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https://github.com/jovanyaldair/shell-script-concepts


https://github.com/jovanyaldair/shell-script-concepts

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README 

        
# Shell Script Concepts



## INTRODUCTION



Shell Script is a script-based language, that's, an file with many instructions that will be executed by shell command interpreter (sh, bash, csh, ksh or zsh) . With her it's possible writte shell-script programs to automate tasks on linux.



## FUNDAMENTALS OF LANGUAGE



### Variables



Variables is a space in memory to store values, they can be numeric or alphanumeric values.



To set a variable, we do `VARIABLE=$VALUE`, eg.: `NAME=$"Maria"`

The `""` on Maria, means it's a alpanumeric value.



To Invoque a variable, we use `$VARIABLE`, eg.: `$NAME`



### Logical operators and comparison and Alphanumeric operators



Logical oprators and comparison and alphanumeric operators, show the shell a condition to be treated, and they can be True or False. They are:



#### Logical operators and comparison



* `-lt` --> Less Than

* `-gt` --> Greater Than

* `-le` --> Less Equal

* `-ge` --> Greater Equal

* `-eq` --> Equal

* `-ne` --> Not Equal



#### Alphanumeric operators



* `=`   --> Text Equal

* `!=`  --> Text Different

* `-n`  --> Text not Null

* `-z`  --> Text Null



1st Eg.: Compare if $A is less than $B



`if [ $A -lt $B ] then`



2nd Eg.: Compare if Text in $A is equal to text in $B



`if [ $A = $B ] then`



### Conditions



#### IF



We use the IF condition, to compare two or more conditions before running a command.



##### Structure



`if [ Condition ]; then command fi`



* [ Condition ] --> The condition to run the Condition

* Command --> The command to run

* fi --> End of if



Eg.: Create a file if $CONTENT variable is empty



-> Go to Scripts folder and see the `example01.sh` file



To run this script type on terminal



* `chmod +x example01.sh`

* `./example01.sh`



#### IF ELSE



##### Structure



`if [ Condition ]; then command_1 else command_2 fi`



Eg.: Check if the folder Download is in your current directory. if yes print a verification message, if not, wait 5 seconds and exit program.



 -> Go to Scripts folder and see the `example02.sh` file



### Logical operators of connections



Logical operators allow to connect two or more conditions created with the comparation operators, and create a new event. They are:



#### AND (-a)



With logical operator (and) the conjuntion results `True` if all the comparation conditions are `True`



#### OR (-o)



With logical operator (or) the conjuntion results `True` if one of comparation conditions are `True`



Eg.: Create a script to say Good Morning, Good Afternoon and Good Night, according to the time.



-> Go to Scripts folder and see the `example03.sh` file.



### Loops



Loops are structures used to repeat any instructions that we want



#### Loop FOR



##### Structure



The structure of `FOR` loop is:



`For (( VARIABLE=START_VALUE; VARIABLE> or  Value to start loop

* <> --> range of te loop

* END_VALUE --> Value to end loop range

* ++/-- --> Increment and decrement of loop



Eg.: Create a script to count to 20



-> Go to Scripts folder and see the `example04.sh` file.