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bash: Tilde Expansion

 
 3.5.2 Tilde Expansion
 ---------------------
 
 If a word begins with an unquoted tilde character ('~'), all of the
 characters up to the first unquoted slash (or all characters, if there
 is no unquoted slash) are considered a TILDE-PREFIX.  If none of the
 characters in the tilde-prefix are quoted, the characters in the
 tilde-prefix following the tilde are treated as a possible LOGIN NAME.
 If this login name is the null string, the tilde is replaced with the
 value of the 'HOME' shell variable.  If 'HOME' is unset, the home
 directory of the user executing the shell is substituted instead.
 Otherwise, the tilde-prefix is replaced with the home directory
 associated with the specified login name.
 
    If the tilde-prefix is '~+', the value of the shell variable 'PWD'
 replaces the tilde-prefix.  If the tilde-prefix is '~-', the value of
 the shell variable 'OLDPWD', if it is set, is substituted.
 
    If the characters following the tilde in the tilde-prefix consist of
 a number N, optionally prefixed by a '+' or a '-', the tilde-prefix is
 replaced with the corresponding element from the directory stack, as it
 would be displayed by the 'dirs' builtin invoked with the characters
 following tilde in the tilde-prefix as an argument (The Directory
 Stack).  If the tilde-prefix, sans the tilde, consists of a number
 without a leading '+' or '-', '+' is assumed.
 
    If the login name is invalid, or the tilde expansion fails, the word
 is left unchanged.
 
    Each variable assignment is checked for unquoted tilde-prefixes
 immediately following a ':' or the first '='.  In these cases, tilde
 expansion is also performed.  Consequently, one may use filenames with
 tildes in assignments to 'PATH', 'MAILPATH', and 'CDPATH', and the shell
 assigns the expanded value.
 
    The following table shows how Bash treats unquoted tilde-prefixes:
 
 '~'
      The value of '$HOME'
 '~/foo'
      '$HOME/foo'
 
 '~fred/foo'
      The subdirectory 'foo' of the home directory of the user 'fred'
 
 '~+/foo'
      '$PWD/foo'
 
 '~-/foo'
      '${OLDPWD-'~-'}/foo'
 
 '~N'
      The string that would be displayed by 'dirs +N'
 
 '~+N'
      The string that would be displayed by 'dirs +N'
 
 '~-N'
      The string that would be displayed by 'dirs -N'
 

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