Tutorial: Control Flow in Bash Scripting

In this tutorial we will provide basic examples of the conditional statements in bash scripting.

Passing Variables to Scripts via Command Line

In the script, we can define the variables with the dollar sign $. By adding a number, we are referring to the variable that we want to pass. So, the $1 is the first variable, the $2 is the second variable and so on. Let’s see an example. Let’s consider the following myexample.sh script.

echo "Hello, I pass the first variable $1 and the second one $2"

And let’s run it by typing:

bash myexample.sh var1 var2

Here, the var1 is assigned to $1 and the var2 to $2 and the output is:

Hello, I pass the first variable var1 and the second one var2

The if-then-else Statement

Below we provide a list of Bash Conditional Expressions

Let’s provide an example of a simple “if statement”. The script ifthenelse.sh is the following and it asks the end-user to enter a number between 30-40 and if the guessed number is equal to 37 (-eq) then it returns “You found it!“, else it returns “Wrong answer!“:

echo "Guess my Age!"
echo "#####################"
echo ""
echo "Enter a Number Between 30 and 40: "
read GUESS

if [ $GUESS -eq 37 ]
then
    echo "You found it!"
else
    echo "Wrong answer!"
fi

Let’s run it by guessing the correct age which is 37 and then again by entering 35.

Note that the else if in Bash scripting is elif as in Python.

The for loop

Let’s say that we want to get the first line of each Bash script in our working directory:

echo "List the first line of all bash scripts within the directory"

SHELLSCRIPTS=`ls *.sh`

for SCRIPT in "$SHELLSCRIPTS"
do
    DISPLAY="`head -n 1 $SCRIPT`"
	echo "File: $SCRIPT - First Lines $DISPLAY"
done

The Case Statement

Let’s provide an example of the case-statement in Bash:

echo "How shall we proceed"
echo "+++++++++++++++"
echo "Press 1 for the main menu"
echo "Press 2 for help"
echo "Press 3 to exit"
echo ""
echo "Press a number between 1 and 3"

read CHOICE

case $CHOICE in
1)
  echo "You are in the main menu";;
2)
  echo "You asked for help";;
3)
  echo "You exit";;
*)
  echo "Choose a number between 1 and 3";;
esac

The while loop

Let’s provide an example of a while loop.

echo "This is a while loop example"

COUNT=1

while [ $COUNT -le 10 ]
do
  echo "This is the loop number $COUNT"
  COUNT="`expr $COUNT + 1`"
done

Execution Operators (&& and ||)

In Bash scripting the AND is && and the OR is ||.The right side of && will only be evaluated if the exit status of the left side is zero (i.e. true). || is the opposite: it will evaluate the right side only if the left side exit status is non-zero (i.e. false). If the test inside evaluates to true, it returns zero; it returns nonzero otherwise. For example:

$ false && echo howdy!

$ true && echo howdy!
howdy!
$ true || echo howdy!

$ false || echo howdy!
howdy!