I tried to add a ruler through View>Ruler but it adds to the current file only. How to make a specific ruler with the size of 79 characters in sublime and make it appear in every file?
If you want to make a ruler appear in every file, then go to Preferences: Settings from the command palette and add the setting "rulers": [79] to your user preferences. This will make the ruler appear at column 79 for every file.
The reason why View -> Ruler -> <Some value> applies only for the current file is because, behind the scenes, it uses a command called set_setting, which sets the given setting (here it's rulers) and it's value only for the current view (Just try to apply a ruler value to a file using View -> Ruler, close the file, open it again and see that it doesn't persist. It's just temporary). Hence, the rulers value is not applied to any other file