bash syntax

evaluating expression

1. shell expansion

1. brace expansion

   1 2 3 4 5

   a{b,c,d} # ab ac ad a{1..2} #a1,a2 1 a{1..5..2} #a1,a3

2. parameter expansion

   1 2	${parameter}; $parameter

3. output command

1
2

  a =  $(cat file )
  a = `cat file`

other

double quote vs single quote, curly bracket, parenthese

1. quote:
   ‘’: keep word literal meaning “”: expand $,,`

   1 2	echo "$(echo hello)" # hello echo '$(echo hello)' # '$(echo hello)'