Shell variables are initialised from environment variables in every shell, you can't get around that.
When the shell starts, for every environment variable it receives that has a valid name as a shell variable, the shell assigns the corresponding shell variable the corresponding value. For instance, if your script is started as:
env VAR_A=xxx your-script
(and has a
#!/bin/bash - she-bang),
env will execute
/bin/bash and pass
VAR_A=xxx to that bash command, and
bash will assign its
$VAR_A variable the value
In the Bourne shell and in the C-shell, if you assign a new value to that shell variable, it doesn't affect the corresponding env variable passed to later commands executed by that shell, you have to use
setenv for that (note however that in the Bourne shell if you
unset a variable, it removes both the shell variable and environment variable).
env VAR=xxx sh -c 'VAR=yyy; other-command'
sh being the Bourne shell, not modern POSIX shells) Or:
env VAR=xxx csh -c 'set VAR = yyy; other-command'
VAR=xxx in its environment, not
VAR=yyy, you'd need to write it:
env VAR=xxx sh -c 'VAR=yyy; export VAR; other-command'
env VAR=yyy csh -c 'setenv VAR yyy; other-command'
other-command to receive
VAR=yyy in its environment.
ksh (and POSIX as a result, and then
bash and all other modern Bourne-like shells as a result) broke that.
Upon start-up those modern shells bind their shell variable to the corresponding environment variable.
What that means is that a script may clobber the environment just by setting one of its variables even if it doesn't export it. Some shells are even known to remove the environment variables it cannot map to shell variables (which is why it's recommended to only use shell-mappable variable names for environment variable names).
That's a main reason why by convention, all uppercase variables should be reserved for environment variables.
To work around that, if you want the commands executed by your script to receive the same environment as the shell interpreting your script received, you'd need to store that environment somehow. You can do it by adding:
at the start of your script, and then run your commands with:
(eval "$my_saved_env"; exec my-other-command and its args)