# What are loops?
**Loops** are programming structures that repeat a block of program statements until a given condition is met.
# Loops in UNIX/Linux
There are three types of *shell loops* in UNIX/Linux:

* *for* loop
* *while* loop
* *until* loop

# *for* loop
The **for loop** operates on a list of items. The statements in the *for* loop are executed for all items in the list. The *for* loop is initialized as follows:
```c
for var in word1 word2 ... wordN
do
   Statement(s) to be executed
done
```
## Flowchart



## Code
The code below demonstrates the use of the *for* loop.

for a in 10 8 6 4 2
do
  echo $a # print a
done


In first iteration of the loop, `var` will be assigned the #key# first value in the list: 10 #key#. In the second iteration, `var` = 8. In the third iteration, `var` = 6, and so on.

# *while* loop
The statements in the **while loop** are executed until the command in the condition becomes false. The *while* loop is initialized as follows:

```shell
while command
do
     Statement(s) to be executed
done
```
## Flowchart

  

## Code
The code below demonstrates the use of the *while* loop.

a=10

while [ $a -gt 0 ]   # a > 0
do
   echo $a  # print a
   a=`expr $a - 2`   # a = a - 1
done

# *until* loop
The statements in the *until* loop are not executed until the command in the condition becomes true. The *until* loop is initialized as follows:
```shell
until command
do
     Statement(s) to be executed
done
```
## Flowchart


## Code
The code below demonstrates the use of the *until* loop.

a=10

until [ $a -lt 2 ]   # a < 2
do
   echo $a  # print a
   a=`expr $a - 2`   # a = a - 1
done

A variable (`a`) is initialized with `a = 10` in line 1. The *until* loop in line 3 executes until the condition (`a < 2`) is false. When `a < 2` becomes true, the loop breaks.

How to implement shell loops in UNIX/Linux

What are loops? Loops are programming structures that repeat a block of program statements until a given condition is met. Loops in UNIX/Linux There are three types of shell loops in UNIX/Linux:  for loop while loop until loop  for loop The for loop operates on a list of items. The statements in the for loop are executed for all items in the list. The for loop is initialized as follows: for var in word1 word2 ... wordN do    Statement(s) to be executed done  Flowchart 