Why is editing a file by renaming the new one safer? Post: 303042321

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Top Forums UNIX for Advanced & Expert Users Why is editing a file by renaming the new one safer? Post 303042321 by Cacializ on Saturday 21st of December 2019 10:55:59 AM

12-21-2019

Registered User

Why is editing a file by renaming the new one safer?

Hello:
I've been reading about ways to edit files from the command line, and I've found two websites which state that the following is the safest way to edit a file:

Code:

command original > new
mv new original

That is, renaming the newer file to the previous one. This is what the websites I mentioned state:

Quote:

Before you start, be warned that editing files is a really bad idea. The preferred way to modify a file is to create a new file within the same file system, write the modified content into it, and then mv it to the original name. This is the only way to prevent data loss in the event of a crash while writing. However, using a temp file and mv means that you break hardlinks to the file (unavoidably), that you would convert a symlink to hard file, and that you may need to take extra steps to transfer the ownership and permissions (and possible other metadata) of the original file to the new file. Some people prefer to roll the dice and accept the tiny possibility of data loss versus the greater possibility of hardlink loss and the inconvenience of chown/chmod (and potentially setfattr, setfacl, chattr...).

(Source: BashFAQ/021 - Greg's Wiki)

Quote:

So, generally speaking, to accomplish almost any editing task on a file, temporary files should be used. Sure, if the file is big, creating a temporary file becomes more and more inefficient, and requires that an amount of available free space roughly the same size of the original file is available. Nonetheless, it's by far the only right and sane way to do the job. Modern machines should have no disk space problems.

[...]

In any case, for our purposes the important thing to remember of these methods is that the old file stays around (whether under its original name or a different one) until the new one has been completely written, so errors can be detected and the old file rolled back. This makes them the preferred method for changing a file safely.

(Source: �In-place�� editing of files << \1)

What I don't understand is: what makes it safer? Why is it safer in the case the machine stops unexpectedly once the file was renamed? What's the logic behind this?

Thanks in advance.

Cacializ

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LEARN ABOUT DEBIAN

potooledit

POEDIT(1)						      General Commands Manual							 POEDIT(1)

NAME

       poedit - program to aid editing gettext po files

SYNOPSIS

       poedit [-a] [-n] <file.po>

       potooledit [-a] [-n] <file.po>

DESCRIPTION

       poedit is a shell script, which uses potool(1), iconv(1) and your favourite editor to help you edit gettext po files.

       It  first  retrieves  only the untranslated entries from the file (or, if -a is specified, reorders the entries so that the translated ones
       are grouped separately from the untranslated ones) and puts them into a temporary file.	Then recodes the file to the charmap of your  cur-
       rent  locale, and runs your favourite editor on the temporary file so you can add new translations. After you have finished, it recodes the
       file back to the original charset and merges the new translations back into the original file.

       poedit is also known as potooledit on Debian systems, to avoid a name clash with an unrelated program of the same name.

OPTIONS

       -a     causes poedit to include already translated entries as well in the temporary file for editing (they are grouped separately, though).
	      Without this option, only the untranslated entries (and the file header) are presented for editing.

       -n     causes poedit not to do any charset recoding before nor after editing the file.
	      You should only use this option if you know what you are doing. Be particularly careful when using this option (without -a) on files
	      that are in an encoding incompatible with your locale's charmap. In that case, the intermediate file to be edited will probably only
	      contain ASCII characters, which means your editor will be free to interpret this as your default locale encoding. This in turn, will
	      make the file invalid when it is merged together with the already translated messages. See Debian bug #297074 for an example of this
	      happening.

SEE ALSO

       potool(1), postats(1), locale charmap(1), msgmerge(1), msgfmt(1).

AUTHOR

       Poedit was written by Zbigniew Chyla and is now being maintained by Marcin Owsiany <porridge@debian.org>.

								September 21, 2007							 POEDIT(1)