Editing Dd (Unix) (section)

=== Data recovery ===
[[Data recovery]] involves reading from a drive with some parts potentially inaccessible. The command is a good fit with this job with its flexible skipping ({{code|seek}}) and other low-level settings. The vanilla {{code|dd}}, however, is clumsy to use as the user has to read the error messages and manually calculate the regions that can be read. The single block size also limits the granularity of the recovery, as a trade-off has to be made: either use a small one for more data recovered or use a large one for speed.

A C program called ''dd_rescue''<ref>{{cite web|url=http://www.garloff.de/kurt/linux/ddrescue/|title=dd_rescue|work=garloff.de|access-date=2006-11-10|archive-date=2001-05-16|archive-url=https://web.archive.org/web/20010516091650/http://www.garloff.de/kurt/linux/ddrescue/|url-status=live}}</ref>  was written in October 1999. It did away with the conversion functionality of {{code|dd}}, and supports two block sizes to deal with the dilemma. If a read using a large size fails, it falls back to the smaller size to gather as much as data possible. It can also run backwards. In 2003, a ''dd_rhelp'' script was written to automate the process of using ''dd_rescue'', keeping track of what areas have been read on its own.<ref>{{cite web|url=http://www.kalysto.org/utilities/dd_rhelp/index.en.html|title=dd_rhelp author's repository|author=LAB Valentin|date=19 September 2011|quote=Important note : For some times, dd_rhelp was the only tool (AFAIK) that did this type of job, but since a few years, it is not true anymore: Antonio Diaz did write a ideal replacement for my tool: GNU 'ddrescue'.|access-date=13 May 2008|archive-date=16 May 2008|archive-url=https://web.archive.org/web/20080516231947/http://www.kalysto.org/utilities/dd_rhelp/index.en.html|url-status=live}}</ref>

In 2004, GNU wrote a separate utility, unrelated to {{code|dd}}, called [[ddrescue]]. It has a more sophisticated dynamic block-size algorithm and keeps track of what has been read internally. The authors of both ''dd_rescue'' and ''dd_rhelp'' consider it superior to their implementation.<ref>{{cite web|url=https://www.gnu.org/software/ddrescue/ddrescue.html|title=Ddrescue - GNU Project - Free Software Foundation (FSF)|work=gnu.org|access-date=2016-07-22|archive-date=2021-07-02|archive-url=https://web.archive.org/web/20210702144117/https://www.gnu.org/software/ddrescue/ddrescue.html|url-status=live}}</ref> To help distinguish the newer GNU program from the older script, alternate names are sometimes used for GNU's ''ddrescue'', including ''addrescue'' (the name on freecode.com and freshmeat.net), ''gddrescue'' ([[Debian]] package name), and ''gnu_ddrescue'' ([[openSUSE]] package name).

Another open-source program called ''savehd7'' uses a sophisticated algorithm, but it also requires the installation of [[Seed7|its own programming-language interpreter]].