Re: [Bug 485507] RFE: add ext4 to "stat -f" output

[Eric Sandeen]

Jim Meyering wrote:
> In http://bugzilla.redhat.com/485507
> 
> Eric Sandeen proposed to add ext4 to the list of names
> currently reported for that type of file system ("ext2/ext3").
> 
> I'm reluctant to change that string, because doing so might
> well break scripts that rely on them:
> 
>     case $(stat -f --format %T .) in
>       ext2/ext3) ;;
>       *) destroy_partition ;;
>     esac
> 
> Sure, script writers might well anticipate this and
> prefer to use the %t directive to get the type in hexadecimal,
> but that's far less readable.
> 
> How much code (and how ugly) would be required to distinguish ext4
> from ext[23]?

it'd need to be able to do some parsing of the ext2/3/4 feature flags,
so it'd need to know some ext2/3/4 details.

lib/blkid/probe.c in e2fsprogs is an example, see probe_ext2,
probe_ext3, etc.  It reads in the superblock and then checks some of the
feature fields, for example:

        /* Ext4 has at least one feature which ext3 doesn't understand */
        if (!(blkid_le32(es->s_feature_ro_compat) &
              EXT3_FEATURE_RO_COMPAT_UNSUPPORTED) &&
            !(blkid_le32(es->s_feature_incompat) &
              EXT3_FEATURE_INCOMPAT_UNSUPPORTED))
                return -BLKID_ERR_PARAM;

so it'd not be rocket science but it might be a lot more fs details than
you'd really want in coreutils, I'm not sure...

-Eric