Bash Scripting

What it is?

Bash is the scripting language we use to communicate with Unix-based OS and give commands to the system.

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Script Execution - Examples

rio@kali[/kali]$ bash script.sh <optional arguments>
rio@kali[/kali]$ sh script.sh <optional arguments>
rio@kali[/kali]$ ./script.sh <optional arguments>

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Shebang

The #!/bin/bash at the beginning of a Bash script is known as a shebang or hashbang. It serves as a directive to the operating system, indicating which interpreter should be used to execute the script.

Conditional Execution

If-Else

if [condition]
then [execution]
else [what would be executed if condition would fail]
fi [closing]

If-Only

if [condition]
then [execution]
fi [closing]

If-Elif-Else

if [first condition]
then [execution]
elif [second condition]
then [execution]
else [what would be executed if conditions would fail]
fi [closing]

Case

case <expression> in
	pattern_1 ) statements ;;
	pattern_2 ) statements ;;
	pattern_3 ) statements ;;
esac

Example::

case $opt in 
	"1") network_range ;;
	"2") ping_host ;;
	"3") network_range && ping_host ;;
	"*") exit 0 ;;
esac

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Comparison Operators

Operator	Explanation
-eq	equal to
-ne	not equal to
-lt	less than
-le	less than or equal to
-gt	greater than
-ge	greater than or equal to

String Operators

Operator	Description
==	is equal to
!=	is not equal to
<	is less than in ASCII alphabetical order
>	is greater than in ASCII alphabetical order
-z	if the string is empty (null)
-n	if the string is not null

File Operators

Operator	Description
-e	if the file exist
-f	tests if it is a file
-d	tests if it is a directory
-L	tests if it is if a symbolic link
-N	checks if the file was modified after it was last read
-O	if the current user owns the file
-G	if the file’s group id matches the current user’s
-s	tests if the file has a size greater than 0
-r	tests if the file has read permission
-w	tests if the file has write permission
-x	tests if the file has execute permission

Special Variables

$#	Number of arguments passed to the script.
$@	List of command-line arguments.
$n	n is number of argument
$$	Id of executing process
$?	Success of command. 0 is success, 1 is a failure

Regular Variables

> variable="Declared without an error."
> echo $variable
> Declared without an error.

Arrays

> domains=(shadow wizard money gang)
> echo ${domains[0]}
> shadow

OR

> domains=(shadow wizard "money gang")
> echo ${domains[2]}
> money gang

Loops

For

for variable in list
do
    # Commands to be executed for each item in the list
done

Example:

fruits=("apple" "orange" "banana")
for fruit in "${fruits[@]}"
do
    echo "I like $fruit"
done

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While

while [ condition ]
do
    # Commands to be executed while the condition is true
done

Example:

count=1 # Count from 1 to 5
while [ $count -le 5 ]
do
    echo $count
    ((count++))
done

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Until

until [ condition ]
do
    # Commands to be executed until the condition becomes true
done

Example:

count=1 # Count from 1 to 5
until [ $count -gt 5 ]
do
    echo $count
    ((count++))
done

Script Termination

Exit Status	Explanation
exit 0	Succesful execution
exit 1	General error condition
exit 2	Specific error condition

Wildcards

In Bash, a wildcard refers to a character or a set of characters that can be used to represent a group of filenames or strings. Wildcards are often used in commands to perform operations on multiple files or strings that match a specified pattern.

Wildcard	Example Usage	Explanation
* (Asterisk)	echo *.txt	Matches all files ending with ".txt".
? (Question Mark)	ls file?.txt	Matches files like "file1.txt", "fileA.txt", etc.
[ ] (Square Brackets)	ls [aeiou]*.txt	Matches any file starting with a vowel and ending with ".txt".
{ } (Brace Expansion)	cp file{1,2,3}.txt dest/	Expands to "file1.txt", "file2.txt", and "file3.txt" and copies to the destination.
!(pattern) (Extended Pattern Matching)	ls !(file*.txt)	Matches all files except those starting with "file" and ending with ".txt".
?(pattern) (Zero or One Occurrence)	ls file?(1).txt	Matches "file.txt" or "file1.txt".
+(pattern) (One or More Occurrences)	ls file+(1).txt	Matches "file1.txt", "file11.txt", etc.
*(pattern) (Zero or More Occurrences)	ls file*(1).txt	Matches "file.txt", "file1.txt", "file11.txt", etc.

Tips & Tricks

1. You Could use tee command for writing output to both standard output and file. If you would use tee -a, it would

2. Use bash -x -v to verbose debugging