Linux Bash Scripting Tutorial

---

Functions

Functions
----------------
A function is a subroutine, code block that implements a set of operations, a "black box" that performs a specified task. or you can call it as a group of commands. shell functions let you to modularize your program by dividing it up into seperate tasks. functions are used wherever there is repetetive code. functions should be declared at the begining of the script. A functions is essentially a code block, which means stdin can be redirected function may have local variables, and may be recursive. Bash uses dynamic scoping model, whereby variables declared with local are visible inside that function and in other functions it calls. A function may use RETURN commands to return an exit status to the caller/calling shell program. it is also possible to have nested functions, i.e functions within functions A Bash function is a block of reusable code designed to perform a particular operation. Once defined, the function can be called multiple times within a script. or you can say group of commands called for later execution. functions are used mainly to avoid repetetive code.

syntax:
     function funtionname{

		command...
	} [redirections]


  or 

function_name() {

	command ....
} [redirections]

Defining & calling functions in Bash

defining function sayHello using syntax 1, it has single command called echo

	#!/usr/bin/bash
	function sayHello {
		echo "Hello World!";
	}

Calling function within srcipt

	sayHello;

defining function sayHello using syntax 2,

	#!/usr/bin/bash
	function sayHello() {
		echo "Hello World!";
	}

	sayHello;

space should be given after function name and before curly braces.

Passing parameters to Functions

is there a way to pass arguments to functions, answer yes,unlike 'C' programming where function accepts parameters,But in Bash functions accepts paramaters using positional arguments($1,$2,$3 etc.,)

passing numbers to bash function

#!/usr/bin/bash
        function sum() {

        s=$(( $1+$2+$3 ))
        echo "sum of numbers :${s}";
        }
        sum  10 20 30

output

sum of numbers :60

find a filename which has lengthy name in a given directory

In this example maxLength function dir variable has directory name,$HOME (shell variable expansion occurs)+Downloads. files variable has only files in a given directory. loop through all files using for loop. extract only file name using basename shell built-in function. If stament checks for is length of the file is greater than the max,if true, sets max value with current file name length,and records filename in name variable.

#!/usr/bin/bash

 function maxLength() {

        dir="$HOME/Downloads" 
        files=`find $dir  -maxdepth 1  -type f`; 

        name='';
        for file in ${files}
        do
                file=`basename "${file}"`
                if [[ ${#file} -gt "${max}" ]];
                then
                        max=${#file};
                        name=${file}
                fi

        done

        echo -e "file in a dir=${dir} \n filename:${name} \n length:${max}";
        }

        maxLength;

Functions and local variables & Global variables

      Variables declared outside the function are called as global variables. Variables declared within the function are called as local variables.
scope of Global variables is from beginning of the script to end of the script. whereas scope of local variables within the function only.

Accessing Global variable name in function test

	name="jhon"
	function test()
	{
		echo $name;
	}
	test;

output:  jhon will be printed

Local variables superceds Global variables,in order to avoid class between local & global variables, different names should be followed.

	name="jhon"
	function test()
	{
		name="peter";
		echo $name;
	}
	test;

output:  "peter" will be printed

Local variables declared within function can be accessed the functions it is calling

	function f1()
	{
		name="jhon";
		echo "name before calling f2 function $name";
		f2;
		echo "name After calling f2 function $name";
	}
	function f2()
	{
		name=${name}" peter"
	}

	f1;

output:

name before calling f2 function "jhon"
name After calling f2 function "jhon peter"

Functions with redirection

       Functions can read input from a file, command or from a Here Document using IO redirection operators

function reading data from a file

	$cat country.txt
Country		Capital 	ISD_CODE

USA		Washington	1
China		Beijing		86
Japan		Tokyo		81
India		Delhi		91

	file="country.txt"
	
	function readFile() {
		
		while read line
		do
			echo $line
		done

	}<${file}

	readFile;

function reading data from a Here Document

        function redir() {
                read line;
                echo $line;

        } <<< $(echo 'one two three')

output:

one two three

adv bash bash ref manual linux shell scripting -