Make a backup file

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>

Java

This example is untested. Please check that it's correct, debug it as necessary, and remove this message.

Works with: Java version 7+

<lang java5>import java.io.PrintWriter; import java.io.FileWriter; import java.nio.file.*;

public class Backup { public static void saveWithBackup(String filename, String... data){ //toRealPath() follows symlinks to their ends Path file = Paths.get(filename).toRealPath(); Path back = Paths.get(filename + ".backup").toRealPath(); Files.move(file, back, StandardCopyOption.REPLACE_EXISTING); try(PrintWriter out = new PrintWriter(new FileWriter(file.toFile()))){ for(String datum:data){ out.println(datum); } } } }</lang>

Tcl

<lang tcl>package require Tcl 8.5

proc backupopen {filename mode} {

   set filename [file normalize $filename]
   if {[file exists $filename]} {

set backups [glob -nocomplain -path $filename ,*] set max [lindex [lsort -dictionary \ [lsearch -all -inline -regexp $backups {,\d+$}]] end] if {$max eq ""} { set n 0 } else { set n [regexp -inline {\d+$} $max] } while 1 { set backup $filename,[incr n] if {![catch {file copy $filename $backup}]} { break } }

   }
   return [open $filename $mode]

}</lang>