> For the complete documentation index, see [llms.txt](https://shahidmobin.gitbook.io/linux/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://shahidmobin.gitbook.io/linux/learn-cybersecurity-with-shahid-mobin/python.md).

# Python

Using python to interact with OS

### Virtual Environment

```
$ pip install virtualenv

$ python3 -m venv env
$ source env/bin/activate
(env) $ deactivate
```

Division (`/`) always returns a float. To do [floor division](https://docs.python.org/3/glossary.html#term-floor-division) and get an integer result (discarding any fractional result) you can use the `//` operator; to calculate the remainder you can use `%`:

In interactive mode, the last printed expression is assigned to the variable `_`. This means that when you are using Python as a desk calculator, it is somewhat easier to continue calculations,

```
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06
```

strings, which can be expressed in several ways. They can be enclosed in single quotes (`'...'`) or double quotes (`"..."`) with the same result [2](https://docs.python.org/3/tutorial/introduction.html#id4). `\` can be used to escape quotes

Two or more *string literals* (i.e. the ones enclosed between quotes) next to each other are automatically concatenated

concatenate variables or a variable and a literal, use +

Note that since -0 is the same as 0, negative indices start from -1.

In addition to indexing, *slicing* is also supported. While indexing is used to obtain individual characters, *slicing* allows you to obtain substring:>>>

```
>>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
'tho'

```

## Regular Expression

{% embed url="<https://docs.python.org/3/howto/regex.html>" %}

the regular expression `test` will match the string `test` exactly. (You can enable a case-insensitive mode that would let this RE match `Test` or `TEST` as well; more about this later.)

Here’s a complete list of the metacharacters; their meanings will be discussed in the rest of this HOWTO.

```
. ^ $ * + ? { } [ ] \ | ( )
```

A **Re**gular **Ex**pression (RegEx) is a sequence of characters that defines a search pattern. For example,

```
^a...s$
```

The above code defines a RegEx pattern. The pattern is: **any five letter string starting with a and ending with s**.

```
import re

txt = "The rain in Spain"
x = re.search("^The.*Spain$", txt)


pattern = '^a...s$'
test_string = 'abyss'
result = re.match(pattern, test_string)

if result:
  print("Search successful.")
else:
  print("Search unsuccessful.")	
```

{% embed url="<https://www.programiz.com/python-programming/regex>" %}

##