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Re: why is eval needed?
On 2022-11-20 07:08, Stephane Chazelas wrote:
zsh doesn't impose anything. "-L 2" is a string made of 4
characters in any shell or programming language. There's no
programming language where "-L 2" means 2 strings -L and 2.
But that's the thing, I'd naively thought the variable could be inserted
in the command string and be 'just' a string of characters, but zsh
imposes that 'tree' must see '-L 2' as a single entity, yes? As shown,
we need word splitting to solve the problem and show tree what it wants
to see. Or, as I was speculating, some way of just flattening the string
back to nothing more than a string of characters -- but then again,
probably tree wouldn't like that either, perhaps the string is always
'packaged' into words? If so, then there's no avoiding that we must
word-split '-L 2' into two words and there's nothing to wish for.
It's what happens when functionality expands by accretion -- simple
structures no longer suffice but they are tweaked rather than
rethought. It's like the difference between the layout of Paris vs.
That is a very weird feature from a language design point of
view. Anybody who doesn't know what the shells looked like
before the Bourne shell would think Stephen Bourne was out of
The dilemma of compatibility! No easy answers. I myself tend towards a
'garage to the dump' mentality but it's not that simple.
My understanding is that the Bourne shell's bizarre IFS handling
and the fact that globbing was performed upon parameter
expansion was an attempt to keep some level of backward
compatibility with that Thompson shell. You see similar things
hapenning in csh from the same era.
The Korn shell (from the early 80s) kept most of that with the
exception that it only did IFS-splitting upon expansions, not on
Holy cow! IFS splitting in text??
Ah! So my little issue is a zsh exclusive? Not complaining tho, I
don't like zsh doing things I didn't ask it to do so if in this case I
need to request a word split that's just fine. After all it's tree
that's being difficult so the problem is rare and easily solved.
The fact that in sh/ksh/bash:
Calls tree with "-L" and "1" as arguments is not that bash
doesn't do some sort of magic "grouping" that zsh would be
doing, but that ksh/bash contrary to zsh does that extra layer
of $IFS-splitting from the Bourne shell on top of the syntax
parsing as the default value of $IFS happens to contain the
That's why in sh/bash/ksh you almost always need to quote
parameter expansions if you intend to pass the contents of a
variable as an argument to a command.
for the kind of thing that can happen if you forget.
Heavy. That's the closest I've come to understanding those vulnerabilities.
If you want to assemble a string, and that string to be
evaluated as shell code, that's what eval is for. eval evaluate
code written in the shell language. That's a way to dynamically
invoke the language interpreter.
Ok, so I understood that correctly. At least partially.
What are you doing there? I have no 'nm' command here. No such command in
nm is a standard development command (though not -D which AFAIK
is a GNU extension). Part of GNU binutils on GNU systems.
Ah, that's why I can't find it. It's hard to find commands so packaged.
Yeah, tides of organization wash here and there. No overall commander
tho, so a bit of chaos is unavoidable.
Note that tree is not part of the GNU project, it's just a
utility written by some guy and shared to the world. There is
*some* level of consistency among utilities in the GNU
toolchest. There is even such a thing as published GNU coding standards.
See for instance
You know, half of it is just cultural adaptation. When I first started
with Linux the first thing I did was decide between zsh and bash and I'm
basically assuming that Linux will be like DOS but slightly more
powerful and zsh will be like C and that all Linux OS apps will be as
flawless as WordPerfect 5.1. CRASH! Now that I'm used to being in a
bazaar not a monastery it's easier to get around. One does get mud on
I'll agree with you that the lack of consistency in API between the
different tools that are available out there can be annoying
(nothing to do with GNU or Linux)
eval evaluates shell code. I struggle to understand what you
don't understand. Maybe you're thinking too much or too little
of what a "command line string" is. A "command line string"
I think at this level of my education it's enough to say that eval looks
at it's arguments as if they were typed on the command line. I haven't
really 'understood' anything, I use eval ad hoc when it seems to solve
problems. Using it with real understanding is a future goal.
For that string to be passed as code to the shell language
interpreter for it to interpret.
Right, I get that:
2 /aWorking/Zsh/Source/Wk 0 $ var="echo howdy"
2 /aWorking/Zsh/Source/Wk 0 $ eval $var
Again, tree has nothing to do with the GNU project.
I dunno, I get it all from Debian and Debian is a GNU/Linux OS so I
don't know more than that.
Thanks for a most educational post! I love these history lessons, they
inform my entire outlook. You can't understand where you are unless you
understand where you've been.
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