Linux Notes

Notes and practices to prepare for various Linux certificates like LPI Linux Essentials, LPIC 101 and Redhat RHCSA exam

Introductory of Linux (LPI Linux Essentials)

Completed Topics:

Connect to the VNC using Linux Academy CentOS7 server

Install CentOS7 w/ Virtual Box

Server Applications

Network Protocols - mostly use ports to transfer data.

Common Network Ports (Connections for Clients)

Port 22 - SSH (Open SSH)
Port 23 - Telnet (telnetd)
Port 25 - SMTP (Sendmail)
Port 53 - DNS (Bind, named)
Port 67 - BOOTP (dnsmasq, dhcpd)
port 80 - HTTP (Apache)
Port 443 - HTTPS (Apache)

Development Languages

Software Development Models

Cathedral Model (Organized, people will see the code once it's released)
The Bazaar Model (less organized, and relies heavily on developers' contributions)

Package Management Tools and Repositories

Each package contains of

Dependency Information
Version Information
Architecture Information
Binary packages are packages that have an executable from source

Two common package systems are usually used today.

RPM (Redhat)
Debian

Debian based package manager uses apt

RPM based package manager = yum

RPM BASED COMMANDS!! Yum is tied to repos as RPM is tied to packages

rpm -ihv - install and shows the progress in verbose mode (i,h,v)
nano-2.2.6-1.x86_64 - package_name-version-buildNumber.architecture
rpm -qi nano - displays the detail information about the package
rpm -e nano - Remove rpm package
yum search apache2 - search package in the yum package manager (apache2 is httpd.x86_64)
yum check-update package name - Check for updates
yum deplist httpd.x86_64 - list dependecies

Useful Commands

ls -p - -p flag indicates they are folders.
ls -R - Displays the contents inside a directory
halt - Shuts down the Linux operating system when super user or root user
reboot - Shutdown & Reboot
init 0 - Just like Shutdown (halt)
init 6 - Just like Reboot
su - switch user
top - lists all the application and processes
netstat - shows the statuses of the network and much more details
route - view a routing table and manipulate routing tables
ifconfig - net configurations, can also modify configuration settings
ip addr - shows ip addresses (only in newer version of Linux)
uname - turns information about Linux
- -s - displays the Linux kernel's name
- -n - displays the host name
- -r - displays the Linux release number
- -v - displays ther version number
- -m - displays the hardware architecture
- -p - displays the processor type
- -i - displays the hardware platform
- -o - displays the OS

Globbing

ls ??.txt - find two letters.txt files
ls *.txt - find all files that ends with .txt
ls [F]*.txt - search for all txt files that starts with capital F
ls f[igh][lae]e*.txt - first letter is f, second character consist of igh, third character with [lae], fourth letter of e.

Obviously, don't use wildcards in the file name!

man

man -k network - search anything related to network

Linux File Systems

bin - consists of executable files
boot - boot related files
dev - various hardware files
etc - text based configuration files and services running
lib - code library for files in the bin/
mnt - external drives
proc - Does not exist in our file system, sudo file system that is dynamically created when accessed.

Commands

grep -in james ./txt2.txt
find . -type f - can work with directories as well, -type d
find . -type f | grep file - find and pipe out the current directory and grep a file that includes file
cut -d" " -f3 file2.txt | cut -c2 - cut delimeter by space of the field 3 of file2.txt then pipe out and cut column 2
wc -w file2.txt | cut -d" " -f1 - only display the counted word, and not the file name

Input Output Redirection

tail /var/log/messages 1> logtemp.txt - redirecting the output of /var/log/messages to a new file called logtemp.txt. Output is redirected by using >
cat wrong_file.txt 2> error.txt - Redirect the stdError output to the error.txt. This copies the error message to the error.txt, whereas 1> redirects the correct stdInput to the new file.
ps >> runningproccess.txt - APPEND the output ps command to the runningproccess.txt file.
- 1 > will replace or create a file, whereas 2 >> will append it to the file.

To log the information, we typically do below command to separate the stdinput and stderror

mount 1> mountfile.txt 2> mounterror.txt - Add correct output to mountfile, if error occurs, logs it to mounterrortxt
sort < words.txt - the less than sign, < will input. In this case, the contents in the words.txt file will be inputted to the sort command

Regex

[asd] - matches any in the bracket
^music - matches the beggining of a file that starts with this word
music$ - matches end of the file
. - any character
grep ^.b abc.txt - anything that begins with somethingb
grep ^...$ abc.txt - grep anything that starts and end with only 3 characters ...
ls file* - will return anything that starts with file. file1, file2, file3 blahblah
grep [ser] ./hostnames - grep files that contains any of the [ser]. Will return files with only er or r.
grep [^ser] ./hostnames - grep files that DOES NOT CONTAIN [ser]. Different from outside of bracket ^

Scripting

if/else configs
Loops

for i in `seq15`
  do
    echo "The current number is $i"
  done
exit 0

91juhwang / Linux-Notes