Info Catalog elisp: Making Backups elisp: Backup Files elisp: Numbered Backups

elisp: Rename or Copy

 
 25.1.2 Backup by Renaming or by Copying?
 ----------------------------------------
 
 There are two ways that Emacs can make a backup file:
 
    • Emacs can rename the original file so that it becomes a backup
      file, and then write the buffer being saved into a new file.  After
      this procedure, any other names (i.e., hard links) of the original
      file now refer to the backup file.  The new file is owned by the
      user doing the editing, and its group is the default for new files
      written by the user in that directory.
 
    • Emacs can copy the original file into a backup file, and then
      overwrite the original file with new contents.  After this
      procedure, any other names (i.e., hard links) of the original file
      continue to refer to the current (updated) version of the file.
      The file’s owner and group will be unchanged.
 
    The first method, renaming, is the default.
 
    The variable ‘backup-by-copying’, if non-‘nil’, says to use the
 second method, which is to copy the original file and overwrite it with
 the new buffer contents.  The variable ‘file-precious-flag’, if
 non-‘nil’, also has this effect (as a sideline of its main
 significance).  Saving Buffers.
 
  -- User Option: backup-by-copying
      If this variable is non-‘nil’, Emacs always makes backup files by
      copying.  The default is ‘nil’.
 
    The following three variables, when non-‘nil’, cause the second
 method to be used in certain special cases.  They have no effect on the
 treatment of files that don’t fall into the special cases.
 
  -- User Option: backup-by-copying-when-linked
      If this variable is non-‘nil’, Emacs makes backups by copying for
      files with multiple names (hard links).  The default is ‘nil’.
 
      This variable is significant only if ‘backup-by-copying’ is ‘nil’,
      since copying is always used when that variable is non-‘nil’.
 
  -- User Option: backup-by-copying-when-mismatch
      If this variable is non-‘nil’ (the default), Emacs makes backups by
      copying in cases where renaming would change either the owner or
      the group of the file.
 
      The value has no effect when renaming would not alter the owner or
      group of the file; that is, for files which are owned by the user
      and whose group matches the default for a new file created there by
      the user.
 
      This variable is significant only if ‘backup-by-copying’ is ‘nil’,
      since copying is always used when that variable is non-‘nil’.
 
  -- User Option: backup-by-copying-when-privileged-mismatch
      This variable, if non-‘nil’, specifies the same behavior as
      ‘backup-by-copying-when-mismatch’, but only for certain user-id
      values: namely, those less than or equal to a certain number.  You
      set this variable to that number.
 
      Thus, if you set ‘backup-by-copying-when-privileged-mismatch’ to 0,
      backup by copying is done for the superuser only, when necessary to
      prevent a change in the owner of the file.
 
      The default is 200.
 

Info Catalog elisp: Making Backups elisp: Backup Files elisp: Numbered Backups