Theory

Linux have one big rule "everything is a file"

Crontab

Cron jobs in Linux allow users to run commands at the specific date and time. To list all scheduled tasks for the user

• crontab -l Look for your crontab file

• crontab -e To edit your crontab file

If this command is run at the first time user will be asked to choose text editor for editing this file.To create a scheduled command user needs to create a records based on the following template

• m h dom mon dow command

Also you could check privileges for files like this, to locate cron jobs:

• /etc/crontab

• /etc/cron.d

• /var/spool/cron/crontabs/root

[Link to make cron date]

Regex Basics

Regular expressions, often referred to as regex or regexp, are a powerful tool for pattern matching and text manipulation. They provide a concise and flexible way to search for, match, and extract specific patterns in text data. Regex is supported in many programming languages and text editors. Here are some of the basics of regex with examples for each feature:

• Literal Characters:

  • A regex can represent literal characters. For example, the regex cat will match the word "cat" in a text.

• Character Classes:

  • Square brackets [] are used to define character classes. For example, [aeiou] matches any vowel, and [0-9] matches any digit.

• Quantifiers:

  • Quantifiers specify how many times a character or group of characters should be repeated.

  • * matches zero or more of the preceding element. For example, ca*t matches "ct," "cat," "caat," and so on.

  • + matches one or more of the preceding element. For example, ca+t matches "cat," "caat," and so on.

  • ? matches zero or one of the preceding element. For example, colou?r matches "color" and "colour."

• Wildcard:

  • The dot . represents any character (except a newline character). For example, c.t matches "cat," "cut," "c8t," etc.

• Anchors:

  • ^ matches the start of a line. For example, ^abc matches "abc" only if it's at the beginning of a line.

  • $ matches the end of a line. For example, xyz$ matches "xyz" only if it's at the end of a line.

• Groups and Alternation:

  • Parentheses () are used to create groups.

  • | represents alternation (OR). For example, (cat|dog) matches either "cat" or "dog."

• Character Escapes:

  • Some characters have special meanings in regex and need to be escaped with a backslash. For example, \. matches a literal period.

• Quantifier Min and Max:

  • Curly braces {} allow you to specify the minimum and maximum number of repetitions. For example, a{2,4} matches "aa," "aaa," and "aaaa."

• Character Shorthands:

  • There are shorthands for common character classes:

    • \d matches any digit (equivalent to [0-9]).

    • \w matches any word character (equivalent to [a-zA-Z0-9_]).

    • \s matches any whitespace character (spaces, tabs, line breaks, etc.).

• Negation:

  • You can use ^ inside a character class to negate it. For example, [^0-9] matches any character that is not a digit.

• Backreferences:

  • You can refer to matched groups later in the regex using backreferences. For example, (abc)\1 matches "abcabc."

• Modifiers:

  • Flags like i (case-insensitive) can be added after the regex to change its behavior. For example, /cat/i matches "cat," "Cat," or "CAT."

Here are some examples of regular expressions and what they match:

• /^abc/ matches "abc" at the beginning of a line.

• /[0-9]+/ matches one or more digits.

• /colou?r/ matches "color" or "colour."

• /[A-Za-z]+/ matches one or more uppercase or lowercase letters.

• /[aeiou]{2,4}/ matches 2 to 4 consecutive vowels.

Logical Operators

Symbols or keywords in Linux that are used to process and compare logical values.

Syntax	Explanation
&&	(AND)
Command1 && Command2	Executes Command2 only if Command1 succeeds (returns a zero exit code). If Command1 fails, Command2 is not executed
II	(OR)
Command1 II Command2	Executes Command2 only if Command1 fails (returns a non-zero exit code). If Command1 succeeds, Command2 is not executed
;	(Semicolon)
Command1 ; Command2	Executes Command2 regardless of the success or failure of Command1. Used for sequential execution of commands
&	(Background Execution)
Command &	Executes the command in the background, allowing you to continue working in the terminal without waiting for the command to complete. You can use the bg or jobs commands to control background processes
!	(NOT)
! Command	Executes the command and returns the reverse exit code. If Command succeeds, ! Command fails, and vice versa
I	(PIPE)
Command1 I Command2	Redirects the output of Command1 to the input of Command2. Used to transfer data between commands and filter the output

Access Rights

Linux has its own scheme for displaying access rights using numbers to make it quick and easy to show which rights are where. Each three digits corresponds to three access rights (read, write, execute) for three different categories of users (owner, group, other). Each of these digits can have a value from 0 to 7, where each value represents a combination of three possible access rights:

0 - (no rights). 1 - (execute). 2 - (write). 4 - (read).

