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Next, we set up a 
while
 loop. A 
while
 loop will continue until a condition is met. 
Here, we're telling it to execute the statements in the block over and over until 
$myvar
 
becomes equal to 
15
. In fact, a 
while
 loop can continue forever if you enter something 
incorrectly, which is known as an 
Infinite Loop
. An infinite loop is dangerous, and 
can cause your server to stop responding. If you used 
-ge 0
 instead, you would've 
created exactly that:
do
With 
do
, we're telling the 
for
 loop to prepare itself to start doing something:
    echo $myvar
Here, we're printing the current content of the 
$myvar
 variable—nothing surprising 
here:
    ((myvar++))
With this statement, we're using what's known as an incrementor to increase the 
value of our variable by 
1
. The double parenthesis tells the shell that we're doing an 
arithmetic operation, so the interpreter doesn't think that we're working with strings:
done
When we're done writing a 
while
 loop, we must close the block with 
done
. If you've 
typed the script properly, it should count from 
1
 to 
15
.
Another type of loop is a 
For Loop
. A 
for
 loop executes a statement for every item in 
a set. For example, you can have the 
for
 loop execute a command against every file 
in a directory. Consider this example:
#!/bin/bash
turtles='Donatello Leonardo Michelangelo Raphael'
for t in $turtles
do
    echo $t
done
Let's take a deeper look into what we've done here:
turtles='Donatello Leonardo Michelangelo Raphael'

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Boosting Your Command-line Efficiency