Control structures

Simple if statement

• The general structure of an if statement

if condition
then
  statements
else
  statements
fi

if test -f blah.py
then
...
fi

if [ -f blah.py ]
then
...
fi

<pre>


You an put then on the same line, but then you have to use a semi

if [ -f blah.py ]; then
...
fi

• if statement after reading user input

echo "Is it morning?  Please answer yes or no"
read timeofday

if [ $timeofday = "yes" ]; then
  echo "good morning"
else
  echo "good afternoon"
fi

• elif
• example: elif that exits with error code when given bad input

echo "Is it morning?  Please answer yes or no"
read timeofday

if [ $timeofday = "yes" ]; then
  echo "good morning"
elif [ $timeofday = "no" ]; then
  echo "good afternoon"
else
  echo "Sorry, $timeoday not recognized.  Enter yes or no"
  exit 1
fi

• See book p36, there is some weird reason you should always double quote variables in contional checks
  • Ex: use "$timeofday" instead of $timeofday
  • Really don't understand this
• If the variable can never be the empty string, you don't need to protect it with double quotes when testing its value. Still, always double quote variables because it's a good habit