Linux Classroom Series – 04/Sept/2020

Reading input from files

• Lets assume we have list of server ip addresses or hosts in the file called as servers.txt.
• We are asked to find which servers are up
• This script is working but it is unable to redirect the output to a text

#!/bin/bash

# Usage: ./checkservers <servers-filepath>
# servers-filepath is a text file with each server in new line

if [ ! -f "$1" ] ;
then
    echo "The input to $0 should be a file"
fi
echo "The following servers are up on $(date +%x)" > checkservers.out
while read server; 
do
    ping -c1 "$server"&& echo "Serverup $server" >> checkservers.out
done < $1

cat checkservers.out

Building Blocks for reusability : Functions

• DRY principle(Don’t Repeat Your Self)

• We will cover the following aspects

  • Introduction
  • Passing Parameters to the functions
  • Variable scope
  • Returning values from functions
  • Recursive functions

• Functions are internally represented as blocks of code in memory as named elements. These elements can be created within shell environment, as well as within the script execution.

• Execute declare -F in the bash. The output of this command might vary with distribution

• Functions can be created using the following two syntaxes

• Syntax 1:

function-name() {
    <code to be executed>
}

• Syntax 2:

function <function-name> {
    <code to be executed>
}

• Lets start with a simple function

show_system_details() {
    echo "Uptime is"
    uptime
    echo "Cpu details"
    lscpu
    echo "User list"
    who
}

is_file() {
    if [ ! -f "$1" ]; then
        echo "$1 is not a file"
        exit 2
    fi
}

backup_file() {
    is_file "$1"
    new_file_loc="${1}.bak" 
    cp $1 $new_file_loc
    echo "file is copied to $new_file_loc"
}

backup_file "/home/ubuntu/1.txt"