Make a backup file

Make a backup file is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

Before writing to a file it is often advisable to make a backup of the original. Creating such a backup file is however also not without pitfalls.

In this task you should create a backup file from an existing file and then write new text to the old file. The following issues should be handled:

• avoid making a copy of the file but instead rename the original and then write a new file with the original filename
• if a copy needs to be made, please explain why rename is not possible.
• keep in mind symlinks, and do not rename or copy the link but the target.

(if there is a link foo -> bar/baz, then bar/baz should be renamed to bar/baz.backup and then the new text should be written to bar/baz)

• it is assumed that you have permission to write in the target location, thus permission errors need not be handled.
• you may choose the backup filename per preference or given limitations.(it should somehow include the original filename however)
• please try to avoid executing external commands, and especially avoid calling a shell script.

Common Lisp

<lang lisp>(defun save-with-backup (filename data)

 (let ((file (probe-file filename)))
   (rename-file file (concatenate 'string (file-namestring file) ",1"))
   (with-open-file (out file
                        :direction :output
                        :if-exists :supersede)
     (with-standard-io-syntax
       (print data out)))))</lang>