if, else, and elif

We can write scripts that can perform certain actions depending on conditions using if, else and elif. The syntax for these things is quite particular so you need to be careful. At a high level, we are looking at...

High Level Psuedo Code - if statement

if [Some form of conditional test]
then
    do thing A
else
    do thing B
fi

Simple if else Script

The easiest way to show this in action is to examine a working script with an if statement in it:

#!/bin/bash

echo "How old are you?"

read age

if [ $age -lt 18 ]
then
  echo "I think you are too young for this game"
else
  echo "Welcome to the club"
fi

Simple if, elif, else Script

The above example only allows for 2 execution branches; one if a condition is true, or the other if not. This is fine if your script is fairly simple, however this can be rather limiting. So, we can add the elif statement to add further conditional checks, providing more granularity to the tool:

High Level Psuedo Code - if else statement

if [ <some test> ]
then
  <perform an action>
elif
then
  <perform this action instead>
else
  <perform yet another different action>
fi

Again, let's examine a working example:

#!/bin/bash

echo "How old are you?"

read age

if [ $age -lt 18 ]
then
  echo "I think you are too young for this game"
elif [ $age -gt 60 ]
then
  echo "Well, arent you the old codger"
else
  echo "Welcome to the club"
fi

In both scripts, you are seeing operators such as -lt and -gt, which are short for Less Than and Greater Than. The table below contains a comprehensive list of operators you can use in your scripts

Operators

Operator	Description
!EXPRESSION	The EXPRESSION is false
-n STRING	STRING length is greater than zero
-z STRING	The length of STRING is zero (empty)
STRING1 = STRING 2	STRING1 is equal to STRING2
STRING1 != STRING 2	STRING1 is not equal to STRING2
INTEGER1 -eq INTEGER2	INTEGER1 is equal to INTEGER2
INTEGER1 -ne INTEGER2	INTEGER1 is not equal to INTEGER2
INTEGER1 -gt INTEGER2	INTEGER1 is greater then INTEGER2
INTEGER1 -lt INTEGER2	INTEGER1 is less than INTEGER2
INTEGER1 -ge INTEGER2	INTEGER1 is greater than or equal to INTEGER2
INTEGER1 -le INTEGER2	INTEGER1 is less than or equal to INTEGER2
-d FILE	FILE exists and is a Directory
-e FILE	FILE exists
-r FILE	FILE exists and has read permission
-s FILE	FILE exists and is not empty
-w FILE	FILE exists and has write permission
-x FILE	FILE exists and has execute permission