cron · diff

v1.0 to v1.0

1 added, 1 removed. Audit A to A.

---

name: cron

- description: >-

+ description: >

Schedule recurring and one-time tasks using cron, crontab, at, and launchd.

Use when the user asks to set up scheduled jobs, configure crontab entries,

write cron expressions, schedule one-time tasks with at, create launchd

plist files on macOS, debug why a cron job is not running, or manage

periodic automated tasks on Unix/Linux/macOS systems.

license: MIT

compatibility: Requires cron/crontab (Linux/macOS) or launchd (macOS)

metadata:

author: zeph

version: "1.0"

---

# Task Scheduling with Cron, At, and Launchd

## Cron Expression Format

```

┌─────────── minute (0-59)

│ ┌───────── hour (0-23)

│ │ ┌─────── day of month (1-31)

│ │ │ ┌───── month (1-12 or JAN-DEC)

│ │ │ │ ┌─── day of week (0-6, SUN-SAT; 7 = SUN on some systems)

│ │ │ │ │

* * * * * command

```

### Special Characters

| Character | Meaning | Example |

|-----------|---------|---------|

| `*` | Any value | `* * * * *` (every minute) |

| `,` | List | `1,15,30` (at 1, 15, 30) |

| `-` | Range | `1-5` (1 through 5) |

| `/` | Step | `*/5` (every 5 units) |

### Common Expressions

| Expression | Schedule |

|------------|----------|

| `* * * * *` | Every minute |

| `*/5 * * * *` | Every 5 minutes |

| `*/15 * * * *` | Every 15 minutes |

| `0 * * * *` | Every hour (at :00) |

| `0 */2 * * *` | Every 2 hours |

| `30 * * * *` | Every hour at :30 |

| `0 0 * * *` | Daily at midnight |

| `0 6 * * *` | Daily at 6:00 AM |

| `0 8,12,18 * * *` | At 8 AM, noon, 6 PM |

| `0 9-17 * * *` | Every hour from 9 AM to 5 PM |

| `0 0 * * 0` | Weekly on Sunday at midnight |

| `0 0 * * 1-5` | Weekdays at midnight |

| `0 0 * * 6,0` | Weekends at midnight |

| `0 0 1 * *` | First day of every month |

| `0 0 1 1 *` | Yearly on January 1 |

| `0 0 1 */3 *` | Every 3 months (quarterly) |

| `0 0 15 * *` | 15th of every month |

| `5 4 * * 0` | Sunday at 4:05 AM |

| `0 22 * * 1-5` | Weekdays at 10 PM |

### Predefined Shortcuts

| Shortcut | Equivalent | Description |

|----------|-----------|-------------|

| `@reboot` | — | Run once at startup |

| `@yearly` / `@annually` | `0 0 1 1 *` | Once a year (Jan 1) |

| `@monthly` | `0 0 1 * *` | Once a month (1st) |

| `@weekly` | `0 0 * * 0` | Once a week (Sunday) |

| `@daily` / `@midnight` | `0 0 * * *` | Once a day |

| `@hourly` | `0 * * * *` | Once an hour |

## Crontab Management

```bash

# Edit current user's crontab

crontab -e

# List current user's crontab

crontab -l

# Remove current user's crontab (careful!)

crontab -r

# Edit crontab for another user (requires root)

sudo crontab -u username -e

# List another user's crontab

sudo crontab -u username -l

# Create crontab from file

crontab mycrontab.txt

# Backup crontab

crontab -l > crontab-backup.txt

# Restore crontab from backup

crontab crontab-backup.txt

```

## Crontab File Format

```bash

# Environment variables (set before jobs)

SHELL=/bin/bash

PATH=/usr/local/bin:/usr/bin:/bin

MAILTO=admin@example.com

HOME=/home/user

# Jobs

# min hour dom month dow command

# Daily backup at 2:30 AM

30 2 * * * /usr/local/bin/backup.sh

# Every 15 minutes: check service health

*/15 * * * * /usr/local/bin/healthcheck.sh

# Weekdays at 9 AM: send report

0 9 * * 1-5 /usr/local/bin/send-report.sh

# Monthly cleanup on the 1st at 3 AM

0 3 1 * * /usr/local/bin/cleanup.sh

# At reboot: start services

@reboot /usr/local/bin/start-services.sh

```

## Output Handling

```bash

# Redirect stdout to file

* * * * * /path/to/script.sh > /var/log/cron-output.log 2>&1

# Append to file

* * * * * /path/to/script.sh >> /var/log/cron-output.log 2>&1

# Discard all output (silent)

* * * * * /path/to/script.sh > /dev/null 2>&1

# Redirect stdout and stderr separately

* * * * * /path/to/script.sh > /var/log/cron.log 2> /var/log/cron-error.log

# Email output (if MAILTO is set)

MAILTO=user@example.com

0 * * * * /path/to/script.sh

# Disable email

MAILTO=""

0 * * * * /path/to/script.sh

# Email only on error

0 * * * * /path/to/script.sh > /dev/null

