Flow Control

If...Else

One of the other very important programming concepts is known as "flow control". Basically, this just means doing different things depending on current conditions. There are several ways to accomplish flow control, but probably the most common is the if...else statement, which looks like this:

if <TEST_SOMETHING>
then
  <DO_THING_1>
else
  <DO_THING_2>
fi

There are lots of different ways to compare values, depending on what type of values you're working with. Here is a page that lists several options: Bash comparison operators. Here's one example:

# Defining value of numeric variable
a=2
        
# if...else to see if value of number is at least 3
if [ $a -lt 3 ]  # Note: this could also be ((a < 3))
then
  echo "$a is less than 3."
else
  echo "$a is NOT less than 3."
fi

Here's an example of combining backticks, command-line arguments, and an if...else statement to write a script that tests if a command-line argument has a certain number of characters.

# Recording length of word provided on command line
# What's going on with the backticks (``) here?
myWordLength=`echo -n $1 | wc -m`

# test if word is at least 5 characters
if [ $myWordLength -lt 5 ]
then
  echo "$1 is shorter than 5 characters."
else
  echo "$1 is at least 5 characters in length."
fi

If...else statements can also be nested inside one another.

if [ $myWordLength -le 7 ]
then
    if [ $myWordLength -ge 3 ]
    then
        echo "Good."
    else
        echo "Not good."
    fi
else
    echo "Not good."
fi

Practice Exercise 1 (If...Else)

(1) Start with the script you wrote above to practice math
(2) Now add two if...else statements to check that:
  - The first number is between 3 and 7
  - The second number is between 10 and 14
(3) If either of these conditions are not met, have your script print an error message to the screen.

If...Elif...Else

Sometimes you may want to evaluate a series of conditions consecutively. In this case, you can use a variation on an if...else statement that has an extra else if in the middle. In these cases, the else if is abbreviated as elif and the structure could look something like this

if (( $1 < 2 ))
then
    echo "less than two"
elif (( $1 < 5 ))
then
    echo "between two and four"
else
    echo "greater than four"
fi

How is this code different than what we saw above? If we put this in a script and passed a command-line argument of 1, what would we get? What if we passed a 4?

You can also string together as many elif tests in a row as you want.

if (( $1 < 2 ))
then
    echo "less than two"
elif (( $1 < 5 ))
then
    echo "between two and four"
elif (( $1 < 8 ))
then
    echo "between five and seven"
else
    echo "greater than seven"
fi

For Loops

One of the most common reasons to write a script is to automate something that is, at a minimum, very tedious to do manually and, at worst, completely impossible otherwise. A versatile way to incorporate repitition into a script is to use a for loop. for loops in bash have the following structure:

for num in 1 two 3 FOUR
do
  echo $num
done

Let's break this down. First, we've defined a new variabled named num. This variable can be named anything you want. In this case, num will iteratively take the value of anything included in the list that follows in. During each iteration, the code in between do and done will be executed. In this case, we will simply print out each of the values our variable takes, one after the other. Later, we will use for loops that have a whole series of commands inside the loop.

Practice Exercise 2 (for loop)

Write a for loop inside a script that follows these steps:

(1) Create a variable called sum and set it to 0.
(2) Loop over the integers 1 through 10.
(3) If the integer is even (2, 4, etc.), add it to sum.
(4) Once the loop is complete, print out sum.

HINT: Remember the modulo (%) operator!!

Double parentheses notation can also be used to write a for loop in a way that doesn't require you to write out every unique element in the list:

for ((num=1;num<=10;num++))
do
  echo $num
done

When written this way, the for loop statement has a structure like this

for (( <START_VALUE> ; <STOP_CONDITION> ; <LOOP_UPDATE> ))

The variable is initialized to the start value, updated according to the loop update, and continues until the stop condition is no longer true. The loop update (num++) here adds 1 to num each time the loop iterates. NOTE: You don't precede variables with $ inside double parentheses.

Sometimes you'll want to loop through a whole series of command-line arguments. To loop through these arguments, you can write $@ in place of the list in your for statement.

for num in $@
do
  echo $num
done

Practice Exercise 3 (for loops, command-line arguments)

Write a script that's similar to the one you wrote above to add even numbers. However,
this time your script should accept any number of integers as command-line arguments
and only sum the even ones. At the end, your script should print out sum.