# Module 5 (Bash Script)
### Intro to Bash Scripting ```bash #!/bin/bash VAR="world" echo "Hello $VAR!" # => Hello world! ``` > ```markup > #!/bin/bash => To Know this script run in env /bin/bash > ``` **Shell Scripting – Shell Variables** * **Valid Variable Names** ``` ABC _AV_3 AV232 ``` * **Invalid variable names** ``` 2_AN !ABD $ABC &QAID ```
### Debugging in Bash Errors will inevitably occur when you’re developing bash scripts. Luckily, debugging the script is quite intuitive. An easy way to check for mistakes early is by running the script using the `-n` parameter. This parameter will read the commands in the script but won’t execute them**, so if there are any syntax errors,** they will be shown on the screen. you can think of it as a dry-run method to test validity of syntax: > bash -n script.sh ipt.sh You can also use the `-x` parameter to turn on verbose mode > bash -x script.sh If you want to start debugging at a given point in the script, you can do this by including the set command in the script itself. > \#!/bin/bash > > set -x > > \--snip-- > > set +x ### Variables The following rules govern the naming of bash variables: • They can include alphanumeric characters. • They cannot start with a numerical character. • They can contain an underscore (\_). • They cannot contain whitespace. ```bash book="black hat bash" echo "This book's name is ${book}" ``` **Unassigning Variables** ``` $book="Black hat bash" unset $book echo "$(book)" ``` **Scoping Variables** **Global** variables are those available to the entire program. But variables in bash can also be **scoped** so that they are only accessible from within a certain block of code. These variables are called local variables and are declared using the **local** keyword. The following script shows how **local** and global variables work: ```bash #!/bin/bash PUBLISHER="No Starch Press" #Global variable print_name(){ local name #local can't print this variable out of this function name="Black Hat Bash" echo "${name} by ${PUBLISHER}" } print_name echo "The variable name = ${name} will not be printed because it is a local variable." ``` > remembering to set the executable permission using chmod, and run it using the following command: ./local\_scope\_variable.sh > > chmod +x script.sh => -x to be execution > **Note** > > Use **`/dev/nul`**l with caution. You may miss out on important errors if you choose to redirect output to it. When in doubt, redirect standard streams such as standard output and standard error to a dedicated log file instead. | Operator | Description | | -------- | ----------- | | + | Addition | | -- | -------------------------- | | - | Subtraction | | \* | Multiplication | | / | Division | | % | Modulo | | += | Incrementing by a constant | | -= | Decrementing by a constant | **Arrays** ```bash #!/bin/bash #set array IP_ADDR=(192.168.1.1. 192.168.1.2 192.168.1.3 192.168.2.1 192.168.2.3) #Print all element in array echo "${IP_ADDR[*]}" #print only the first element echo "${IP_ADDR[0]}" ``` ### Control Operators
OperatorDescription
&Sends a command to the background.
&&Used as a logical AND. The second command in the expression will beevaluated only if the first command evaluated to true.
( and )Used for command grouping
;Used as a list terminator. A command following the terminator will run after the preceding command has finished, regardless of whether it evaluates to true or not.
;;Ends a case statement.
|Redirects the output of a command as input to another command.
||Used as a logical OR. The second command will run if the first one evaluates to false.
**Redirection Operators**
OperatorDescription
>Redirects stdout to a file
>>Redirects stdout to a file by appending it to the existing content
&> or >&Redirects stdout and stderr to a file
&>>Redirects stdout and stderr to a file by appending it to the existing content
<Redirects input to a command
<<Called a here document or heredoc, redirects multiple input lines to a command
|Redirects output of a command as input to another command

### **If / Else/ Elif Statements** * Syntax ```shell if[] then . . . . fi ``` Example add Tow Numbers and sum: ```bash #!/bin/bash read -p "Enter number 1: " num1 #read to input from user read -p "Enter number 2: " num2 if sum=$((num1 + num2)) echo "The sum of $num1 and $num2 is: $sum" # echo to output | print #Using condition #!/bin/bash read -p "Enter number 1: " num1 # input form user read -p "Enter number 2: " num2 if [ $num1 -gt $num2 ]; then sum=$((num1 + num2)) echo "The sum of $num1 and $num2 is: $sum" elif [ $num1 -eq 0 ] && [ $num2 == 0 ]; then echo "num1 = 0 num2 = 0 cant calculated" else echo "$num1 is not greater than $num2, so no sum is calculated." fi ``` ### **Function** **this is function showuptime** ```bash #!/bin/bash showuptime(){ up=$(uptime -p | cut -c4-) since=$(uptime -s) cat << EOF ---- this machine has been up for ${up} it has been runnin ${since} ---- EOF } showuptime Outout:- this machine has been up for 3 hours, 39 minutes it has been runnin 2024-06-25 21:16:47 ``` Using **`local`** varibale ```bash #!/bin/bash up="before" since="after" showuptime(){ local up=$(uptime -p | cut -c4-) #Local vat since=$(uptime -s) #Global var cat << EOF ---- this machine has been up for ${up} it has been runnin ${since} ---- EOF } showuptime echo ${up} echo ${since} output:- his machine has been up for 3 hours, 39 minutes it has been runnin 2024-06-25 21:16:47 ---- before it has been runnin 2024-06-25 21:16:47 ``` ### AWK In Bash Scripting 
Ex1:-
echo "Just enter your name: abdo " | awk -F: '{print $2}'

output:- 
abdo

Ex2:-
echo "Just enter your name, abdo ,ali" | awk -F, '{print $2,$3}'

output:- 
abdo ali
> ## [Bash Scripting Tutorial for Beginners](https://www.youtube.com/watch?v=tK9Oc6AEnR4) ### EX1: Sub\_finder.sh ```bash #!/bin/bash for sub in $(cat subdomian.txt) do if [[ $(ping -c 1 $sub 2> /dev/null) ]] then echo $sub >> valid_sub.txt echo "$sub ++++++ Pong" else echo "$sub ------ Error" fi done for ip in $(cat valid_sub.txt) do host $ip echo host $ip | cut -d " " -f 4 | head -n 1 >> ips.txt done for sc in $(cat ips.txt) do nmap $sc done ```