2. Linux Primer for Network Engineers

# Navigating the Linux file system:
Root file system refer to by / aka root directory
sudo = super do, executes a command with root privileges

# Sudo:
Debian/Ubuntu doesn’t allow logging in as root
Redhat/fedora allow a user to log in as root by default
You can configure who can run sudo in the sudoers file
If you are at user@machinesname:~$ running ‘sudo su’ puts you in root shell root@machinename:/# (Ctrl + D to return)

Remember: in Linux terminal $ is an indication that current user has no root privileges, while # indicates you’re root

# Path and Directories:
pwd = prints current working directory
ls = lists content of current directory
man <command> = shows manual of a command

A path is a file or directory's location on a filesystem. There are two types of paths in Linux:
Relative path: relative to the current working directory
Absolute path: relative to the root directory, starts with / e.g: /home/username/Desktop

Note: Linux terminal is case sensitive. So home is not the same as Home

# cd command:
'cd ..' changes to parent directory of current directory
'cd' or 'cd ~' back to home directory (home = /home/username)
'cd .. / ..' back two directories. You can add dots and /s as you wish
'cd -' changes to parent directory of current directory and prints parent directory

# Linux distributions and package managers:
Linux operating systems are distributed as complete collections of software built-around the Linux kernel and some form of package management system. These collections are called distributions (distros), and though there are almost 300 various distributions available today, most are derived from one of two main operating systems: Red Hat and Debian
Distributions are commonly developed to meet specific end-user requirements. For example, Kali Linux is focused on penetration and security testing while OpenWrt is generally focused on home router functionality. CentOS is built for stability, whereas Fedora is considered less stable because it is a testing ground for new features

The two main Linux OS are:
Red Hat Enterprise Linux RHEL = Commercial OS. Derivatives are Fedora and CentOS
Debian = Free and open source, developed under the Debian project. Most popular derivatives are Ubuntu, which has derivatives as well such as Linux Mint and Gnome

# Package management:
Packages are convenient methods to deliver software
Package tool maintains a database of installed applications and their file locations

Two main CLI tools required to install, update and remove software. Installing software package:
RHEL:
rpm = installs a local rpm package (package has a .rpm file extension)
yum = downloads and installs a package from repositories. Also updates and removes
        
Debian:
dpkg = installs a local rpm package (package has a .dpkg file extension)
apt & apt-get = downloads and installs a package from repositories. Also updates and removes

# Working with files and directories:
touch command:
updates time stamp and create empty file
updates time stamp of existing file w/o overwriting it
e.g: touch newfile.txt OR touch ./newfile.txt (./ indicates file is created in current working directory)
    
make directory command:
mkdir = creates a new directory e.g: mkdir newDirectory
mkdir -p = creates a new directory and its sub directories e.g: mkdir -p /newDirectory/newChildDirectory

Remove command:
rm = removes a file
rm -r = removes a directory that contains non-empty directories
rmdir = removes empty directory
rm -rf = removes a directory and contents including write-protected files
    
Copy and move commands:
cp = copies a file e.g: cp file1 file2
cp -r = copies an entire directory
mv = moves or renames a file. eg: mv nexus.txt Testing     (Testing is new directory to move to).mv nexus.txt home/cisco/nexus home/cisco/Testing/
mv nexus.cfg nexus123.cfg  (renames a file. notice destination directory is same as current)
    
Viewing files commands:
more = allows you to view the contents of a file one page at a time and requires you to press the spacebar to advance to the next page of contents. But you can't use up arrow to scroll up
less = allows you to view the contents of the file but it allows the user the ability to scroll up and down and perform a keyword search
cat = (short for concatenate) is used to stream the entire contents of the file without pausing. It is useful for files with just a few lines. Also, cat is used to add contents to a file
e.g: cat router1.txt
     cat cisco/platforms/routers/router1.txt
     cat router2.txt >> router1.txt  (merges two files in router1.txt and leaves router2.txt as is)
     head = displays the first 10 lines in a file
     tail = displays the last 10 lines in a file
     diff = compares content of two files

File permissions:
permissions are based on two factors:
specific user or group
specific action (read (r), write (w), execute (x))
e.g: ls -l config.txt (shows permission of config.txt)
-rw-rw-r-- 1 deeplearning deeplearning 3547 Oct 22 12:04 config.txt
that output is derived from drwxrwxrwx
d = file type
first rwx = owner's permission
second rwx = owner's group permission
3rd rwx = permission of others (others means not the owner nor members of group the owner is a member of)

commands to change permissions:
chmod u(+/-)(rwx),g(+/-)(rwx),o(+/-)(rwx) filename    = this command sets the permission ona given file for current user, group and others
e.g: chmod u+rwx,go+rx  = owner may read, write, and execute the file. All others may read and execute the file
chmod rwx+ugo = no restricitons at all
chmod u+rw,u-x,go-rwx = The owner may read and write a file. All others have no rights

Linux processes:
viewing running processes:
top = provides a real-time look at the local processor activity
htop = provides a real-time look at the local processor activity in easily readable format / graph. Also provides options to search and kill process
ps = lists running processes associated with current user and terminal session, but offers many more arugments such as seeing processes running as certain user
ps aux = provides an exhaustive list of running processes

Note: arguments are additive which mean that they can be strung together to customize an output. For example, in order to display an exhaustive list of running processes that are sorted by processor and memory utilizations, the syntax would be: ps aux --sort=-pcpu,+pmem

Terminating processes:
kill = ends a running process, used along with process id. e.g: kill 2442. kill sends a signal to application to terminate
kill -KILL pid = sends a signal to Linux kernel to terminate a process. last resort. e.g: kill -KILL 2442
killall = kills all processes associated with an application. e.g: killall firefox
pkill = end a process by name

Bonus command:
tree: draws a tree of current directory structure.
e.g: tree /home/deeplearning/Desktop
/home/deeplearning/Desktop
├── baseSwitchConfig.txt
├── CoAC_PY.py
├── swcfg
├── switchconfig.py
└── Untitled Document 1.py