Re: [Help-bash] eval backgrounds with & in variable string

[Eduardo A . Bustamante López]

On Fri, Mar 14, 2014 at 12:29:28PM -0700, Jesse Molina wrote:
> 
> Behold my glory:
> 
> [~/work/eval_testing]
> address@hidden>eval echo 'one&uno&ichi'
> [3] 14201
> [4] 14202
> one
> -bash: uno: command not found
> -bash: ichi: command not found
> [3]   Done                    echo one
> [4]   Exit 127                uno
> 
> 
> 
> 
> Okay, that's definitely not what I wanted.  Eval seems to have
> evaluated my "&" characters into being a split and then tries to
> background those commands.
Yes, as expected.

> 
> Can I "set" something to make this work, or am I going to have to
> escape these on input?
Yes: echo 'one&uno&ichi' # this works

if you want to use variables:
string='one&uno&ichi'; echo "$string"

if you want to use user input:
IFS= read -r -p 'Provide string> ' string; echo "$string"


I cannot think of a reason why you would want to use eval there.
Consider that eval takes a string and evaluates it as code. So, if
you have:

eval echo 'one&uno&ichi'

by shell parsing rules, that line will become the following words:

['eval', 'echo', 'one&uno&ichi'] # note that the quotes aren't really
                                   there, it's just to denote the
                                   start and end of the word in my
                                   representation of the vector of
                                   words.

So bash takes the first word as a command, and since it's the built
in 'eval', it'll *again* interpret the rest of the arguments,
apparently concatenated to a single string, so it becomes:

background(['echo', 'one']); background(['uno']); ['ichi'];

by shell parsing rules. If you want to avoid this, then you'll have
to put a whole new layer of quotes, so that the second evaluations
takes the inner quotes, which are not used by the first evaluation,
like this:

eval echo '"one&uno&ichi"'

which becomes:

['eval', 'echo', '"one&uno&ichi"']

and then ['echo', 'one&uno&ichi']

But, again, why are you using eval here?
> 
> 
> 
> While on the subject of eval, does anyone else have fun examples,
> known pitfalls, and other interesting things?
Well, there's nothing fun in using eval. It's just a way of using
data as code, which is a recipe for disaster (improper escaping will
lead to code injection, the nested quotes are hard to get, you're
introducing unnecessary overhead, and so on).

-- 
Eduardo Alan Bustamante López