IF / ELSE

if [ condition ]
then
  # do something
else
  # do something else
fi

Be very particular of the whitespace around [ and ]. It is [ condition ] and not [condition]

OR

if [ condition ]; then
  # do something
else
  # do something else
fi

where you can place then at the end of the same line as your if condition, separated by a semi-colon ;.

For example:

# Take user input
read input
    
# See if user provided twitter
if [ "$input" == "twitter.com" ]; then
  # show error
  echo "can not ping this!"
else
  # ping three times
  ping -c3 ${input}
fi

ELIF

ELIF is ELse IF.

# Take user input
read input
    
# See if user provided twitter
if [ $input == "twitter.com" ]; then
  # show error
  echo "can not ping this!"
elif [ $input == "google.com"  ]; then 
  echo "Why always Google? -_-"
else
  # ping three times
  ping -c3 ${input}
fi