Bash is astonishingly fussy about spacing and punctuation.
Nothing astonishing about it, really, if you ponder for a few moments. Every computer language is precise about its use of punctuation and spacing. Computers are infamously literal devices.
The UNIX command "man bash" gives detailed explanations of the backquote character ` (on U.S. keyboards, on the same key as the tilde or squiggle ~ , usually at the far left of the top key row) and the straight quote character ' (next to the ENTER key on U.S. keyboards).
Do a search (triggered by the slash or virgule character / followed by the search term then ENTER) for "command substitution". Keep searching through the man page on each term (by hitting / without a new search term then ENTER) until you see what you want. Use "g" to go to the top of the man page for a new search. Do another for "single quote".
You might want to spend some time really reading through this man page. It could help you make yourself a bash guru.
There's a good book from O'Reilly, _Learning the bash Shell, 2nd Edition_, <http://www.oreilly.com/catalog/bash2/index.html>.
More generally, there's _Linux in a Nutshell_ from O'Reilly <http://www.oreilly.com/catalog/linuxnut4/>. It has a chapter on bash.
There are many other books about UNIX, Linux, shell programming and bash in particular.
