https://www.youtube.com/watch?v=GK8R3SjG3B4&list=PLuHgQVnccGMA8iwZwrGyNXCGy2LAAsTXk&index=21
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Create repository (which has .git) in current directory
git init
vim f1.txt
git add f1.txt
git commit -m "1"
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vim f1.txt
git commit -am "2"
File which is not performed "add" is not performed by automatic "add"
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git log
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git branch
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git branch exp
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git branch
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git checkout exp
git branch
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git log
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Currently, master branch = exp branch
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vim f1.txt
git add f1.txt
git commit m "3"
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git log
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git checkout master
git log
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cat f1.txt
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git checkout exp
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git log
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cat f1.txt