Re: New Linux User needs some guidance

["Thomas Schmitt" <scdbackup@gmx.net>]

Hi,

after not finding in the man page a description of cp behavior with
existing target file, i looked up POSIX

  https://pubs.opengroup.org/onlinepubs/9699919799/utilities/cp.html
  "3. If source_file is of type regular file, [...]
     a. [...] if dest_file exists, the following steps shall be taken:
      [...]
      ii. A file descriptor for dest_file shall be obtained by performing
          actions equivalent to the open() function defined in the System
          Interfaces volume of POSIX.1-2017 called using dest_file as the
          path argument, and the bitwise-inclusive OR of O_WRONLY and
          O_TRUNC as the oflag argument.
  "

open() is descibed by man 2 open:
  "The  argument  flags  must  include  one of the following access modes:
   O_RDONLY, O_WRONLY, or O_RDWR.  These request opening  the  file  read-
   only, write-only, or read/write, respectively.
   [...]
      O_TRUNC
          If  the file already exists and is a regular file and the access
          mode allows writing (i.e., is O_RDWR or  O_WRONLY)  it  will  be
          truncated to length 0.  If the file is a FIFO or terminal device
          file, the O_TRUNC flag is ignored.   Otherwise,  the  effect  of
          O_TRUNC is unspecified.
   "

So bitrot could indeed happen if open(2) decides to change its unspecified
behavior for block devices.
(I have difficulties to imagine what one must smoke in order to come to
 the idea of unlinking the device file and creating a data file instead.)


Have a nice day :)

Thomas