Python Script:

1. Add some lines into a blank file:　

　 echo "hello world" >> text.txt.

　 Everytime before create a script file. we have some steps:

　　a. Create a .script file by using mkdir text.script

　　b. add some lines into the script to make sure the file is allowed to run under the shell:

　　　　echo ‘if __name__ == "__main__":\n    print("Welcome to a Python script")‘ > script.py # here we are using single bracket on each side of echo word.Pay attention that there are four space before print.

2.  We can use python2 to call the script:

　　python text.script　

　  Or use python3 to call the script:

　　python3 text.script　

 　Here, we can use python3 or python2 to call the script. But sometimes, if several project want to call a same library in different version. 

3.  Why we want to use virtual environment:

　So if we use virtual environment:

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