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, thenbar/bazshould be renamed tobar/baz.backupand then the new text should be written tobar/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.
Some examples on this page assume that the original file already exists. They might fail if some user is trying to create a new file.
Common Lisp
Appends a version number and increments it on each backup <lang lisp>(defun parse-integer-quietly (&rest args)
(ignore-errors (apply #'parse-integer args)))
(defun get-next-version (basename)
(flet ((parse-version (pathname)
(or (parse-integer-quietly
(string-left-trim (file-namestring basename)
(file-namestring pathname))
:start 1) 0)))
(let* ((files (directory (format nil "~A,*" (namestring basename))))
(max (reduce #'max files :key #'parse-version)))
(merge-pathnames (format nil "~a,~d" (file-namestring basename) (1+ max))
basename))))
(defun save-with-backup (filename data)
(let ((file (probe-file filename)))
(rename-file file (get-next-version file))
(with-open-file (out file :direction :output)
(print data out))))</lang>
Java
<lang java5>import java.io.PrintWriter; 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(file.toFile())){ for(String datum : data){ out.println(datum); } } } }</lang>
<lang java5>import java.io.File; import java.io.IOException; import java.io.PrintWriter;
public class Backup{ public static void saveWithBackup(String filename, String... data) throws IOException{ File orig = new File(filename); //getCanonicalPath() follows symlinks to their ends File backup = new File(orig.getCanonicalPath() + ".backup");
orig.renameTo(backup); PrintWriter output = new PrintWriter(orig); for(String datum : data){ output.println(datum); } output.close(); } }</lang>
Locomotive Basic
AMSDOS has automatic one-level backups which also work from Locomotive BASIC: If e.g. the file test.bas is saved, the data gets written to test.$$$. When the file is closed a preexisting test.bas gets renamed to test.bak and finally test.$$$ is renamed to test.bas. (These backups affect all file types, not just BASIC source code.)
Pike
<lang Pike>string targetfile = "pycon-china"; targetfile = System.resolvepath(targetfile); mv(targetfile, targetfile+"~"); Stdio.write_file(targetfile, "this task was solved at the pycon china 2011");</lang>
Python
<lang Python> import os targetfile = "pycon-china" os.rename(os.path.realpath(targetfile), os.path.realpath(targetfile)+".bak") f = open(os.path.realpath(targetfile), "w") f.write("this task was solved during a talk about rosettacode at the PyCon China in 2011") f.close() </lang>
Ruby
This version does not overwrite the backup file if it exists. <lang ruby>def backup_and_open(filename)
filename = File.realpath(filename)
bkup = filename + ".backup"
backup_files = Dir.glob(bkup + "*").sort_by do |f|
f.match(/\d+$/)
$&.nil? ? 0 : $&.to_i
end
backup_files.reverse.each do |fname|
if m = fname.match(/\.backup\.(\d+)$/)
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
1.upto(12) {|i| backup_and_open(ARGV[0]) {|fh| fh.puts "backup #{i}"}}</lang>
Example:
$ echo "original" > 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 10 Nov 11 11:41 original -rw-rw-rw-+ 1 glennj mkgroup-l-d 10 Nov 11 11:41 original.backup -rw-rw-rw-+ 1 glennj mkgroup-l-d 10 Nov 11 11:41 original.backup.1 -rw-rw-rw-+ 1 glennj mkgroup-l-d 9 Nov 11 11:41 original.backup.10 -rw-rw-rw-+ 1 glennj mkgroup-l-d 9 Nov 11 11:37 original.backup.11 -rw-rw-rw-+ 1 glennj mkgroup-l-d 9 Nov 11 11:41 original.backup.2 -rw-rw-rw-+ 1 glennj mkgroup-l-d 9 Nov 11 11:41 original.backup.3 -rw-rw-rw-+ 1 glennj mkgroup-l-d 9 Nov 11 11:41 original.backup.4 -rw-rw-rw-+ 1 glennj mkgroup-l-d 9 Nov 11 11:41 original.backup.5 -rw-rw-rw-+ 1 glennj mkgroup-l-d 9 Nov 11 11:41 original.backup.6 -rw-rw-rw-+ 1 glennj mkgroup-l-d 9 Nov 11 11:41 original.backup.7 -rw-rw-rw-+ 1 glennj mkgroup-l-d 9 Nov 11 11:41 original.backup.8 -rw-rw-rw-+ 1 glennj mkgroup-l-d 9 Nov 11 11:41 original.backup.9 $ cat original original.backup original.backup.1 original.backup.2 original.backup.3 original.backup.4 original.backup.5 original.backup.6 original.backup.7 original.backup.8 original.backup.9 original.backup.10 original.backup.11 backup 12 backup 11 backup 10 backup 9 backup 8 backup 7 backup 6 backup 5 backup 4 backup 3 backup 2 backup 1 original
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>