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

|'''Time'''

|'''Narration'''

| 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 as 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 equal 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'''.

| Let's take another expression. Type:

'''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:

| opening square bracket space '''4''' space hyphen '''eq''' space '''5''' space closing square bracket semicolon space '''echo''' space dollar sign and a question mark. Press '''Enter'''.

| 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'''.

| Lastly, end the '''if''' loop with '''fi.'''

| The basic rules of '''condition''' are:

| 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 '''condition'''.

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

| Let us see an example on '''if''' statement.

| Come back to our '''terminal'''.

| 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.

| This is the first line of '''Bash shell script''' known as '''shebang line.'''

| 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 whether '''count''' is equal to hundred.

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

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

| Now '''save''' the file by pressing''' Ctrl + S'''.

| 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.

|  Now switch back to our '''slides'''.  

| On the next line, type commands.

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

| And again type some other commands.

| on the next line, type '''fi''' to end '''if''' block.

| Come back to the '''terminal'''.

| I will open the file '''ifelse.sh'''.

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

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

| As '''abc123''' is a string, it should be written within double-quotes.

| The '''read''' command reads one line of data from the '''standard input.'''

| In this case, '''standard input''' is our keyboard.

| Hyphen '''s''' is for '''silent mode'''

| which means the entered password will not be displayed as we type.

| Hyphen '''p''' is for '''prompt'''.

| It will display a string “'Enter password: ” before it takes '''input''' from user.

| '''mypassword''' is a variable.

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

| If the password matches, it displays the message:

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

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

|  Now '''save''' the file by pressing '''Ctrl, s'''.

| Come back to our '''terminal''', make the file executable. Type:'''chmod''' space '''plus x''' space '''ifelse.sh'''. Press '''Enter.'''

| Type: '''dot slash ifelse.sh .''' Press ''' Enter.'''

| '''Enter password''': 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'''.

| It is displayed as: '''Password accepted'''.

| This brings 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 and '''if-else '''statement.

|  As an assignment-

| Write a script, take your name as an input.

| It should check this name with your system's username.

| 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'''.

| Watch the video available at the link shown below.

| It summarizes the Spoken Tutorial project.

| If you do not have good bandwidth, you can download and watch it.

|  The Spoken Tutorial Project team:

| Conducts workshops using spoken tutorials.

| Gives certificates to those who pass an online test.

| Spoken Tutorial project is a part of the "Talk to a Teacher" project.

| It is supported by the National Mission on Education through ICT, MHRD, Government of India.

| More information on this mission is available at the link shown below.

| This is Ashwini from IIT Bombay, signing off.