Linux Classroom Series – 28/Aug/2020

Variable Scopes in Shell scripts

• Lets create two shell scripts
  • script1.sh

  #!/bin/bash
  
  name="Learning Thoughts"
  echo "This is from script 1 name= ${name}"
  ./script2.sh
  

  • script2.sh

  #!/bin/bash
  
  echo "This is script2"
  echo "Value of name is ${name}"
  

• From script1 lets call script2 and try to see if the variable defined in script1 is available for use in script2
• lets execute script1.sh
• The value of name is not available in script2. The default scope of variable is the same script file.
• So lets find out if there is any other way to pass the variable from script1 to external scripts,
  1. add Arguments to script2

  ./script2.sh 
  # replace this with
  ./script2.sh $name
  

  2. export the variable using export statement in script1. so lets change script1.sh

  #!/bin/bash
  
  name="Learning Thoughts"
  echo "This is from script 1 name= ${name}"
  export name
  ./script2.sh
  

  

Debugging your scripts

• As the script grows and decision paths are included with conditional statements, we start using looping structures etc, we may need some level of debugging to analyse the scripts.
• This can be done as bas provides two options for us
  • -v option
  • -x option
• Create a bash script debugdemo.sh with the following content

#!/bin/bash

echo "the zeroth argument is $(basename $0)"
echo "Hello $*"

• Now execute this script with the following command

bash -v ./debugdemo.sh khaja ibrahim

• -v option displays the verbose output from bash
• -x option, which displays the commands as they are executed and is most commonly used.
• Now lets run the script

bash -x ./debugdemo.sh khaja ibrahim

• This way shows how the evaluated and is very helpful to know the decision branch that has been chosen by the script.
• Install bash debug plugin in visual studio code and we can debug the shell script.
• Now we can debug the shell script
• This approach works with visual studio code installed on the linux desktops or mac.

Enhancing interactive scripts

• Lets understand how to limit number of characters entered
• so limit number of characters to be entered, lets run the same command with one more option
• Lets understand how to control visibility of entered text, If we request for sensitive content like pin/password etc, its not a good approach to show the text while user is typing, Shell gives an option of hiding text when the user is typing. This can be achieved by adding a -s option to read command

Next Steps:

• How to make my shell scripts understand options (named parameters)

./downloadfile.sh --url <>  --location-to-save /home/ubuntu/test.txt