Make a backup file

From Rosetta Code
Revision as of 16:19, 11 November 2011 by rosettacode>Glennj (add Ruby)
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>

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>

Ruby

This version does not overwrite the backup file if it exists. <lang ruby>def backup_and_open(filename)

 filename = File.readlink(filename) if File.symlink?(filename)
 bkup = filename + ".backup"
 Dir.glob(bkup + "*").reverse.each do |fname|
   if m = fname.match(/\.backup\.(\d+)$/)
     File.rename(fname, "%s.%d" % [bkup, m[1].to_i + 1])
   elsif fname == bkup
     File.rename(bkup, bkup + ".1")
   end
 end
 File.rename(filename, bkup)
 File.open(filename, "w") {|handle| yield handle}

end

backup_and_open(ARGV[0]) {|fh| fh.puts "new text"} backup_and_open(ARGV[0]) {|fh| fh.puts "some more new text"}</lang>

Example:

$ echo "original" > test.file
$ ruby backup.rb test.file
$ ls -l test.file*
-rw-rw-rw-+ 1 glennj mkgroup-l-d 19 Nov 11 11:19 test.file
-rw-rw-rw-+ 1 glennj mkgroup-l-d  9 Nov 11 11:19 test.file.backup
-rw-rw-rw-+ 1 glennj mkgroup-l-d  9 Nov 11 11:18 test.file.backup.1
$ cat test.file
some more new text
$ cat test.file.backup
new text
$ cat test.file.backup.1
original
$ touch original
$ ln -s original linkfile
$ ruby backup.rb linkfile
$ ls -l linkfile* original*
lrwxrwxrwx  1 glennj mkgroup-l-d   8 Nov 11 11:22 linkfile -> original
-rw-rw-rw-+ 1 glennj mkgroup-l-d  19 Nov 11 11:22 original
-rw-rw-rw-+ 1 glennj mkgroup-l-d   9 Nov 11 11:22 original.backup
-rw-rw-rw-+ 1 glennj mkgroup-l-d   0 Nov 11 11:22 original.backup.1

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

   }
   return [open $filename $mode]

}</lang>