bash exercise

3.5.2 Tilde Expansion

echo "~"       # only begin with an unquoted character is considered a tilde-prefix
echo ~
touch t.txt
ls ~+/t.txt      # use the shell variable PWD replaces the tilde-prefix
mkdir td
cd td
ls ~-/t.txt      # use the OLDPWD variable replaces the tilde-prefix

3.5.3 Shell Parameter Expansion

3.7 Arrays

$ declare -a array               		# declare a indexed array
declare -a arr[2]                       # 2 is ignored
$ echo ${array[@]} 						# all members of the array name
array=([0]=2 [1]=3)  				    # the form [subscript]=string .
$ echo ${array[*]}                      # 
array=(2 3 4 5 6)					    # index array do not require anything but string
IFS=","                                 # set IFS
echo "${array[*]}"   					# expands to a single word with the value of each array member separated by the first character of the IFS varaible
echo "${array[@]}"                      # expands each element of name to a separated word.
echo ${#{array[@]}                      #expands to the length of ${name[subscript]}
echo ${array[-2]}                       # intepreted ass relative to one greater than the maximum index of name 
echo ${array}  							# Referencing an array varaible without a subscript is equivalent with a subscript of 0 
echo ${noexist[2]} 						#  Any reference to a variable using a valid subscript is legal, and bash will create an array if necessary.
echo ${!array[@]}  						# obtain the keys of an array 
echo ${!array[*]}  						# the same to @
unset array[0]   						# destroy the array element  
unset array     						# destroy arrays



$ declare -A Array 						# declare a associative array
Array[a]=123					 		#using the [subscript]=value to assign a value 
Array+=([c]=7 [d]=8)             		# use the operator+ 

declare -aA arrOrArr  					# -A takes precedence

read -a stdarr   						#  assign a list of words read from the standard input to an array.

filename expands