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 NETBSD

gendiff

GENDIFF(1)						      General Commands Manual							GENDIFF(1)

NAME

       gendiff - utility to aid in error-free diff file generation

SYNOPSIS

       gendiff <directory> <diff-extension>

DESCRIPTION

       gendiff	is  a  rather  simple script which aids in generating a diff file from a single directory.  It takes a directory name and a "diff-
       extension" as its only arguments.  The diff extension should be a unique sequence of characters added to the end of all	original,  unmodi-
       fied files.  The output of the program is a diff file which may be applied with the patch program to recreate the changes.

       The  usual  sequence  of events for creating a diff is to create two identical directories, make changes in one directory, and then use the
       diff utility to create a list of differences between the two.  Using gendiff eliminates the need for the  extra,  original  and	unmodified
       directory copy.	Instead, only the individual files that are modified need to be saved.

       Before editing a file, copy the file, appending the extension you have chosen to the filename.  I.e. if you were going to edit somefile.cpp
       and have chosen the extension "fix", copy it to somefile.cpp.fix before editing it.  Then edit the first copy (somefile.cpp).

       After editing all the files you need to edit in this fashion, enter the directory one level above where your source code resides, and  then
       type

	   $ gendiff somedirectory .fix > mydiff-fix.patch

       You should redirect the output to a file (as illustrated) unless you want to see the results on stdout.

SEE ALSO

       diff(1), patch(1)

AUTHOR

       Marc Ewing <marc@redhat.com>

4th Berkeley Distribution					  Mon Jan 10 2000							GENDIFF(1)