File Rights

To set permissions using numbers, you simply add the number values for each category together: For example, `chmod 755 file.txt' sets the following permissions:

1. The owner has read, write, and execute permissions (4 + 2 + 1 = 7).

2. Group has read and execute permissions (4 + 1 = 5).

3. Other users have read and execute permissions (4 + 1 = 5).

Directory Rights

For example, in the directory, it displays drwxr-xr-x. The first character d is the type of object. In this case, d indicates that it is a directory. The rest of the rwxr-xr-x string consists of nine characters that represent access rights for different categories of users (owner, group, and others). This part of the string can be considered as three sets of three characters each.

The first set rwx represents the access rights for the object owner:

1. r Owner has the right to read the file or view the contents of the directory.

2. - Owner has the right to write or edit the file or create new files in the directory.

3. x Owner has the right to execute the file (for directories, this means the ability to access it as a directory).

The second set of r-x represents the access rights for a user group:

1. r Group has the right to read the file or view the contents of the directory.

2. - Group does not have the right to write the file or create new files in the directory.

3. x Group has the right to execute the file (for directories).

The third set of r-x represents access rights for other users (users who are not owners and do not belong to the group):

1. r Other users have the right to read the file or view the contents of the directory.

2. - Other users do not have the right to write the file or create new files in the directory.

3. x Other users have the right to execute the file (for directories).

Data Streams, Redirects

In Linux, there is such a thing as data streams, which are used to transmit input/output data in programs. Each of them has its own descriptor, a numeric identifier that helps to interact with them more easily

• STDIN - Descriptor 0. Input stream

• STDOUT - Descriptor 1. Output stream

• STDERR - Descriptor 2. Error stream

• The output in the output streams is separate and in parallel.

REDIRECT( > ) is redirect of the command output to the file (ls > dirs.txt). Redirect by default is through the stdout stream, but it can be changed. cat unexistedfile.txt 2> error.txt Reverse redirect ( < ) is redirect of the output stream through stdin so that the input is not through the keyboard but through the file

Examples

2>&1 If we redirect the output from stderr, we output to the same place as the output from stdout. And with this we can output both simple output and errors in one file, one stream.

0>&1 By using 0>&1, you are actually telling the shell to read stdin from the same source as stdout. This can be useful in cases where you need to merge or copy input and output streams for interactive input, for example in reverse shells.

File signatures

File signatures, commonly referred to as "magic bytes", are specific byte sequences at the beginning of a file that identify or verify its content type and format. These bytes often have corresponding ASCII characters, allowing for easier human readability when inspected. The identification process helps software applications quickly determine whether a file is in a format they can handle, aiding operational functionality and security measures.

• In cyber security, file signatures are crucial for identifying file types and formats. You'll encounter them in malware analysis, incident response, network traffic inspection, web security checks, and forensics. Knowing how to work with these magic bytes can help you quickly identify malicious or suspicious activity and choose the right tools for deeper analysis.

Here is a list of some of the most common files and their magic:

File Format	Magic Bytes	ASCII
PNG image file	89 50 4E 47 0D 0A 1A 0A	%PNG
GIF image file	47 49 46 38	GIF8
Windows and DOS executables	4D 5A	MZ
Linux ELF executables	7F 45 4C 46	.ELF
MP3 audio file	49 44 33	ID3

• You could look all magic bytes here [LINK]

Mount

Mount command is used to attach (or "mount") file systems to a directory structure, making storage devices like hard drives, USB drives, or network file systems accessible under a specific directory (mount point). The mounting process allows users and the system to read/write data from/to the storage device.

Regular Mount

mount /dev/sda1 /mnt

Authenticated Mount

sudo mount -t cifs -o username=ven17,password=superkek123,domain=. //13.13.13.13/Share /mnt/Share