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[parted-Bugs][313055] fdisk -l is not as read-only as I'd hoped

From: parted-bugs
Subject: [parted-Bugs][313055] fdisk -l is not as read-only as I'd hoped
Date: Mon, 21 Mar 2011 12:46:29 +0000

Bugs item #313055, was opened at 2011-03-21 13:46 by Edward Welbourne
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Status: Open
Priority: 3
Submitted By: Edward Welbourne (eddy-guest)
Assigned to: Nobody (None)
Summary: fdisk -l is not as read-only as I'd hoped 
Category: Unspecified
Group: Version < 1.6.25
Resolution: None

Initial Comment:
I have a partition I made ages ago, that I've not been using - and I've 
forgotten which file-system type I put on it (if any).  I initially supposed 
I'd used ext3 (since that's what I used for all the *other* partitions created 
at the same time) but mounting it as such failed, saying it wasn't an ext3 
So I googled and found http://nst.sourceforge.net/nst/docs/user/ch04s03.html 
which suggested using fdisk -l; the man page said it lists the partition table 
on a device - which sounded promising.
However (once I'd duly logged in as root to have sufficient privilege ...), 
when I ran fdisk -l on it, (after its copyright preamble) it said: <quote>

Device contains neither a valid DOS partition table, nor Sun, SGI or OSF 
Building a new DOS disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.

Disk /dev/dm-1: 740 GB, 740135370240 bytes
255 heads, 63 sectors/track, 89983 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes

    Device Boot      Start         End      Blocks   Id  System
... and "Building a new DOS disklabel." definitely comes under the heading of 
unwanted behaviour vigorously in conflict with the principle of least surprise. 
 I thought I was running a read-only command - I do not have delusions of 
knowing what I'm doing, so I start by gathering information using commands that 
won't mess with things, until I've satisfied myself that I have enough of a 
clue that it's not entirely stupid to try what I think I should do next.  
Having one of my information-gathering commands make changes was a scary shock.

Now, OK, it's only made changes in memory, so it hasn't *really* changed the 
device, but all the same it's not really appropriate for a "list the contents" 
command to *modify* the thing it's meant to be listing.
Being a complete innocent, as concerns disk partitioning, I'm left in the 
unpleasant situation of not knowing what to do about the "in memory" disk label 
that I don't want and probably (but I don't know) need to get rid of if I want 
to find out what's actually there.  So the fact that it's only in memory is 
*not* entirely harmless !  (The message could beneficially be expanded to say 
what I need to do to tell fdisk to undo the in-memory changes, i.e. restore the 
status quo ante, in whatever situations it *is* appropriate for it to take this 
action and produce this message.)

(... and, of course, producing the headers for a table, when there's no data to 
put under them as a table, is a waste of output.  But that really is harmless.)

fdisk -v says: <quote>
GNU Fdisk 1.2.4


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