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Re: Command substitution with null bytes generates warning

From: Greg Wooledge
Subject: Re: Command substitution with null bytes generates warning
Date: Tue, 20 Sep 2016 14:22:52 -0400
User-agent: Mutt/

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?

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?

If you are reading a file that uses \0 as a separator between fields,
then how it is *OK* for your script to mash the fields all together
into one giant mess?  Why aren't you reading the fields separately?

Take for example the file /proc/1/cmdline on Debian 8 (with mostly
default settings):

address@hidden:~$ cat -vtue /proc/1/cmdline

I count 4 NUL bytes there, each one presumably terminating a field
of some kind.

If you simply use $(< /proc/1/cmdline) in some naive script running
under bash 2.0 ~ 4.3 then you get something like


Again, how is a script that produces this as a value not simply buggy?

Perhaps what you really wanted was something like:

{ read -r -d '' a; read -r -d '' b; read -r -d '' c; read -r -d '' d; } \
   < /proc/1/cmdline

which gives:

address@hidden:~$ declare -p a b c d
declare -- a="/lib/systemd/systemd"
declare -- b="--system"
declare -- c="--deserialize"
declare -- d="16"

I don't know what these fields *mean*, but that's OK, because I'm not
the one using them in a script.  But this would be a much better way
of getting them, than a blind command substitution.

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