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RE: bash 4.4 null byte warning!!


From: address@hidden
Subject: RE: bash 4.4 null byte warning!!
Date: Mon, 10 Apr 2017 05:08:10 +0000

Hi Greg/Chet Ramey,
 
Thank  you so much for the response! It gave me an idea on what has to be done next.
 
We have been using the same script all these years and had no issues of broken commands when used with the lower versions of bash which is why I have been thinking whether to rewrite the script for the new bash(4.4) or edit the bash source.
 
The reason behind using –print0 is to use the resultant output to find a binary
 
Below are the lines of code followed by the find command:
 
FIND_RPM="$(find $linIdx -type d -name "${2}" -print0 2>/dev/null)"
        [ -n "$FIND_RPM" ] && \
            [ -s "${FIND_RPM}/bin" ] && \
            bdt_msg 4 "[D] Found %s in %s component area." "$2" "$linIdx" && \
               
               
 
>> If you want to suppress the warning, you'll have to edit the source and rebuild.
Do you have a guide on how to do this?
 
Or else I have been planning to add tr to remove the nuls as suggested by Greg.
 
Regards,
Emlyn Jose.
 
---------------------------------------------------------------------------------------------------------
-----Original Message-------------------------------------------------------------------------------
From: Chet Ramey [mailto:address@hidden]
Sent: Saturday, April 8, 2017 7:48 AM
To: Emlyn Jose (GIS); address@hidden
Cc: address@hidden
Subject: Re: bash 4.4 null byte warning!!
 
On 4/6/17 3:47 AM, address@hidden wrote:
> Hi,
>
>
>
> We are trying to use the bash 4.4 downloaded from
> http://ftp.gnu.org/gnu/bash/ on our RHEL 6.2 machine.
>
> But while using a script that has this command :
>
> FIND_RPM=`find /opt/RPM/components -type d -name enum-1.1.6-print0`
>
> It throws a warning as below:
>
> bash:warning:command substitution:ignored null byte ininput
 
Yes.  It drops the null bytes because C strings can't handle them.  I received bug reports asking why bash silently transforms the command substitution output and added the warning while not changing the behavior.
 
If you want to suppress the warning, you'll have to edit the source and rebuild.
 
This does beg the question of why you're using -print0 without something on the receiving end to handle the null bytes.
 
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/
 
 
--------------------------------------------------------------------------------------------------------------------------------
-----Original Message------------------------------------------------------------------------------------------------------
From: Greg Wooledge [mailto:address@hidden]
Sent: Thursday, April 6, 2017 8:15 PM
To: Emlyn Jose (GIS)
Cc: address@hidden
Subject: Re: bash 4.4 null byte warning!!
 
On Thu, Apr 06, 2017 at 07:47:35AM +0000, address@hidden wrote:
> FIND_RPM=`find /opt/RPM/components -type d -name enum-1.1.6 -print0`
> It throws a warning as below:
>
> bash: warning: command substitution: ignored null byte in input
 
Your command is broken, and bash is warning you of this.  Why do you want to silence the warning instead of fixing the command?
 
If your find returns a single result, you get a filename followed by NUL, and the NUL is discarded, leaving you a filename inside the command substitution (and therefore the variable).  You could achieve the EXACT same result by dropping the -print0.  Then, if you get a single result from find, you get the filename followed by newline, and the command substitution discards the trailing newline, leaving you just the filename in the command substitution and therefore the variable.
 
On the other hand, if your find command returns MULTIPLE results, then you have file1\000file2\000 and the command substitution drops the NUL bytes, leaving you with file1file2 in your variable.  At least if you dropped the -print0 you would have file1\nfile2 in your variable, which is still wrong, but not as wrong as file1file2.
 
> Is there any patch available for this or is there any way to make bash silently drop this warning(which has been the behavior on the lower versions of bash)??
 
If you REALLY want to continue to run a broken command, you can explicitly use tr to remove the NULs before the command substitution ever sees them.
 
var=$(find ... -print0 | tr -d '\0')
 
That will prevent the warning and allow you to continue running a broken script without letting your coworkers catch on to the bugs (or whatever your issue with the warning is).
 
If you actually want to FIX the script, the results of find should be read into an array, since there can be more than one.  Every place you use your variable currently, you'd have to rewrite that to handle an array with potentially multiple elements.
 
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