<|im_start|>system
You are a helpful AI assistant.<|im_end|>
<|im_start|>user
What is the best way to run a bash script periodically on ubuntu?<|im_end|>
<|im_start|>assistant
There are several ways to run a bash script periodically on Ubuntu, but one of the most common methods is to use the cron utility. Here are the steps to set up a cron job:

1. Open a terminal window.

2. Enter the command crontab -e to edit your user's crontab file.

3. If prompted, select an editor to use with crontab.

4. In the crontab file, add a new line for the job you want to run. The format of a cron job line is as follows:
``` 
*     *     *   *    *        command to be executed
-     -     -   -    -
|     |     |   |    |
|     |     |   |    +----- day of the week (0 - 6) (Sunday is 0)
|     |     |   +------- month (1 - 12)
|     |     +--------- day of the month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)
``` 
For example, to run a bash script every day at 3 AM, the cron job line would look like this:
``` 
0 3 * * * /path/to/script.sh
``` 
5. Save and exit the crontab file.

Your bash script will now run automatically according to the schedule you specified in the cron job line. You can view your scheduled cron jobs by running the crontab -l command in the terminal.<|im_end|>
<|im_start|>user
How can I execute the script every Thursday on odd days and odd months?<|im_end|>
<|im_start|>assistant
You can use the cron scheduling syntax to execute a script every Thursday on odd days and odd months. The syntax for cron is as follows:
``` 
*     *     *   *    *        command to be executed
-     -     -   -    -
|     |     |   |    |
|     |     |   |    +----- day of the week (0 - 6) (Sunday=0)
|     |     |   +------- month (1 - 12)
|     |     +--------- day of the month (1 - 31)
|     +----------- hour (0 - 23)
+------------- min (0 - 59)
``` 
To execute the script every Thursday on odd days and odd months, you can use the following cron expression:
``` 
0 0 * * 4 [ $(( $(date +%d) % 2)) -ne 0 ] && [ $(( $(date +%m) % 2)) -ne 0 ] && /path/to/your/script.sh
``` 
This expression will execute the script at midnight (0:00) on every Thursday (4), and will check whether the current day of the month ($(date +%d)) and month ($(date +%m)) are odd numbers before running the script ([ $(( $(date +%d) % 2)) -ne 0 ] && [ $(( $(date +%m) % 2)) -ne 0 ]). If either the day or the month is even, the script will not be executed. You will need to replace /path/to/your/script.sh with the actual path to your script.<|im_end|>
<|im_start|>user
Thanks! Is there a way to schedule irregular intervals with crontab? Like running something every three days? Or skipping months? Running once a week one month and then twice a week the next month?

If that can be done with crontab, please write the line or lines that are needed. If that has to be done with a script that gets called by crontab, please write both the crontab line(s) and the script.<|im_end|>
<|im_start|>assistant
Sure! Here's a an example crontab line that runs a command every 3 days at midnight:
```
0 0 */3 * * <command>
```

Here's another example runs at noon on the first day of each month, skipping April through August.
```
# '-' specifies an inlcusive range of times, while ',' specifies joining two ranges or numbers
0 12 0 0-2,9-11 * <command>

# Can also be written as
0 12 0 jan-mar,sep-dec * <command>
```

A simple way to run a command once a week one month and twice a week another month is to add two entries for a command:
```
# Run at noon on Sunday in January...
0 12 * jan sun <command>

# ... and at noon on Sunday and Wednesday in February
0 12 * feb sun,wed <command>
```<|im_end|>