Bash Scripting for Beginners (Part 4— Decision Making)

Let’s continue with if/else and case/esac statements.

If Statements

If statements allow us to make decisions in our Bash scripts. They allow us to decide whether or not to run a piece of code based upon conditions that we may set.

Format:

if [ <condition> ]
then
<do something>
fi

if…else statement

Here is a simple if statement that check to see if the condition is true or not. Depending on the result, it will do one of two things. If the condition is true than if block will run. Otherwise, else block will run. If we would like to check an expression then we may use the double brackets.

#!/bin/bash
AGE=36
if [[ $AGE -gt 10 ]]
then
  echo "You are greater than 10."
else
  echo "You are smaller than 10."
fi

Output:

You are greater than 10.

if…elif…else statement

You can have one or more elif clauses in the statement. The else clause is optional. The conditions are evaluated sequentially. Once a condition returns True, the remaining conditions are not performed.

if [ <condition> ]
then
<do something>
elif
then
<do something>
else
<do something>
fi

Example:

#!/bin/bash

echo -n "Enter a number: "
read NUM

if [[ $NUM -gt 10 ]]
then
  echo "The variable is greater than 10."
elif [[ $NUM -eq 10 ]]
then
  echo "The number is equal to 10."
else
  echo "The number is less than 10."
fi

Output:

$ ./test.sh 5
The number is less than 10.

Nested if statements

It is possible to place an if statement inside yet another if statement. This is called nesting. Indenting is an important part of writing good, clean code. But not necessary. The aim is to improve readability.

#!/bin/bash
num=120
if [ $num -gt 100 ]
then
    echo Hey that\'s a large number.
    if (( $num % 2 == 0 ))
    then
        echo And is also an even number.
    fi
fi

Output:

Hey that's a large number.
And is also an even number.

Arithmetic Comparison

Below is the list of parameters used for numeric comparisons.

• num1 -eq num2 check if 1st number is equal to 2nd number
• num1 -ge num2 checks if 1st number is greater than or equal to 2nd number
• num1 -gt num2 checks if 1st number is greater than 2nd number
• num1 -le num2 checks if 1st number is less than or equal to 2nd number
• num1 -lt num2 checks if 1st number is less than 2nd number
• num1 -ne num2 checks if 1st number is not equal to 2nd number

String Comparison

Below is the list of parameters used for string comparisons.

• var1 = var2 checks if var1 is the same as string var2
• var1 != var2 checks if var1 is not the same as var2
• var1 < var2 checks if var1 is less than var2
• var1 > var2 checks if var1 is greater than var2
• -n var1 checks if var1 has a length greater than zero
• -z var1 checks if var1 has a length of zero

Note: When using > or < symbols in scripts, they should be used with escape character as “/>” or “/<“.

Example:

#!/bin/bash

name=aoyilmaz
if [ $USER = $name ]
        then
                echo "User exists"
        else
                echo "User not found"
fi

Output:

$ ./test.sh aoyilmaz
User exists

File Comparison

Below is the list of parameters used for file comparisons.

• -b filename Block special file
• -c filename Special character file
• -d directoryname Check for directory existence
• -e filename Check for file existence
• -f filename Check for regular file existence not a directory
• -G filename Check if file exists and is owned by effective group ID
• -g filename true if file exists and is set-group-id
• -k filename Sticky bit
• -L filename Symbolic link
• -O filename True if file exists and is owned by the effective user id
• -r filename Check if file is a readable
• -S filename Check if file is socket
• -s filename Check if file is nonzero size
• -u filename Check if file set-ser-id bit is set
• -w filename Check if file is writable
• -x filename Check if file is executable
• file A -nt file B checks if file A is newer than file B
• file A -ot file B checks if file A is older than file B

This script checks if the directory exists, if not create the directory.

#!/bin/bash
directory="./aoy"

if [ -d $directory ]
then
	echo "Directory exists"
else 
	mkdir ./aoy
fi

Boolean Operators

If you need to compare two things and you don’t want to use nested if statements, you can use boolean operators instead.

• and — &&
• or — ||

#!/bin/bash
if [ $1 == 'aoyilmaz' ] || [ $2 == '123' ]
then
    echo Login confirmed.
else
    echo Login failed
fi

Output:

$ ./test.sh aoyilmaz 123
Login confirmed.

Case Statement

The bash case statement is generally used to simplify complex conditionals when you have multiple different choices.

Format:

case EXPRESSION in
PATTERN_1)
    STATEMENTS
    ;;
PATTERN_2)
    STATEMENTS
    ;;
PATTERN_N)
    STATEMENTS
    ;;
*)
    STATEMENTS
    ;;
esac

Example:

#!/bin/bash
case choice in
    1)
        echo You chose 1.
        ;;
    2)
        echo You chose 2.
        ;;
    3)
        echo You chose 3.
        ;;
    *)
        echo You did not choose anything.
        ;;
esac

Output:

$ ./test.sh 3
You chose 3.

I will continue in the next article.