```

## Environment in Cron

Cron jobs run with a minimal environment. The PATH is usually just `/usr/bin:/bin`.

```bash

# Set PATH in crontab

PATH=/usr/local/bin:/usr/bin:/bin:/home/user/.local/bin

# Or use full paths in commands

0 * * * * /usr/local/bin/python3 /home/user/scripts/task.py

# Or source profile in command

0 * * * * . /home/user/.profile && /home/user/scripts/task.sh

# Or wrap in bash with login shell

0 * * * * /bin/bash -lc '/home/user/scripts/task.sh'

# Set environment variables inline

0 * * * * LANG=en_US.UTF-8 /home/user/scripts/task.sh

# Set in crontab file

LANG=en_US.UTF-8

NODE_ENV=production

```

## Locking (Preventing Overlap)

```bash

# Using flock (recommended)

*/5 * * * * flock -n /tmp/myjob.lock /path/to/script.sh

# flock with timeout (wait up to 60 seconds for lock)

*/5 * * * * flock -w 60 /tmp/myjob.lock /path/to/script.sh

# Using PID file (manual)

*/5 * * * * /path/to/script-with-pidcheck.sh

# In script:

# PIDFILE=/tmp/myjob.pid

# if [ -f "$PIDFILE" ] && kill -0 $(cat "$PIDFILE") 2>/dev/null; then exit 0; fi

# echo $$ > "$PIDFILE"

# trap "rm -f $PIDFILE" EXIT

```

## System Cron Directories

```bash

# System-wide crontab

/etc/crontab

# Drop-in directories (scripts placed here run automatically)

/etc/cron.d/ # custom cron files (crontab format with user field)

/etc/cron.hourly/ # scripts run hourly

/etc/cron.daily/ # scripts run daily

/etc/cron.weekly/ # scripts run weekly

/etc/cron.monthly/ # scripts run monthly

# Format in /etc/crontab and /etc/cron.d/* includes a user field:

# min hour dom month dow user command

*/5 * * * * root /usr/local/bin/check.sh

```

## at — One-Time Scheduled Tasks

```bash

# Schedule a command for a specific time

echo "/path/to/script.sh" | at 14:30

# Schedule for a specific date and time

echo "/path/to/script.sh" | at 14:30 2024-12-25

# Schedule relative to now

echo "/path/to/script.sh" | at now + 5 minutes

echo "/path/to/script.sh" | at now + 2 hours

echo "/path/to/script.sh" | at now + 1 day

echo "/path/to/script.sh" | at now + 1 week

# Interactive mode (type commands, press Ctrl+D to finish)

at 15:00

# > /path/to/script.sh

# > echo "Done" | mail user@example.com

# > <Ctrl+D>

# Other time expressions

echo "cmd" | at midnight

echo "cmd" | at noon

echo "cmd" | at teatime # 4 PM

echo "cmd" | at tomorrow

echo "cmd" | at "10:00 AM Jul 4"

# List pending jobs

atq

at -l

# View job contents

at -c JOB_NUMBER

# Remove a job

atrm JOB_NUMBER

at -d JOB_NUMBER

# batch — run when system load is low

echo "/path/to/heavy-task.sh" | batch

```

## Launchd (macOS)

Launchd is the macOS replacement for cron. Jobs are configured via XML property list (plist) files.

### Plist File Template

```xml

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN"

"http://www.apple.com/DTDs/PropertyList-1.0.dtd">

<dict>

<key>Label</key>

<string>com.example.myjob</string>

<key>ProgramArguments</key>

<array>

<string>/usr/local/bin/script.sh</string>

</array>

<key>StartInterval</key>

<!--

<key>StartCalendarInterval</key>

<dict>

<key>Minute</key>

</dict>

-->

<key>StandardOutPath</key>

<string>/tmp/myjob.stdout.log</string>

<key>StandardErrorPath</key>

<string>/tmp/myjob.stderr.log</string>

<key>RunAtLoad</key>

<true/>

<key>EnvironmentVariables</key>

<dict>

<string>/usr/local/bin:/usr/bin:/bin</string>

</dict>

</plist>

```

### Calendar Schedule Keys

```xml

<key>StartCalendarInterval</key>

<dict>

<key>Minute</key>

</dict>

<key>StartCalendarInterval</key>

<array>

<dict>

<key>Weekday</key><integer>1</integer>