M&P Technology - Issue - Solution

W e will discuss in detail about the Shell input/output redirections. Most Unix system commands take input from your terminal and send the resulting output back to your terminal. A command normally reads its input from the standard input, which happens to be your terminal by default. Similarly, a command normally writes its output to standard output, which is again your terminal by default. Output Redirection The output from a command normally intended for standard output can be easily diverted to a file instead. This capability is known as output redirection. If the notation > file is appended to any command that normally writes its output to standard output, the output of that command will be written to file instead of your terminal. Check the following  who  command which redirects the complete output of the command in the users file. $ who > users Notice that no output appears at the terminal. This is because the output has been redirected from the default standard output dev

W e will discuss in detail about the shell functions. Functions enable you to break down the overall functionality of a script into smaller, logical subsections, which can then be called upon to perform their individual tasks when needed. Using functions to perform repetitive tasks is an excellent way to create  code reuse . This is an important part of modern object-oriented programming principles. Shell functions are similar to subroutines, procedures, and functions in other programming languages. Creating Functions To declare a function, simply use the following syntax − function_name () { list of commands } The name of your function is  function_name , and that's what you will use to call it from elsewhere in your scripts. The function name must be followed by parentheses, followed by a list of commands enclosed within braces. Example Following example shows the use of function − #!/bin/sh # Define your function here Hello () { echo "Hello World" } # Invoke yo

There are various operators supported by each shell. We will discuss in detail about Bourne shell (default shell) in this chapter. We will now discuss the following operators − Arithmetic Operators Relational Operators Boolean Operators String Operators File Test Operators Bourne shell didn't originally have any mechanism to perform simple arithmetic operations but it uses external programs, either  awk  or  expr . The following example shows how to add two numbers − #!/bin/sh val = `expr 2 + 2` echo "Total value : $val" The above script will generate the following result − Total value : 4 The following points need to be considered while adding − There must be spaces between operators and expressions. For example, 2+2 is not correct; it should be written as 2 + 2. The complete expression should be enclosed between ‘ ‘, called the backtick. Arithmetic Operators The following arithmetic operators are supported by Bourne Shell. Assume variable  a  holds 10 and variable  b  h

W e will discuss in detail about special variable in Unix. In one of our previous chapters, we understood how to be careful when we use certain nonalphanumeric characters in variable names. This is because those characters are used in the names of special Unix variables. These variables are reserved for specific functions. For example, the  $  character represents the process ID number, or PID, of the current shell − $echo $$ The above command writes the PID of the current shell − 29949 The following table shows a number of special variables that you can use in your shell scripts − Command-Line Arguments The command-line arguments $1, $2, $3, ...$9 are positional parameters, with $0 pointing to the actual command, program, shell script, or function and $1, $2, $3, ...$9 as the arguments to the command. Following script uses various special variables related to the command line − #!/bin/sh echo "File Name: $0" echo "First Parameter : $1" echo "Second Parameter :

W e will discuss shell loop control in Unix. So far you have looked at creating loops and working with loops to accomplish different tasks. Sometimes you need to stop a loop or skip iterations of the loop. we will learn following two statements that are used to control shell loops− The break statement The continue statement The infinite Loop All the loops have a limited life and they come out once the condition is false or true depending on the loop. A loop may continue forever if the required condition is not met. A loop that executes forever without terminating executes for an infinite number of times. For this reason, such loops are called infinite loops. Example Here is a simple example that uses the  while  loop to display the numbers zero to nine − #!/bin/sh a = 10 until [ $a - lt 10 ] do echo $a a = `expr $a + 1` done This loop continues forever because  a  is always  greater than  or  equal to 10  and it is never less than 10. The break Statement The  break  statement

  Amazon SNS is a service used for push notification. In this chapter, we will explain working of AWS Lambda and Amazon SNS with the help of an example where will perform the following actions − Create Topic in SNS Service and use AWS Lambda Add Topics to CloudWatch Send SNS text message on phone number given. Requisites To create Topic in SNS Service and use AWS Lambda Add Topics to CloudWatch, we need not follow the steps given below − Create Topic in SNS Create Role for permission in IAM Create AWS Lambda Function Publish to topic to activate trigger Check the message details in CloudWatch service. To send SNS text message on phone number given, we need to do the following − Add code in AWS Lambda to send message to your phone. Example In this example, we will create a topic in SNS. When details are entered in the topic to publish, AWS Lambda is triggered. The topic details are logged in CloudWatch and a message is sent on phone by AWS Lambda. Here is a basic block diagram which exp