add hello world dag

from airflow import DAG

from airflow.operators.bash import BashOperator

from datetime import datetime, timedelta

default_args = {

    'owner': 'airflow',

    'depends_on_past': False,

    'email': ['bozhang@nao.cas.cn'],

    'email_on_failure': False,

    'email_on_retry': False,

    'retries': 1,

    'retry_delay': timedelta(minutes=5),

}

with DAG(

    'dag_hello_world',

    default_args=default_args,

    description='A simple tutorial DAG',

    schedule_interval=timedelta(days=1),

    start_date=datetime(2023, 1, 1),

    catchup=False,

    tags=['example'],

) as dag:

    t1 = BashOperator(

        task_id='print_date',

        bash_command='date',

    )

    t2 = BashOperator(

        task_id='sleep',

        bash_command='sleep 5',

        retries=3,

    )

    t1 >> t2