Check if a program exists from a bash script

$ command -v foo >/dev/null 2>&1 || { 

  echo >&2 "I require foo but it's not installed.  Aborting."; exit 1;  

}

OR 
$ type foo >/dev/null 2>&1 || { 

  echo >&2 "I require foo but it's not installed.  Aborting."; exit 1;  

}

OR 
$ hash foo 2>/dev/null || { 

  echo >&2 "I require foo but it's not installed.  Aborting."; exit 1;  

}