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 CENTOS

hardlink

hardlink(1)						      General Commands Manual						       hardlink(1)

NAME

       hardlink - Consolidate duplicate files via hardlinks

SYNOPSIS

       hardlink [-c] [-n] [-v] [-vv] [-h] directory1 [ directory2 ... ]

DESCRIPTION

       This manual page documents hardlink, a program which consolidates duplicate files in one or more directories using hardlinks.

       hardlink  traverses  one or more directories searching for duplicate files.  When it finds duplicate files, it uses one of them as the mas-
       ter.  It then removes all other duplicates and places a hardlink for each one pointing to the master file.  This allows for conservation of
       disk space where multiple directories on a single filesystem contain many duplicate files.

       Since hard links can only span a single filesystem, hardlink is only useful when all directories specified are on the same filesystem.

OPTIONS

       -c	 Compare  only	the  contents of the files being considered for consolidation.	Disregards permission, ownership and other differ-
		 ences.

       -f	 Force hardlinking across file systems.

       -n	 Do not perform the consolidation; only print what would be changed.

       -v	 Print summary after hardlinking.

       -vv	 Print every hardlinked file and bytes saved. Also print summary after hardlinking.

       -h	 Show help.

AUTHOR

       hardlink was written by Jakub Jelinek <jakub@redhat.com>.

       Man page written by Brian Long.

       Man page updated by Jindrich Novy <jnovy@redhat.com>

BUGS

       hardlink assumes that its target directory trees do not change from under it.  If a directory tree does change, this may result in hardlink
       accessing  files  and/or directories outside of the intended directory tree.  Thus, you must avoid running hardlink on potentially changing
       directory trees, and especially on directory trees under control of another user.

																       hardlink(1)