====== Shell Scripting ======
Execute shell script: 
  ./script.sh
  bash script.sh
  source script.sh

Read user input: 
  read

Echo text to screen: 
  echo

Arithmetic operations: 
  let
  expr

Conditional statements: 
  if
  case

Loops: 
  for
  while

Array operations: 
  declare 
  read -a

Check file/directory existence: 
  [file] -e 
  [file] -d 
  [file] -f

Check file permissions: 
  [file] -r 
  [file] -w 
  [file] -x
  
Check file type: 
  [file] -b 
  [file] -c 
  [file] -L
  [file] -S