${#A}  length of A
${A#*/}  strip of everthing upto and including / from A
${A##*/} greedy strip
${A%.out}  strip suffix .out from A
${A%%/*}  greedy suffix strip
$* all the arguments
$@  all the arguments
$$ PID
$?  return code
$- arguments to shell
$# num of args
$! last background process number
$0 name of command
${A-word}  A if non-null, else word
${A=word}  A if non-null, else set A=word
${A+word}  word if A, else nothing
${A?mesg} A if set else mesg and exit
${!prefix*}
${!prefix@}
${!name[@]}
${!name[*]}
${A/pattern/string}
${A//pattern/string}
(ls -y || ls) 2> /dev/null 
{ }
(( ))
[[ ]]
for (( i=0 ; $i<5 ; i++ )) ; do echo $i ; done
:  do nothing
arrays: declare local readonly unset

${A:0:2}  first 2 chars of A
${A:5:3}  chars 6,7,8 of A