I have a python program which I need to run at a particular day of a month, so I am using crontab for this task and create a shell script to run this python program.

This is part of my shell script:

```
#!/bin/bash
filepath='file2018'
cd ${filepath}
python3 file.py

```

When I run the crontab which executes the shell script, the log file shows the following error:

```
line 9: python3: command not found 

```

I'm really confused about why this error occurs because I have already install python3 and I can run python3 directly from the command line. 

Besides, if I replace python3 with python, the shell script works! My python version is python2, but I have to use python3 for this program, so I have to use python3 instead of python.

My operating system is Linux CentOS.

Hope someone can give me some tips!