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  Git for beginners: The definitive practical guide
  
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I don’t get what the Git command when adding files to the stage with the use of a period (or full stop, single dot) means:

$ git add .

What does this do?

git add . adds / stages all of the files in the current directory. This is for convenience, and can still be used if you have certain files you don’t want to add by using a .gitignore

A tutorial for .gitignore is located here.

A deeper look into git add . vs git add -A vs. git add -u is located here and it might answer your question if you wanted more control of how you add all of the files / wanted to know how git add . works.

git,git-add