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Advanced if usage

if/then/else constructs

This is used to take one course of action if the condition is true, and another if condition is false.

Syntax

if [ condition ]
then
  statement belongs to condition is true
else
  statement belongs to condition is false
fi

Example 

#!/bin/bash
number1=5
number2=6
if [ $number1 -eq $number2 ]
then
  echo "$number1 is equal to $number2"
else
  echo "$number1 is not equal to $number2"
fi
if_else

Getting input from commandline in runtime 

first_person_age=$1
second_person_age=$2
if [ $first_person_age -gt $second_person_age ]
then
  echo "first_person age $first_person_age is elder than second_person age $second_person_age"
else
  echo "first_person age $first_person_age is younger than second_person age $second_person_age"
fi
if_else_arg

NOTE

$0 refers to the name of the script

$1, $2 ... $n refers to the positional parameters

$# refers to the number of command line arguments

$@ refers to all the positional parameters passed to a script as separate words

$* refers to all the positional parameters passed to a script as single words

Examples 

#!/bin/bash
first_person_name=$1
second_person_name=$2
if [ $first_person_name = $second_person_name ]
then
  echo "$first_person_age is elder than $second_person_age"
else
  echo "$first_person_age is younger than $second_person_age"
fi
In below snapshot, $0 => if_else.sh, $1 => 25 and $2 => 8 (Positional parameters) if_else_arg 

#!/bin/bash
echo "Total number of arguments are $number"
In below snapshot, $# counts the number of arguments passed to the shell script $# 

#!/bin/bash
for number in "$@"
do
  echo "Argument $number"
done
In below snapshot, each argument is considered as separate word $@ 

#!/bin/bash
for number in "$*"
do
  echo "Argument $number"
done
In below snapshot, all arguments are considered as one word $*

if/then/elif/else constructs

Syntax

if [ condition ]
then
  statement belongs to condition is true
elif [ some_condition ]
then
  statement belongs to some_condition is true  
else
  statement belongs to condition is false
fi

NOTE: Multiple elif block can be possible.

Example

#!bin/bash
month_number=$1

if [ $# -ne 2 ]
then
  echo "No month number given. Please enter a month number as a command line argument."
  echo "eg: ./print-month-number 5"
  exit
fi

if [[ $month_number -lt 1 ]] || [[ $month_number -gt 4 ]]
then
  echo "Invalid month number given. Please enter a valid number - 1 to 12."
  exit
fi

if [ $month_number -eq 1 ]
then
  echo "First quarter"
elif [ $month_number -eq 2 ]
then
  echo "Second quarter"
elif [ $month_number -eq 3 ]
then
  echo "Third quarter"
elif [ $month_number -eq 4 ]
then
  echo "Fourth quarter"
fi
if_elif_else

Nested if statements

Syntax

if [ condition ]
then
  statement belongs to condition is true
elif [ some_condition ]
then
  statement belongs to some_condition is true
  if [ anothe_condition]
  then
    statement belongs to another_condition is true
  else
    statement belongs to another_condition is false
  fi
else
  statement belongs to condition is false
fi

Example

#!/bin/bash
year=`date +%Y`

if [ $[$year % 400] -eq "0" ]; then
  echo "This is a leap year.  February has 29 days."
elif [ $[$year % 4] -eq 0 ]; then
        if [ $[$year % 100] -ne 0 ]; then
          echo "This is a leap year, February has 29 days."
        else
          echo "This is not a leap year.  February has 28 days."
        fi
else
  echo "This is not a leap year.  February has 28 days."
fi
nested_if

Using the exit statement

  1. exit statement terminates execution of the entire script. It is used if the user input is incorrect, if a statement did not run successfully or if some other error occurred.

  2. exit statement takes an optional argument. This argument is the integer (exit status code), which is passed back to the parent and stored in the $? variable.

  3. A zero exit status code means that the script ran successfully

  4. Any other exit status code may be used to pass back different messages to the parent, so that different actions can be taken according to failure or success of the child process

  5. If no argument is given to the exit command, the parent shell uses the current value of the $? variable.

NOTE

exit is different from return. exit is used to stop the script whereas return is used to stop the function.

Example

#!/bin/bash

if [ $# != 1 ]; then
  echo "Usage: $0 10"
  exit 1
fi
if [ $1 -eq 10 ] ; then
  echo "Given argument is 10"
else
  echo "Given argument is NOT 10"
  exit 10
fi
if_exit

Using case statements

case statement will be useful when you are working on menu-driven script.

Syntax

In the below code snippet, if any of the "option" matches with "value", then the respect block of statements will be executed. If none of the "option" matches with "value", then * block of statements will be executed. 

case < value > in
  option) statement
    ;;
  option) statement
    ;;
  *) statement
    ;;
esac

NOTE

If any of the block executed, then the rest will be omitted. In other words, at any point of time, ONLY ONE block will be executed (If * is included)

Example

#!/bin/bash
space=`df -h | awk '{print $5}' | grep % | grep -v Use | sort -n | head -1 | cut -d "%" -f1 -`

case $space in
[1-6]*)
  Message="All is quiet"
  ;;
[7-8]*)
  Message="Start thinking about cleaning out some stuff.  There's a partition that is $space % full"
  ;;
9[1-8])
  Message="Better hurry with that new disk...  One partition is $space % full"
  ;;
99)
  Message="I'm drowning here!  There's a partition at $space %!"
  ;;
*)
  Message="I seem to be running with an nonexistent amount of disk space"
  ;;
esac

echo $Message as $space disk is used
case