Difference between revisions of "BASH/C2/Basics-of-Shell-Scripting/English-timed"

|* '''System variables'''

|* '''User defined variables''' and

|* Accepting user input via keyboard.

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

|* '''Ubuntu Linux 12.04''' Operating System and  

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

| * '''Bash variables''' provide temporary storage for information.

|* These '''variables''' can be used within the lifespan of the program.

|* There are two types of variables:  

# '''System variables'''

# '''User defined variables '''.

|* '''BASH_VERSION '''

 

|* '''HOSTNAME '''

|* '''HOME etc.'''

| Let us open the terminal by pressing '''Ctrl Alt''' and '''T''' keys simultaneously  on your keyboard.

|Let me clear the prompt.

| Now, type: '''echo''' space within double quotes '''dollar sign''' '''HOSTNAME'''.

|and now press '''Enter.'''

|The '''hostname''' of the system will be displayed.

| Now let's find out the full path of '''home'''directory.

| Type:  '''echo''' space within double quotes '''dollar sign HOME'''(in capital).  

|The full path of user's '''home''' directory will be displayed.

| Now, type:

 

|'''echo''' space within double quotes '''HOME''' (in capital).

|This will display only the '''HOME''' not the value of '''HOME''' variable.  

|So, it is necessary to use '''dollar sign( '$') '''at the beginning of every variable to display its value.

|Let us switch back to our slides.

|* These '''variables''' are created and maintained by users.

|* It is always a good idea to avoid '''uppercase''' for the names of '''user defined variables'''.  

|* This makes it easy to differentiate between '''user defined '''and '''system variables'''.

| Type: '''username equal to sign sunita'''.

|Please note that there should not be any blank space between '''username''', '''equal to '''sign and '''sunita'''.

|Now, press '''Enter.'''

 

| To display the value of '''variable''' '''username''',

|Type: '''echo''' space within double quotes '''dollar sign username'''.

|Let us switch back to our '''slide'''.

|'''unset'''- the value of '''variable''' can be unset by using the '''unset''' '''command'''.

|Let us check. Type: '''echo''' space within double quotes '''dollar sign username ''', press '''Enter.'''

| * In '''Shell '''script,''' user defined variables''' can be declared '''globally''' or '''locally. '''

|* By default, all '''variables''' are '''global.'''

|* This means, their values remain the same inside and outside the '''function'''.  

| Let us learn how to declare '''variables''' '''globally''' and '''locally.'''

| Switch to the '''terminal '''and type:  

|and & (ampersand) is used to free up the prompt.

| Type the code as shown here, in your''' g_(underscore)variable.sh '''file.

| Let me explain the code now.

| The first line with the '''hash '''and '''exclamation '''symbol, is a '''shebang''' or a '''bang''' line.

| '''username=sunita '''is the '''userdefined variable '''and it is declared '''globally.'''

| '''echo '''will display the '''string''' '''outside function: ''' and  

|'''dollar username''' will print the value of the variable '''username.'''

|This is how we defined a '''function''' in '''BASH''' '''script'''.  

|Here another message '''inside function '''will be displayed, along with the value of '''username.'''  

|This is our code. Now let's execute it.

|Now type: '''dot slash g_(Underscore)variable.sh'''

| Observe the output.

 

|Outside the function, '''username '''takes the value '''sunita.'''  

| This is because '''username''' was declared '''globally '''outside the function.

| Next, let us learn how to declare a '''variable''' '''locally'''.

| Type the code as shown here, in your '''l_(underscore)variable.sh '''file.

| Let me explain the code.

| The code is the same as before, except for an extra line of code inside the function.

| Inside the '''function''' block, we have a line-'''local''' '''space username equals to jack'''

| This assigns a new '''value''' for the variable '''username locally'''.

|Let's make file executable.

|By typing '''chmod''' '''space plus x space l_variable.sh'''

|Press '''Enter.'''

 

|The output is displayed.

|This is because '''username''' is assigned this value '''locally,''' within the function.

|The '''read''' '''command''' is used to accept input from the keyboard.

|Press '''Enter.'''

 

| Type the code as shown in your''' read.sh '''file.

| press '''Enter.'''

 

| Type: '''dot slash read.sh'''  press '''Enter.'''

|Let us go back to our slide and summarize.  

|* '''System variables '''

 

|* '''User defined variables'''

|* Accepting user input via keyboard.

|  As an assignment-

 

| Write a simple '''Bash''' program to get the following system variables.

|* '''pwd ''' and * '''logname'''.

|* Write a simple '''Bash''' program

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