Error while calling a .sql script from shell script

You're mixing up named and positional substitution variables, and possibly confusing them with shell variables a bit too. You can refer to the argument passed on the SQL*Plus command line positionally:

select file_name,bytes/1024/1024,maxbytes/1024/1024
  from dba_data_files
 where tablespace_name=upper('&1');
/

If you wanted to you could define a named substitution variable; possibly overkill here but can be useful for clarity in bigger scripts:

define TAB_NAME=&1

select file_name,bytes/1024/1024,maxbytes/1024/1024
  from dba_data_files
 where tablespace_name=upper('&TAB_NAME');
/

or to simplify later references a bit:

define TAB_NAME=upper('&1')

select file_name,bytes/1024/1024,maxbytes/1024/1024
  from dba_data_files
 where tablespace_name=&TAB_NAME;
/

Note that in this version the reference to &TAB_NAME is not enclosed in single quotes, as they are applied during the substitution.

I can be helpful to set verify on while testing/debugging, then set verify off for real use to hide the noise. You might also find it useful to include the -s flag in the sqlplus call to hide the banner text. You don't even need a heredoc in this case, you can run the script directly from the command line too:

sqlplus -s "/ as sysdba" @$LOC/tab.sql $TAB_NAME

(Connecting as sys for routine queries isn't ideal; it would be better to have a normal user with privileges to see the data dictionary tables you need to query, but that's a different topic...)