[Help-bash] Unexpected behaviour in job control inside subshell environment

[Diego Augusto Molina]

Hi, I was looking for a way to test if the "sleep" command could be 
coerced in some way to tell if the given arguments would be ok. The 
thing is I want to test if an argument received by a function is of a 
suitable format for that command. Different "sleep" implementations 
accept different forms of arguments so I wanted to make sleep command 
tell me if the argument is ok. So I came up with the following idea:

SLEEP_ARG=wrong-value;
(
  sleep "10${SLEEP_ARG}" &
  sleep 0.1; # Optional but recommended
  jobs -r 1;
  if [ $? -eq 0 ]; then
    kill -9 $!;
    exit 0;
  else
    exit 1;
  fi;
) &> /dev/null;

The hypothesis is that any acceptable argument for "sleep" that 
represents the time to sleep could have prepended any other number to 
make sure it's a big enough time to test if it's running in background 
or if it has exited immediately.
So, if the argument is right then it will sleep in background and "jobs 
-r 1" will exit successfully, which would cause the first branch of the 
"if" statement to kill the background process and exit the subshell 
successfully. If the argument is wrong then "sleep" will exit 
immediately and "jobs -r 1" will exit unsuccessfully, causing the second 
branch of the "if" statement to take control and exit the subshell 
unsuccessfully. The optional but recommended "sleep 0.1" ensures the 
first "sleep" command has the time to exit unsuccessfully before bash 
delivers control to the "jobs -r 1" statement after forking the first 
"sleep" to background. The downside is that I'm already assuming that 
"sleep" can receive decimal values.
Note that all stdout and stderr of the subshell will be lost completely 
since all I need is to know if the argument is correct or not.

But the effect is odd: the subshell always returns successfully and I 
can verify that "jobs -r 1" detects the first "sleep" as running even 
when it isn't. Proof of concept:

SLEEP_ARG=wrong-value;
(
  sleep "10${SLEEP_ARG}";
  sleep 3; # Obscenely high
  jobs -r 1;
  echo "Exit status: $?";
);

Whether or not all this is of any use (or just overthinking utterly 
simple things) is not as important as the fact that (I guess) it should 
work. I'm probably settling for somthing simpler like assuming that the 
argument should conform to some regexp.

Cheers.