the PATH Problem",
|
||
"body": "<p>Two common failure modes:</p><p><strong>Wrong owner:</strong> A cron script in <code>/etc/cron.daily/</code> must be owned by root. If it is owned by another user, run-parts may skip it.</p><p><strong>Missing PATH:</strong> Cron does not source <code>.bashrc</code> or <code>.profile</code>. Commands that work interactively may fail in cron because the PATH only contains <code>/usr/bin:/bin</code>. Always use full paths in cron scripts, or set PATH explicitly at the top of the script.</p>",
|
||
"code": "#!/bin/bash\nPATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin\n..."
|
||
},
|
||
{
|
||
"heading": "Checking If a Cron Ran",
|
||
"body": "",
|
||
"code": "# Check syslog or the cron-specific log\ngrep CRON /var/log/syslog | tail -20\ncat /var/log/cron.log # if separate cron log is configured\n\n# Check journald\njournalctl -u cron --since \"1 hour ago\""
|
||
},
|
||
{
|
||
"heading": "Capturing Cron Output",
|
||
"body": "<p>By default, cron mails output to the user. On servers with no mail configured, errors disappear silently. Redirect to a log file instead:</p>",
|
||
"code": "0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1"
|
||
}
|
||
]
|
||
}
|