Variables and User Input

What is a variable

• Variable is a standard building block in scripting languages, They allow you to store information, so that we can retrieve/change multiple times during the execution of script.
• Assigning the value variable <variable_name>=<variable_value>

name="Learning Thoughts"

• Referencing the variable ${variable_name} or $variable_name

echo $name
echo ${name}
echo "Name == ${name}"

• Now lets try to create a simple shell script

#!/bin/bash

#####################################################################
# Author: Shaik Khaja Ibrahim
# Version: v1.0.0
# Date: 25-Aug-2020
# Description: This is shell script demonstrating variables
# Usage: ./variabledemo.sh
#####################################################################

name="Khaja Ibrahim"
org_name="Learning Thoughts"

# Here we are interpolating the variables
echo "This is ${name} from ${org_name}"

# All the variables are treated as text by linux until and unless you are explicit
number=5
echo $number + 1

• Now execute the script
• Now shell script is treating numbers as text, if we want shell script to consider variable number as integer, we need to use and expression

$(( ${number} + 1 ))

• Now considering the script to be

#!/bin/bash

#####################################################################
# Author: Shaik Khaja Ibrahim
# Version: v1.0.0
# Date: 25-Aug-2020
# Description: This is shell script demonstrating variables
# Usage: ./variabledemo.sh
#####################################################################

name="Khaja Ibrahim"
org_name="Learning Thoughts"

# Here we are interpolating the variables
echo "This is ${name} from ${org_name}"

# All the variables are treated as text by linux until and unless you are explicit
number=5
echo $number + 1

echo "lets use arthimetic expression"
echo $(( ${number} + 1 ))

• In shell scripting we following naming conventions, for

  • variables: lowercase with underscores

  my_number=6
  message="how are you"
  

  • Constants: UPPERCASE

  LOCATION="/home"
  

  • Do not create variables with names which already have some meaning PATH, USER, LANG, SHELL, HOME & so on. If the Purpose is similar try to prefix SCRIPT_, for example define as shown below.

  SCRIPT_PATH over PATH
  SCRIPT_USER over USER
  

• So far, we have been dealing with static scripts, so let’s introduce the concept of user inputs in shell scripting

Basic Inputs

• At a very basic level, everything you enter on the command line after the script can be used as input

./<script>.sh Hello

• Lets create a script called as printname.sh with the following script

#!/bin/bash

#####################################################################
# Author: Shaik Khaja Ibrahim
# Version: v1.0.0
# Date: 25-Aug-2020
# Description: This script demonstrates basic user inputs
# Usage: ./printname.sh <pass name>
#####################################################################
name=${1}
echo "Hello ${name}"

echo "0 = ${0}  1 = ${1}"

• Now execute the script
• Here ${1} stands for the first positional argument
• Generally we will have two kinds of arguments
  • positional arguments

  cp 1.txt 2.txt # 1.txt & 2.txt are two positional arguments
  
  

  • named arguments

  ping --count 4 google.com
  # google.com is a positional argument
  # where as 4 is a named argument as it value for a name called as count
  

• The script shown above is depending on positional arguments.