Re: Command substitution with null bytes generates warning

[L. A. Walsh]

Greg Wooledge wrote:
> I probably shouldn't even bother at this point, but morbid curiosity
> compels me to foolish ends.
>
> What are you DOING with these files that contain NUL bytes that makes
> it permissible to simply drop the NUL bytes on the floor, with no
> explicit step like tr -d \\0 to remove them?
>   
Usually to write a quick & dirty 1-off script to read something
and give me some info.

If I want to improve upon it or have it handle the null data better,
I'd probably use perl.  But as an example see attached.  I've had a few
versions of this script (this one is dated 2011).
If you run it in a dir where there is binary
data, you are almost certain to see have some nulls in the data.

I used something this script to look at values in /proc or /sys.
Run as a normal user -- I get many unreadables, but enough interesting
things to help me understand what to do and how to use some of the values:

/sys/class/net/br0> /tmp/showvals
addr_assign_type:              1
addr_len:                      6
address:                       00:15:17:bf:be:b2
/tmp/showvals: line 63: printf: `invalid format character
brforward:                    '`

f
#ʧ 

7ridge/ageing_time:            30000
bridge/bridge_id:              8000.001517bfbeb2
bridge/default_pvid:           1
bridge/flush:                  <Unreadable>
bridge/forward_delay:          1500
bridge/gc_timer:               23319
bridge/group_addr:             1:80:c2:0:0:0
bridge/group_fwd_mask:         0x0
bridge/hash_elasticity:        4
bridge/hash_max:               512
bridge/hello_time:             200
bridge/hello_timer:            0
bridge/max_age:                2000
----
etc... brforward sure looks like binary, for example.

One can do the same on windows in /proc/registry (in cygwin) --
lots of binary values  that display just fine, in spite of dropping
nulls.
This is just 1 example...But since these are knockoff scripts, I often
don't remember how/where to find them ... this one was by luck, last
modified ~ 5 years ago.

> How is your script anything but buggy, if it is relying on a program
> (shell) to drop certain input bytes, when there is no documentation
> stating that it does this?
>   
How is it buggy?  ... works fine for the listed requirements! ;-)


Remember What I said about you being "too advanced" to relate to
some of these things: case in point.  ;-)


#!/bin/bash  

# designed for displaying config vals stored in files;
# recursively display values (or 1st line) of files in cur dir
# or given on cmd line;  
# dirs given or below targets will be expanded;
#
# (c) 2011 L. Walsh (lawalsh@tlinx.org); 
# License: copy and distribute freely, but for this source:
#   * leave attribution on unmodified or slightly modified copies. (OR)
#               * leave 'honorable mention' if this is > 30% of any final work 
(OR)
#   * Else, Do as thow wilt... and enjoy!
#       (Hopefully, that's not too onerous a license...)


# this is writtin 'split', so if one wants auto-priv-raising,
# it can be done once and not on every recursive call

# If you want to force root to be used when running this
# instead of raising privs as needed, written / split this file can be 
'sourced' and the function
# will define to mem and so if 'sudo' is needed

USE_SUDO=1
# to prevent automatically trying sudo, uncomment next line
#USE_SUDO=''

sudop=$([[ -n $USE_SUDO && ! -n $_SUDO ]] && echo '/usr/bin/sudo')
shopt -s expand_aliases extglob
getfacl=$(type -P getfacl)
if [[ -n $getfacl ]]; then
        function _showvals_tread {
                local fn=$1
                local -a acc
                readarray acc < <($getfacl  -c $fn)
                ans="$(echo ${acc[*]##*:*([^r])})"
                if [[ -n $ans ]]; then
                        return 0 
                else    
                        return 1
                fi      
        }
else
        function _showvals_tread {      #use broken test...
                test -r $1
        }
fi
export -f _showvals_tread
#type _showvals_tread

#export sudoed_showvals='
prefx=${prefx:-""}
function showvals {
        local -i width=30
        for n in "${@}" ; do
                [[ -z $n || $n == \* ]] && return 0
                if [[ -d $n && ! -L $n ]] ; then
                        [[ $n == .  ]]  && return 0
                        (cd "$n"; shift; 
                                export prefx="${prefx:-""}${prefx:+/}$n"; 
shift; showvals * )
                elif [[ -f $n ]] ; then
                        if _showvals_tread "$n" ; then  
                                read -t .1 -r val <"$n"
                                printf -v  val -- "$val"
                        else
                                val='<Unreadable>'
                        fi
                        key="${prefx:+$prefx/}$n:";keyl="${#key}"
                        [[ -n "key" ]] && keylen=${#key}
                        width=$((width<keylen&&keylen<50 ? keylen : width))
                        printf  "%-*s %-.40s\n" "$width" "$key" "$val"
                fi
        done
        return 0
};
export -f showvals
showvals  #'


function showvials {
        if [[ $EUID!=0  && -n "$_SUDO" ]]; then 
                (local s=$_SUDO; export -u _SUDO;  
                $s /bin/bash --norc -c "$sudoed_showvals \"$@\"" )
        else
                eval "$sudoed_showvals $@"
        fi
}

export -f showvals

if [[ $EUID != 0 && -x /usr/bin/sudo ]]; then 
        if sudo grep "$(id -un)" /etc/sudoers &> /dev/null; then
                export _SUDO="$sudop"
        fi
fi

hash -r &>/dev/null

if [[ $0 =~ showvals ]] ;then
        showvals ${@:-*}
else 
        echo "(showvals defined)"
fi