conditional statements if...then if...then...else

We can check the conditional statement in bash scripting

1. if..then..fi statement (Simple If)

2. if..then..else..fi statement (If-Else)

3. if..elif..else..fi statement (Else If ladder)

4. if..then..else..if..then..fi..fi..(Nested if)

File Operations


-s    file exists and is not empty
-f    file exists and is not a directory
-d    directory exists
-x    file is executable
-w    file is writable
-r    file is readable

Numeric Comparison

expr1 -eq expr2   Returns true if the expressions are equal
expr1 -ne expr2   true if the expressions are not equal
expr1 -gt expr2   true if expr1 is greater than expr2
expr1 -ge expr2   Returns true if expr1 >= to expr2
expr1 -lt expr2   Returns true if expr1 is less than expr2
expr1 -le expr2   Returns true if expr1 is >= expr2
! expr1    Negates the result of the expression

-lt	<
-gt	>
-le	<=
-ge	>=
-eq	==
-ne	!=

String Comparison

Str1 = Str2    Returns true if the strings are equal
Str1 != Str2   Returns true if the strings are not equal
-n Str1        Returns true if the string is not null
-z Str1        Returns true if the string is null

=	equal
!=	not equal
<	less then
>	greater then
-n s1	string s1 is not empty
-z s1	string s1 is empty

if..then..fi

if [ expression ]
then
   Statement(s) to be executed if expression is true
fi

Here Shell expression is evaluated. If the resulting value is true, given statement(s) are executed.If expression is false then no statement would be not executed. Most of the times you will use comparison operators while making decisions.

#!/bin/sh
a=10
b=20
if [ $a == $b ]
then
   echo "$a is equal to $b"
fi

if [ $a != $b ]
then
   echo "a is not equal to b"
fi

#!/bin/bash
count=100
if [ $count -eq 100 ]
then
  echo "Count is 100"
fi

#!/bin/bash
if [ $? -eq 0 ]
then
echo 'Success!'
  else
echo "Failed!!!!"
fi

#!/bin/bash
count=`wc -l /root/nopass | cut -f1 -d" "`
echo $count
if [ "$count" > "150" ]
then
echo "File has more than 150 lines"
fi

#!/bin/bash
gender="female"
if [[ "$gender" == f* ]] #syntax differ when * comes
then
echo "Welcome, Madame.";
fi

#!/bin/bash
if [ "$(whoami)" != 'root' ]; then
   echo "You have no permission to run $0 as non-root user."
    exit 1;
fi

Note: you can give the system commands in two ways: 
        1. inside tilde `` Or using second method
        2. $()

if..then..else..fi

; semi-colon is a command terminator in bash

Use single quote when you want to literally print everything inside the single quote.
echo 'Hostname=$HOSTNAME ;  Current User=`whoami` ; Message=\$ is USD'

Use double quotes when you want to display the real meaning of special variables.
echo "Hostname=$HOSTNAME ;  Current User=`whoami` ; Message=\$ is USD"

Double quotes will remove the special meaning of all characters except the following:

$ Parameter Substitution.
` Backquotes
\$ Literal Dollar Sign.
\´ Literal Backquote.
\” Embedded Doublequote.
\\ Embedded Backslashes.

Before special variables use escape characters

#!/bin/sh
# This is some secure program that uses security.

VALID_PASSWORD="secret" #this is our password.

echo -n "Please enter the password:"
read PASSWORD

if [ "$PASSWORD" == "$VALID_PASSWORD" ]; then
        echo "You have access!"
else
        echo "ACCESS DENIED!"
fi

If..then..else if..then..else if..then..else [Nested if]

#!/bin/sh

# Prompt for a user name...
echo -n "Please enter your name:"
read USERNAME

# Check for the file.
if [ -s ${USERNAME}_DAT ]; then
        # Read the age from the file.
        AGE=`cat ${USERNAME}_DAT` # This is how you execute a command and put                                                # the text output from the command into a                                                    # variable.
        echo "You are $AGE years old!"
else
        # Ask the user for his/her age
        echo "How old are you?"
        read AGE

 if [ "$AGE" -le 2 ]; then
  echo "You are too young!"
 else
  if [ "$AGE" -ge 100 ]; then
   echo "You are too old!"
  else
          # Write the age to a new file.
          echo $AGE > ${USERNAME}_DAT
         fi
        fi
fi

If..elif (else if) 

if [ expression 1 ]
then
   Statement(s) to be executed if expression 1 is true
elif [ expression 2 ]
then
   Statement(s) to be executed if expression 2 is true
elif [ expression 3 ]
then
   Statement(s) to be executed if expression 3 is true
else
   Statement(s) to be executed if no expression is true
fi

