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It's a standard part of shell syntax that you can use `~' at the beginning of a file name to stand for your own home directory. You can use `~user' to stand for user's home directory.
Tilde expansion is the process of converting these abbreviations to the directory names that they stand for.
Tilde expansion applies to the `~' plus all following characters up to whitespace or a slash. It takes place only at the beginning of a word, and only if none of the characters to be transformed is quoted in any way.
Plain `~' uses the value of the environment variable HOME
as the proper home directory name. `~' followed by a user name
uses getpwname to look up that user in the user database, and
uses whatever directory is recorded there. Thus, `~' followed
by your own name can give different results from plain `~', if
the value of HOME is not really your home directory.