CentOS - Mar 2022 - Need fstab-decode for CentOS 8

On 2/28/22 8:46 AM, Robert Nichols wrote:
> On 2/28/22 1:22 AM, centos at niob.at wrote:
>> Am 28.02.22 um 05:45 schrieb Robert Nichols:
>>> On 2/27/22 12:26 PM, centos at niob.at wrote:
>>>> Am 27.02.22 um 04:33 schrieb Robert Nichols:
>>>>> Does anything for CentOS 8 provide the function of the
fstab-decode utility?
>>>>> Entries in /proc/mounts and /etc/fstab can have escape
sequences for certain special characters, and I need to decode that.
>>>>
>>>> Preface: Never heard of fstab-decode before. Researching the
command made me really wonder why it was invented. Especially since I have never
seen an /etc/fstab with "escape sequences" or "special
characters" since at least 1990 (If I am wrong: Please show me such a fstab
file).
>>>>
>>>> So why not just use:
>>>>
>>>> ?????umount $(awk '$3 == "vfat" {print $2}'
/etc/fstab)
>>>>
>>>> instead of the seemingly canonical use of fstab-decode
>>>>
>>>> ?????fstab-decode umount $(awk '$3 == "vfat" {
print $2 }' /etc/fstab)
>>>
>>> Those samples break if the mount point directory name contains
spaces, tabs, or whatever other characters I don't know about that also get
represented by escape sequences. I'm not actually using it with /etc/fstab,
but with /proc/mounts which uses the same convention. I can control /etc/fstab
and avoid the problem, but I cannot control how some auto-mounted foreign
filesystem might be named. I have a script that needs to be robust in the face
of such names.
>>>
>> Get creative! Unix administration is a creative job. Having said this:
>>
>> Using white space within mount points is asking for trouble anyway. If
you really want this in the most generic way, then do the unquoting with
something like this:
>>
>> ???? awk '$3 == "vfat" {print $2}' /etc/fstab | perl
-pl000 -e 's/\\([0-7]{3})/chr(oct($1))/eg' | xargs -0 -n 1 -r umount
>>
>> This seems to be the unixy way to do this.
> 
> Yes, white space in mount points is asking for trouble, but if someone
automounts a USB flash drive filesystem which has a label that includes white
space (e.g.: "USB DISK", like the VFAT preformat on some that I have
bought) or other "funny" characters, that label gets used as the mount
point directory.
> 
> Indeed, I can re-invent the wheel if that wheel is lost in the sands of
time.
It turns out that particular wheel is best resurrected from the fstab-decode.c
file in an old initscripts source package. The encoding is nonstandard, and the
above perl code would not handle it correctly. Handling the unlikely oddball
case seems like severe paranoia, but this could turn out to be, "That line
of code you thought would never be executed just might save the day that one
time when it _does_ get executed."

-- 
Bob Nichols     "NOSPAM" is really part of my email address.
                 Do NOT delete it.

On 3/1/22 05:53, Robert Nichols wrote:
> It turns out that particular wheel is best resurrected from the 
> fstab-decode.c file in an old initscripts source package. The encoding 
> is nonstandard, and the above perl code would not handle it correctly.

It's pretty close.? It won't handle double backslash, and its use of 
xargs is incorrect.

If you prefer a version that you don't need a C compiler to use, here's 
a pure bash implementation:

#!/bin/sh

declare -a cmdline
eol=$'\n'

for arg in "$@"
do
 ? arg="${arg//\011/??? }"
 ? arg="${arg//\012/$eol}"
 ? arg="${arg//\040/ }"
 ? arg="${arg//\134/\\}"
 ? arg="${arg//\\/\\}"
 ? cmdline+=("$arg")
done

"${cmdline[@]}"