#!/bin/sh

a=10
b=20

if [ $a == $b ]
then
   echo "a is equal to b"
elif [ $a -gt $b ]
then
   echo "a is greater than b"
elif [ $a -lt $b ]
then
   echo "a is less than b"
else
   echo "None of the condition met"
fi

#!/bin/sh

# Prompt for a user name...
echo "Please enter your age:"
read AGE

if [ "$AGE" -lt 20 ] || [ "$AGE" -ge 50 ]; then
 echo "Sorry, you are out of the age range."
elif [ "$AGE" -ge 20 ] && [ "$AGE" -lt 30 ]; then
 echo "You are in your 20s"
elif [ "$AGE" -ge 30 ] && [ "$AGE" -lt 40 ]; then
 echo "You are in your 30s"
elif 

\[ "$AGE" -ge 40 ] && [ "$AGE" -lt 50 ]; then
 echo "You are in your 40s"
fi

 Nested if/else


#!/bin/bash

# Declare variable choice and assign value 4
choice=4

# Print to stdout
 echo "1. Bash"
 echo "2. Scripting"
 echo "3. Tutorial"
 echo -n "Please choose a word [1,2 or 3]? "

# Loop while the variable choice is equal 4
# bash while loop

while [ $choice -eq 4 ]; do

# read user input
read choice

# bash nested if/else
if [ $choice -eq 1 ] ; then

        echo "You have chosen word: Bash"

else

        if [ $choice -eq 2 ] ; then
                 echo "You have chosen word: Scripting"
        else

                if [ $choice -eq 3 ] ; then
                        echo "You have chosen word: Tutorial"
                else
                        echo "Please make a choice between 1-3 !"
                        echo "1. Bash"
                        echo "2. Scripting"
                        echo "3. Tutorial"
                        echo -n "Please choose a word [1,2 or 3]? "
                        choice=4
                fi
        fi
fi

done

********************************************

Exmp1:

#!/bin/bash
# Author: 
# Date: 
# Purpose: 

password="IF"
echo -n "Please enter your password:"
read passwd

if [ $password = $passwd ]; then
 echo "You have the access to run this script"
else
 echo "Access Denied"
exit
fi

if [ -d ~/IF ]; then
echo "Directory exists. creting file"
 cd /home/ec2-user/IF ; touch iftesting
else
 echo "directory does not exists...Creating directory"
  mkdir /home/ec2-user/IF
exit
fi


Exmp2:

#!/bin/sh

# Prompt for a user name... 

echo "Please enter your name:"
read USERNAME

# Check for the file.
 

if [ -s ${USERNAME}_DAT ]; then
        # Read the age from the file.
        AGE=`cat ${USERNAME}_DAT`
        echo "You are $AGE years old!"
else
        # Ask the user for his/her age
        echo "How old are you?"
        read AGE

 if [ "$AGE" -le 2 ]; then
  echo "You are too young!"
 else
  if [ "$AGE" -ge 100 ]; then
   echo "You are too old!"
  else
          # Write the age to a new file.
          echo $AGE > ${USERNAME}_DAT
         fi
        fi
fi

Exmp3:

#!/bin/sh
# Prompt for a user name... 

echo "Please enter your age:"
read AGE

if [ "$AGE" -lt 20 ] || [ "$AGE" -ge 50 ]; then
 echo "Sorry, you are out of the age range."
 

elif [ "$AGE" -ge 20 ] && [ "$AGE" -lt 30 ]; then
 echo "You are in your 20s"
 

elif [ "$AGE" -ge 30 ] && [ "$AGE" -lt 40 ]; then
 echo "You are in your 30s"
 

elif [ "$AGE" -ge 40 ] && [ "$AGE" -lt 50 ]; then
 echo "You are in your 40s"
fi

Exmp4:

#!/bin/bash
# Purpose: Detecting Hardware Errors
# Author: 
# Note : The script must run as a cron-job.
# Last updated on : 
# -----------------------------------------------
 
# Store path to commands
LOGGER=/usr/bin/logger
FILE=/var/log/mcelog
 
# Store email settings
AEMAIL="jeffinm@gmail.com "
ASUB="H/W Error - $(hostname)"
AMESS="Warning - Hardware errors found on $(hostname) @ $(date). See log file for the details /var/log/mcelog."
OK_MESS="OK: NO Hardware Error Found."
WARN_MESS="ERROR: Hardware Error Found."
 
 
# Check if $FILE exists or not
if test ! -f "$FILE" 
then   
 echo "Error - $FILE not found or mcelog is not configured for 64 bit Linux systems."
 exit 1
fi
 
# okay search for errors in file
error_log=$(grep -c -i "hardware error" $FILE)
 
# error found or not?
if [ $error_log -gt 0 ]
then    # yes error(s) found, let send an email
 echo "$AMESS" | email -s "$ASUB" $AEMAIL
else    # naa, everything looks okay
 echo "$OK_MESS"
fi