NotebookSave

NotebookSave[notebook]
saves the current version of the specified notebook.

NotebookSave[notebook, "file"]
saves the notebook in the specified file.

NotebookSave[]
saves the current version of the current evaluation notebook in a file.

NotebookSave[notebook] saves the notebook in a file whose name is given by the notebook object notebook.

NotebookSave writes out the Mathematica expression corresponding to the notebook, together with Mathematica comments which make it easier for the front end to read the notebook in again.

Save a new document to a specified file:

Modify a previously saved document and save it to the same place:

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