Difference between revisions of "BASH/C2/Conditional-execution/English-timed"

| Dear friends, Welcome to the spoken tutorial on '''Conditional execution in Bash.'''

|  In this tutorial, we will learn

| * Use of '''test''' command

| and '''Conditional''' Statements

| If not, for relevant tutorials please visit our website which is are shown.

|  For this tutorial I am using  

| * '''Ubuntu Linux 12.04 ''' OS  

| and * '''GNU Bash''' version '''4.1.10'''

| '''GNU Bash '''version '''4''' or above is recommended for practice .

| * '''test''' is a '''built-in''' command, which returns the '''exit status.'''

| * It returns '''0''' Zero for '''True''' and '''1''' One for False.

| *  '''Return '''value''' depends on the evaluation of the expression.

| * One can get the '''return''' status by typing '''Dollar and question mark sign ($?)'''  

| * An expression can be evaluated in two ways-

| # One by using the keyword ''' test'''

| And Other by using the expression enclosed within square brackets.

|  Now open the '''terminal''' by pressing '''Ctrl+Alt and '''T''' keys simultaneously.

| Type: '''test space 4 space hyphen eq space 4 semicolon space echo space dollar sign and a question mark. Press ''' Enter'''

| It returns ''' zero ''' which means ''' true'''  

| '''4''' is equals to '''4'''

| Next type:

| '''opening square bracket space 4 space hyphen eq space 4 space closing square bracket semicolon space echo space dollar sign and a question mark. Press '''Enter'''.

|  It returns '''zero''' which means ''' true'''  

'''test space 4 space hyphen eq space 5 semicolon space echo space dollar sign question mark .'''

Press''' Enter'''

| It returns '''one''' which means '''false''' .

|  Now let's write the same expression within square brackets, type:

| It also returns '''one''' which means '''false'''  

| i.e.''' 4''' is not equal to '''5'''

| Please type '''man space test''' and explore its usage.

| Now let us go back to our slides.

| '''if space opening square bracket space expression space closing square bracket semicolon space then'''

| On the next line,type '''commands''' or  '''statements''' that you want to execute.

| The Basic rules of condition are:

| Always keep spaces between the brackets and the expression.

| Always terminate the line using '''semicolon''' before keyword “'''then”'''.

| '''Semicolon''' is used to terminate the statement or an expression.

|  It is recommended'' ''to quote string variables, if you use them in conditions.

| Don't forget to close the conditional block with “'''fi”.'''

| I will open an already existing script file named '''simpleif.sh'''

| This Bash script displays the message “count is 100” when the '''count''' equals to 100

| '''An integer 100''' is assigned to a '''variable count.'''

| Note that, there should not be any space between''' count, <nowiki>=</nowiki>''' and '''100.'''

| This expression checks whethe '''count''' is '''equal to hundred.'''

| Here ''' -eq ''' is '''comparison operator''' .

| If the condition is true, it will display the message '''count is 100'''

| To make the file executable, type: '''chmod space plus x space simpleif.sh''' and Press '''Enter'''.

| Let me clear the prompt.

| Now type dot slash '''simpleif.sh''' Press '''Enter'''.

| Try changing the value of '''variable '''count ''' and execute the script.

|  Switch back to slides.  

| On the next line, type '''commands'''

| On the next line, there is '''else''' statement

| And again type '''some other commands'''.

| This is the '''shebang line'''

| Here,'''abc123''' is stored in the variable '''PASS'''

| It is stored in a variable''' mypassword'''

| If the password matches, it will display the'' ''message

| else it will display '''“Access denied”'''

| '''fi '''is the end of '''if-else loop'''

|  Now save the file by pressing “'''Ctrl s”''' .

 

'''chmod''' space '''plus x''' space '''ifelse.sh''' .Press '''Enter.'''

| I will type '''abc''' press '''Enter'''

| As the password entered is wrong, it displays the message as ''' “Access denied”'''

| Let's execute again, but this time we will enter password as '''abc123'''

| This bring us to the end of this tutorial.

| Come back to our slides and summarize.

| In this tutorial we have covered Usage of test command , Simple ''' if''' statement & '''if else '''statement .

|  As an assignment

| If the username matches, it should greet you by displaying “'''Hello'''”

| # Else, it should display “'''Try again'''”

| HINT: Your system's username is stored in a variable '''$USER'''

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| It summarizes the Spoken Tutorial project  

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|  The Spoken Tutorial Project Team

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