#Finding the executable
*	can be used at any place, at prompt $ * can expand all files out

#Quoting with the Shell
$	dollor sign is a meta-character and tells the shell the next word is a variable
"	Quoted characters do not have a special meaning, double quote expand 
'	single quote keep not expand it

check out the following command
rm -i file1 file2	# one command 3 parameters, shell treate -i file1 file2 EQually
			# however commnad treate -i differently and commnad take - as an option
			# because it start as a hyphen and the other two as filename

space			# Normally, a space delineates arguments. To include a space in a filename, you must quote it.
\			# escape.You quote a string, escape a meta-character. " and esc: all special characters are treated as regular characters.

you want to remove a file with a space in this fileName,
there are different ways to do it

$ touch 'file with a space'
$ rm 'file with a space' # or 
$ rm  file' with a space'

In other words, when reading a shell script, you must remember the current quoting state.
The quote toggles the state.

command substuding
--
`		#old way
$()		#new way

single or double quotation mark
--
The quotation marks in the shell are NOT used to define a string. 
There are used to disable or enable interpretation of meta-characters.

# these 5 are the same
echo abcd
echo 'abcd'
echo ab'c'd
echo a"b"cd
echo 'a'"b"'c'"d"

assign
--
Multiple assignments can be placed on one line:
A=1 B=2 C=3 D=4

a= x #FAIL,since a is assigned as space

a=one; echo $a	#a=one --> a='one', so result is one
a=two echo $a	#expands meta (ie $) before executing the built-in commands, so a=two echo one --> one
a=three echo $a >$a

Using curly braces with variables
# also check https://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18
# for 2.6.2 Parameter Expansion
--
$d=				#init it to empty
$d=$(expr ${d:-0} + 1)		# :- undefined then set to
unset d; d=$(expr ${d:-0} + 1)	#as one line