==================
Linux and Linux API
==================

Linux
=====

Linux is one of the most popular operating systems for servers and cloud-based infrastructures. It gives access to a robust CLI (Command Line Interface), a scripting environment, essential tools and utilities, strong security features, and powerful diagnostic tools for troubleshooting. In order to master the art of delivering high-quality software and infrastructure, it is required for a DevOps Engineer to master Linux.

The REPL: Read iput, Evaluate command, Print input, Loop back to read input.

Opening Terminal (macOS):

.. code-block:: python
   :caption: ``pwd - print working directory``

   $ pwd
   /Users/TeslaPython

.. code-block:: python
    :caption: ``ls - list``

    $ ls
    Applications
    Desktop
    Documents
    Downloads
    library
    Movies
    Music
    Music
    Pictures
    Public

.. code-block:: python
    :caption: ``list -l(long) and -h(human-readable)``

    $ ls -l -h
    total 8
    drwx------@  5 TeslaPython  staff   160 Mar 25 10:26 Applications
    drwx------+ 19 TeslaPython  staff   608 Mar 18 21:20 Desktop
    drwx------+  6 TeslaPython  staff   192 Feb  1 22:22 Documents
    drwx------@ 50 TeslaPython  staff  1600 Apr  4 11:35 Downloads
    drwx------@ 88 TeslaPython  staff  2816 Mar 16 19:28 Library
    drwx------   3 TeslaPython  staff    96 Dec 30 10:02 Movies
    drwx------+  4 TeslaPython  staff   128 Dec 30 11:08 Music
    drwx------+  5 TeslaPython  staff   160 Jan 25 18:39 Pictures
    drwxr-xr-x+  4 TeslaPython  staff   128 Dec 30 10:02 Public

Now, we explain this:

.. image:: _images/linux_explained.png
   :alt: Linux Image
   :align: center

.. code-block:: python
    :caption: ``cd - change directory``

    $ pwd
    /Users/TeslaPython
    $ cd projects
    $ pwd 
    /Users/TeslaPython/projects

.. code-block:: python
    :caption: ``find - find files``

    $ find / TeslaPython

It means the above will search "/" the whole system with the name "TeslaPython". Keep in mind that when you are not executing this as the all powerful root (administrator) user, "find" will not have permissions to list the contents of many directories, so you will receive output like find: '/root': Permission denied

.. code-block:: python
    :caption: ``fing - find files``

    $ find / -type d -name TeslaPython
    
The above will search "/" the whole system for a directory "-type d" with the name "TeslaPython"

.. code-block:: python
    :caption: ``less - Reading files``

    $ less teslapython.txt
    alabaster==1.0.0
    asgiref==3.8.1
    babel==2.17.0
    cachetools==5.5.2
    certifi==2025.1.31
    
For example, the above will read "teslapython.txt" file, to quit hit q.

.. code-block:: python
    :caption: ``mkdir - make a directory``

    $ mkdir TeslaPython
    $ ls
    TeslaPython

.. code-block:: python
    :caption: ``rmdir - remove (empty) directories``

    $ rmdir TeslaPython
    $ ls
    (nothing)

.. code-block:: python
    :caption: ``rm - remove (not empty) files and directories``

    $ rm filename
    
.. code-block:: python
    :caption: ``rm -rf remove (not empty) files and directories``

    $ rm -rf filename

The above will let you delete directories that are critical to the operation of your system. You'd like to delete the root directory, which contains everything on the system. -rf means recursively force.

Move or rename files and directories. First, we'll create a file using touch:

.. code-block:: python
    :caption: ``mv - move or rename files and directories``

    $ touch teslapython.txt
    $ ls
    teslapython.txt

Then, we'll rename the file in place, this command would overwrite any existing file name teslapython.txt:

.. code-block:: python
    :caption: ``mv - move or rename files and directories``

    $ touch teslapython.txt docsteslapython.txt
    $ ls
    docsteslapythonlab.txt

To move the file to a new directory, we'll create a new directory and then move the file there:

.. code-block:: python
    :caption: ``mv - move or rename files and directories``

    $ mkdir teslapythondir
    $ mv docsteslapython.txt teslapythondir/
    $ ls teslapythondir/
    docsteslapython.txt

Remember you can always skim the relevant information using specific flags, "man" (man means manual) will print something like this:

.. code-block:: python
    :caption: ``man - getting help``

    $ man find
    NAME
     find – walk a file hierarchy

    SYNOPSIS
     find [-H | -L | -P] [-EXdsx] [-f path] path ... [expression]
     find [-H | -L | -P] [-EXdsx] -f path [path ...] [expression]

    DESCRIPTION
     The find utility recursively descends the directory tree for each path listed, evaluating an expression (composed of the “primaries” and “operands” listed below) in terms of each file in
     the tree.

     The options are as follows:

     -E      Interpret regular expressions following the -regex and -iregex primaries as extended (modern) regular expressions rather than basic regular expressions (BRE's). The re_format(7) manual page fully describes both formats.

Or using "apropos"

.. code-block:: python
    :caption: ``apropos - display online manual documentation pages``

    $ apropos find
    git-bisect(1)            - Use binary search to find the commit that introduced a bug
    git-cherry(1)            - Find commits yet to be applied to upstream
    git-merge-base(1)        - Find as good common ancestors as possible for a merge
    git-name-rev(1)          - Find symbolic names for given revs
    git-pack-redundant(1)    - Find redundant pack files
    strings(1)               - find the printable strings in a object, or other binary, file
    BIO_find_type(3ssl), BIO_next(3ssl), BIO_method_type(3ssl) - BIO chain traversal

Or using "whatis"

.. code-block:: python
    :caption: ``whatis - display online manual documentation pages``

    $ whatis find
    git-bisect(1)            - Use binary search to find the commit that introduced a bug
    git-cherry(1)            - Find commits yet to be applied to upstream
    git-merge-base(1)        - Find as good common ancestors as possible for a merge
    git-name-rev(1)          - Find symbolic names for given revs
    git-pack-redundant(1)    - Find redundant pack files
    strings(1)               - find the printable strings in a object, or other binary, file
    SPI_cursor_find(3)       - find an existing cursor by name
    XkbFindOverlayForKey(3)  - Find the alternate name by using the primary name for a key that is part of an overlay
    XkbLookupKeyBinding(3)   - Find the string bound to a key by XRebindKeySym
    XkbLookupKeySym(3)       - Find the symbol associated with a key for a particular state
    XkbTranslateKeySym(3)    - Find the string and symbol associated with a keysym for a given keyboard state

Connecting to  Remote Servers via SSH

SSH stands for Secure Shell. It is a cryptographic network protocol that allows secure communication between two systems. The default port for SSH is 22

.. code-block:: python
    :caption: ``ssh``

    $ ssh -p 443 teslapython@216.24.57.252

Linux API
=========

Why Linux API?

- Development
- Malware analysis
- Vulnerability analysis and exploitation
- Reversing
- Diagnostics
- Debugging
- Monitoring
- Memory forensics
- Crash and hang analysis
- Secure coding 
- Static code analysis
- Trace and log analysis

What Linux API?

- Source code prespective
- ABI (Application Binary Interface) perspective: Libraries and Syscalls

`Downloading WinDbg`_ or `Debugging Tools for Windows`_

.. _Downloading WinDbg: https://learn.microsoft.com/en-gb/windows-hardware/drivers/debugger/
.. _Debugging Tools for Windows: https://learn.microsoft.com/en-us/windows-hardware/drivers/debugger/debugger-download-tools