I am following a video tutorial and that guy did this:

$ pip freeze --local > requirement.txt
$ cat requirement.txt

this is to export all these packages with their versions in another project, but how:

• what is pip freeze? and
• what is that requirement.txt? are we supposed to export all those packages in a txt file, how can .txt file do that does it grab their name?

• what is the word cat in second line? my machine can't understand either of those, but in his computer they were working, my machine says:

  

  As you can see cat is not recognized.

Then I look in virtualenv directory to search for requirement.txt and I find this

Yes, requirement.txt is zero bytes, nothing in it. What is the problem?

Next I googled for what is pip freeze and what is cat? I couldn't find a simple definition for cat. but here is pip freeze

Usage : pip freeze [options] Description Output installed packages inrequirements format.

packages are listed in a case-insensitive sorted order.

What is "requirements format"? Is that a text file?

Then I came to this question in Stack Overflow: How to freeze packages installed only in the virtual environment?

Does he means how do I preserve packages with a layer of ice in my virtual environment?

Pip is a package manger for Python modules. The command pip freeze outputs all installed modules (including version numbers). The --local flag prevents Pip from printing globally installed packages in a virtual environment.

Usually, a Python program depends on other modules. You can put those required modules in a text file (requirements.txt by convention) so that other people can install those dependencies by running pip install -r requirements.txt. You can conveniently create such a file using pip freeze.

On a Linux machine, cat is used to output the contents of a file. You can use type on Windows.

The requirements format looks like this:

docutils==0.11
Jinja2==2.7.2
MarkupSafe==0.19
Pygments==1.6
Sphinx==1.2.2

Each lines consist of a python module name and a corresponding version.

https://pip.pypa.io/en/stable/reference/pip_freeze/
https://pip.readthedocs.io/en/1.1/requirements.html