how to write python in linux terminal?

Answer 1

$ sudo apt-get install python3
... (Downloading)

$ python3
python 3.7.1
>>>print("Hello world!")
Hello world!

Answer 2

# Install
# Gentoo
$ emerge -av dev-lang/python
# Debian
$ sudo apt install python3

# run python
$ python3

Answer 3

If you have python script you can run it like
$r>python3 test.py
where test.py is name of file
also if you have python2.7 scripts you can simply do
$r>python test.py
as python2.7 comes installed default also u can't remove python2.7 your latest python version will run parallel to 2.7