FlowControl

There is already a note here where I explained how if-else clauses control the flow of the script, which are also part of flow control.

Loops:

Loops are components that can increase efficiency and enable error-free processing. Each control structure is either a branch or a loop. Logical expressions of Boolean values usually control the execution of a control structure. These control structures include:

Branches:
- If-Else Conditions
- Case Statements
Loops:
- For Loops
- While Loops
- Until Loops

For loops

The for loop is executed on each pass for precisely one parameter, which the shell takes from a list or from another data source. The for loop runs as long as it finds corresponding data. For example, for loops are often used when we need to iterate over many values in an array. This can be used to scan different hosts or ports.

#!/bin/bash

for variable in 1 2 3 4
do
	echo $variable
done

#!/bin/bash
for variable in file1 file2 file3
do
	echo $variable
done

#!/bin/bash
for ip in "10.10.10.170 10.10.10.174 10.10.10.175"
do
	ping -c 1 $ip
done

We can do the same thing in the shell:

sosoBoy@htb[/htb]$ for ip in 10.10.10.170 10.10.10.174; do ping -c 1 $ip;done

PING 10.10.10.170 (10.10.10.170): 56 data bytes
64 bytes from 10.10.10.170: icmp_seq=0 ttl=63 time=42.106 ms

--- 10.10.10.170 ping statistics ---
1 packets transmitted, 1 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 42.106/42.106/42.106/0.000 ms
PING 10.10.10.174 (10.10.10.174): 56 data bytes
64 bytes from 10.10.10.174: icmp_seq=0 ttl=63 time=45.700 ms

--- 10.10.10.174 ping statistics ---
1 packets transmitted, 1 packets received, 0.0% packet loss
round-trip min/avg/max/stddev = 45.700/45.700/45.700/0.000 ms

(Check here to revisit what #ttl and #hop are)

while loops

The while loop follows the following principle:

A statement is executed as long as a condition is fulfilled (true).
A while loop needs some sort of a counter to orientate itself when it has to stop executing the commands it contains.
we can also use the command "break," which interrupts the loop when reaching this command like in the following example:

#!/bin/bash
count=0

while [ $counter lt 10 ]
do
	#Increase counter by 1
	((counter++))
	echo "Counter: $counter"
	
	if [ $counter == 2 ]
	then
		continue
	elif [ $counter == 5 ]
	then
		break
	fi
done

Execution:

sosoBoy@htb[/htb]$ ./WhileBreaker.sh 
Counter: 1 
Counter: 2 
Counter: 3 
Counter: 4
Counter: 5

until loop

The code inside a until loop is executed as long as the particular condition is false.

#!/bin/bash
count=0

until [ $counter -eq 10 ]
do
	#Increase counter by 1
	((counter++))
	echo "Counter: $counter"
done

Execution:

sosoBoy@htb[/htb]$ ./Until.sh
Counter: 1 
Counter: 2 
Counter: 3 
Counter: 4 
Counter: 5 
Counter: 6 
Counter: 7 
Counter: 8 
Counter: 9 
Counter: 10

Do exercise: