# crontab

### **Structure of a** `crontab` Entry

Each entry in a `crontab` file is made up of two main parts:

1. **Schedule specification**: Defines when the job should run.
    
2. **Command to execute**: Specifies the script, command, or executable to run.
    

#### **Basic Syntax**

```bash
bashCopy code* * * * * command_to_execute
```

| **Field** | **Value Range** | **Description** |
| --- | --- | --- |
| `Minute` | `0-59` | The minute when the task runs. |
| `Hour` | `0-23` | The hour of the day (24-hour format). |
| `Day of Month` | `1-31` | The day of the month. |
| `Month` | `1-12` | The month of the year. |
| `Day of Week` | `0-6` (`0` = Sunday) | The day of the week. |

---

### **Special Characters in** `crontab` Fields

| **Character** | **Usage** |
| --- | --- |
| `*` | Matches every value in the field. |
| `,` | Separates multiple values. |
| `-` | Specifies a range of values. |
| `/` | Specifies step values (increments). |

---

### **Examples with** `*` and Other Characters

1. **Run every minute**
    
    ```bash
    bashCopy code* * * * * /path/to/script.sh
    ```
    
    * Runs the command every minute of every hour, every day.
        
2. **Run at 5 AM daily**
    
    ```bash
    bashCopy code0 5 * * * /path/to/script.sh
    ```
    
    * Runs the command at exactly 5:00 AM.
        
3. **Run on the 1st of every month at midnight**
    
    ```bash
    bashCopy code0 0 1 * * /path/to/script.sh
    ```
    
    * Executes at midnight on the first day of each month.
        
4. **Run every Sunday at 2 PM**
    
    ```bash
    bashCopy code0 14 * * 0 /path/to/script.sh
    ```
    
    * Executes at 2:00 PM every Sunday.
        
5. **Run every 5 minutes**
    
    ```bash
    bashCopy code*/5 * * * * /path/to/script.sh
    ```
    
    * Executes every 5 minutes.
        
6. **Run every hour between 9 AM and 5 PM**
    
    ```bash
    bashCopy code0 9-17 * * * /path/to/script.sh
    ```
    
    * Executes at the start of every hour from 9 AM to 5 PM.
        
7. **Run on weekdays (Monday to Friday) at 10:30 PM**
    
    ```bash
    bashCopy code30 22 * * 1-5 /path/to/script.sh
    ```
    
    * Executes at 10:30 PM Monday through Friday.
        
8. **Run on the 15th and 30th of every month at noon**
    
    ```bash
    bashCopy code0 12 15,30 * * /path/to/script.sh
    ```
    
    * Executes at 12:00 PM on the 15th and 30th of each month.
        

---

### **Special Macros in** `crontab`

| **Macro** | **Equivalent Schedule** | **Description** |
| --- | --- | --- |
| `@reboot` | Runs once at system startup. | Example: Start a service on reboot. |
| `@yearly` | `0 0 1 1 *` | Runs once a year (midnight, Jan 1). |
| `@monthly` | `0 0 1 * *` | Runs once a month (midnight, 1st day). |
| `@weekly` | `0 0 * * 0` | Runs once a week (midnight, Sunday). |
| `@daily` | `0 0 * * *` | Runs once a day (midnight). |
| `@hourly` | `0 * * * *` | Runs every hour. |

---

### **Common Issues and Troubleshooting**

1. `cron` Job Not Running
    
    * Ensure the script has execution permissions:
        
        ```bash
        bashCopy codechmod +x /path/to/script.sh
        ```
        
    * Check the cron logs:
        
        ```bash
        bashCopy codegrep CRON /var/log/syslog
        ```
        
2. **Environment Variables**
    
    * `cron` may not have access to your user's environment variables. Explicitly set paths or source environment files in your script.
        
3. **Output Redirection**
    
    * Redirect output to capture logs or silence errors:
        
        ```bash
        bashCopy code* * * * * /path/to/script.sh >> /path/to/logfile.log 2>&1
        ```