How to Validate Integer Input using Shell script

Method 1:
#!/bin/bash
# SCRIPT:  validinteger.sh
# USAGE:   validinteger.sh [ Input value to be validated ]
# PURPOSE: validate integer input, allow negative integers also
#
#             \\\\ ////
#            \\  - -  //
#                @ @
#        ---oOOo-( )-oOOo---
#
# Using the $? exit status variable or integer comparison operators, a
# script may test if a parameter contains only digits, so it can be
# treated as an integer.
#
#####################################################################
#                      Arguments Checking                           #
#####################################################################

if [ $# -eq 0 ]
then
    echo -n "Enter input to test: "
    read Number
else
    Number=$1
fi

# You can also use bellow one liner
#[ $# -eq 0 ] && { echo -n "Enter input:";read Number; } || Number=$1

#####################################################################
#                     Main Script Starts Here                       #
#####################################################################

# Check the input is an integer or not

if [ $Number -ne 0 -o $Number -eq 0 2>/dev/null ]
then

# An integer is either equal to 0 or not equal to 0.
# 2>/dev/null suppresses error message.

    echo "Supplied Input $Number is an Integer"

else

    echo "Supplied Input $Number is not an Integer."
fi

# You can compare number itself also

#  if [ $Number -eq $Number 2> /dev/null ]
#  then
#    echo "Supplied Input $Number is an Integer"
#  else
#    echo "Supplied Input $Number is not an Integer."
#  fi

OUTPUT:
# chmod 755 validinteger.sh
# ./validinteger.sh -12345
Supplied Input -12345 is an Integer
# ./validinteger.sh 12345A
Supplied Input 12345A is not an Integer.
# ./validinteger.sh
Enter input to test: 876549
Supplied Input 876549 is an Integer
# ./validinteger.sh
Enter input to test: -345k123
Supplied Input -345k123 is not an Integer.

Method 2 : 

#!/bin/bash
# SCRIPT:  validinteger2.sh
# USAGE:   validinteger2.sh [ Input value to be validated ]
# PURPOSE: validate integer input, allow negative integers also
#
#
#             \\\\ ////
#            \\  - -  //
#                @ @
#        ---oOOo-( )-oOOo---
#
# In this method if you want to allow negative integers, it gets a
# tiny bit more complicated. You must have basic knowledge of string
# manipulation methods.
#
#####################################################################
#                      Arguments Checking                           #
#####################################################################

if [ $# -eq 0 ]
then
    echo -n "Enter input to test: "
    read Number
else
    Number=$1
fi

#####################################################################
#                     Main Script Starts Here                       #
#####################################################################

# Check first character is -

if [ "${Number:0:1}" = "-" ]    #extract first character
then
    RemNumber="${Number:1}"     #extract except first char
else
    RemNumber="$Number"
fi

# You can extract substring using bellow method also

#if [ "$(echo $Number|cut -c 1)" = "-" ]
#then
#    RemNumber=$(echo $Number|cut -c 2-)
#else
#    RemNumber="$Number"
#fi

if [ -z "$RemNumber" ]
then
    echo "Supplied Input $Number is not an Integer"
else

    NoDigits="$(echo $RemNumber | sed 's/[[:digit:]]//g')"

    if [ -z "$NoDigits" ]
    then
        echo "Supplied Input $Number is an Integer"
    else
        echo "Supplied Input $Number is not an Integer."
    fi
fi

OUTPUT:
# chmod 755 validinteger2.sh
# ./validinteger2.sh -987612
Supplied Input -987612 is an Integer
# ./validinteger2.sh -A987612
Supplied Input -A987612 is not an Integer.
# ./validinteger2.sh
Enter input to test: -123456789
Supplied Input -123456789 is an Integer
# ./validinteger2.sh
Enter input to test: A234K78
Supplied Input A234K78 is not an Integer.

Method 3: 
#!/bin/bash
# SCRIPT:  validinteger3.sh
# USAGE:   validinteger3.sh [ Input value to be validated ]
# PURPOSE: validate integer input, allow negative integers also
#
#                           \\\\ ////
#                          \\  - -  //
#                              @ @
#                      ---oOOo-( )-oOOo---
#
# In this method input value validated using ASCII value range of
# numbers (48 - 57).
# Sample script provided by Bond, Thank you Bond.
#
#####################################################################
#                      Arguments Checking                           #
#####################################################################

if [ $# -eq 0 ]
then
    echo -n "Enter input to test: "
    read Number
else
    Number=$1
fi

# You can also use bellow one liner
#[ $# -eq 0 ] && { echo -n "Enter input:";read Number; } || Number=$1

#####################################################################
#                     Main Script Starts Here                       #
#####################################################################

# Check first character is - ?
# This is another method to extract substring using Substring Removal
# method.

  if [ "${Number%${Number#?}}" = "-" ]  #extract first char
  then
      RemNumber="${Number#?}"           #extract all except first char
  else
      RemNumber="$Number"
  fi

if [ -z "$RemNumber" ]
then
    echo "Supplied Input $Number is not an Integer"
else

    Length=${#RemNumber}              # Counting length of the string

    for ((i=0; i < Length; i++))
    do

       char=${RemNumber:$i:1}          # Reads character one by one

       code=`printf '%d' "'$char"`     # Get ASCII value of char

       # Compare code with ASCII value range of Intergers ( 48 - 57 )

       if [ $code -lt 48 ] || [ $code -gt 57 ]
       then
           echo "Supplied Input $Number is not an Integer"
           exit 1
       fi

    done

    echo "Supplied Input $Number is an Integer."

fi

OUTPUT:
$ sh validinteger3.sh -1234
Supplied Input -1234 is an Integer.
$ sh validinteger3.sh -12-12
Supplied Input -12-12 is not an Integer
$ sh validinteger3.sh 
Enter input to test: -
Supplied Input - is not an Integer
$ sh validinteger3.sh 
Enter input to test: 123a456
Supplied Input 123a456 is not an Integer