wgu Course computer science

D281 - Linux Foundations

Linux Foundations prepares learners for the LPI Linux Essentials certification, and is an introduction to Linux as an operating system as well as an introduction to open-source concepts and the basics of the Linux command line. Learners will gain skills in identifying the fundamentals of open-source software and to develop resources for data access and security.

Resources

• YouTube playlist
• Cisco course
• Study guide
• Reddit overview

What is Linux?

• Usually when people say "Linux" they mean GNU/Linux
  • Combination of software
  • Defines the operating system

FLOSS and FOSS

• FLOSS
  • Free/Libre/Open Source Software
  • Libre is all about freedom; do what you want
• FOSS
  • Free and Open Source Software
• Open source
• FSF
  • GNU came from this
• Licenses range from permissive to more restrictive (copyleft)

Operating systems

• Linux distributions
  • Bundling of software (web browser, email client, package manager, etc.) to make a useful system
  • Run the Linux kernel
    • Kernel is like an air traffic controller, applications are the airplanes
    • Kernel decides which program gets which block of memory, starts/stops applications, handles displaying text, etc.
    • Abstracts things away for applications to access them (eg. writing to storage)
  • Debian based
    • More community focused than some other distros
    • Ubuntu + Ubuntu variants
      • Most popular Debian distro
    • Any .deb based systems
  • Red Hat based
    • RHEL
      • Red Hat Enterprise Linux
    • Fedora
      • Shorter release cycles than RHEL
    • CentOS
      • Recompiles RHEL and gives them away for free
      • No paid support like RHEL
    • RPM (Red Hat Package Manager) based systems
  • Other
    • Arch
    • Slackware
    • SUSE
      • Derived from Slackware
    • Android
      • Most popular Linux distro
      • Uses Dalvik virtual machine
        • Provides a good platform for mobile devices
      • Lacks typical packages like GNU so incompatible with desktop Linux distros
    • Embedded systems
    • Linux From Scratch
      • Teaches you to build a distro from the ground up
• In Linux, file extensions are only useful to the user, the OS ignores them

Decision points

• Role
  • Desktop vs server
• Function
• Life cycle
• Stability
• Compatibility
• Cost
• Interface

Linux

• Life cycle
  • Distros are broadly enthusiast or enterprise
  • Enterprise distros are stable, have good support, regular releases, etc.

Working in Linux

Major applications

• Typically three categories of software
  • Server applications
    • No interaction with I/O (monitor, keyboard)
    • Interacts with other computers
  • Desktop applications
    • Users directly interact with them
  • Tools
    • Make it easier to manage computer systems

Server applications

Email servers

• Mail Transfer Agent (MTA)
  • User to transfer messages to other systems
• Mail Delivery Agent (MDA)
  • Sorts the email in a user's mailbox
  • Invoked from the final MTA in the chain
• POP/IMAP server
  • Post Office Protocol (POP) and Internet Message Access Protocol (IMAP) allow email clients to get emails from email servers

File sharing

• Samba
  • Windows-centric
  • Allows a Linux machine to share files and work on a Windows domain
• Netatalk
  • Allows a Linux machine to perform as an Apple Macintosh file server
  • Network File System (NFS)

Desktop applications

• Email
  • Thunderbird
  • Evolution
  • KMail
• Creative
  • Blender
  • GIMP (GNU Image Manipulation Program)
  • Audacity
• Productivity
  • LibreOffice

Console tools

Shells

• Accepts commands from a user and passes them to the Linux kernel to execute
• Provides a language for interacting with the environment
• Two main shell families:
  • Bourne shell
  • C shell
• More modern shells:
  • Bourne Again Shell (bash)
    • Default on most systems
  • tsch

Text editors

• Fully featured:
  • vi/vim
  • emacs
• Simpler:
  • pico
  • nano

Package managers

• Adding, updating, or removing packages usually requires administrative access

Debian

• .deb extension
• dpkg
  • Lower level tool for managing Debian packages
• apt-get
  • Advanced Package Tool
  • Frontend for dpkg
  • Makes management of packages easier

RPM

• Standard package management system according to Linux Standards Base
• .rpm extension
• rpm
  • Backend to the package management system
  • Install, update, query, or remove packages
• yum and up2date
  • Frontends for RPM
  • Automate resolving dependency issues

Linux in the cloud

• Powers 90% of the public cloud workload
• Most virtual servers are based on some version of the Linux kernel
• Why is Linux uniquely suited for the cloud?
  • Flexibility
    • Modular design
    • Huge ecosystem
  • Accessibility
  • Cost-effective
    • Power efficient operating system
    • Kernel is completely free (as are many applications)
  • Manageability
    • Very widespread, lots of talent in the industry know how to use Linux
    • Can be managed by automated programs
  • Security
    • One of the most secure and reliable operating systems
    • Open source lends itself to security vulnerabilities being discovered and fixed quickly
  • Virtualization

Open source software and licensing

• Open source users have a right to obtain the source code
• Increases accountability for bugs, security vulnerabilities, and compatibility issues
  • Becomes a shared responsibility

Licensing

• Three components
  • Ownership
  • Money transfer
  • Licensing
• End User License Agreement (EULA)
  • Legal document that must be accepted to install software
• GNU General Public License v2 (GPLv2)
  • Source code must be made available to those who ask
  • Anyone can make changes to the code
    • If you distribute your changes, they must also be GPLv2 licensed
  • No one can charge for distributing the code
• Berkeley Software Distribution (BSD)
  • Simpler than GPL
  • Can redistribute source/binaries if you maintain copyright notices and don't imply original creator endorses your code
    • "Do what you want, don't take credit"

Creative Commons

• Tries to carry the intentions of FOSS licenses to non-software things
• Available conditions:
  • Attribution (BY)
    • Creator must be given credit even if they don't endorse the use
  • ShareAlike (SA)
    • Can copy, distribute, perform, and modify if they do so under the same terms
  • NonCommercial (NC)
    • Can't do anything commercially with the work
  • NoDerivatives (ND)
    • Creator must provide permission to modify
• Licenses
  • Attribution (CC BY)
    • Similar to BSD
    • Can use the content in any way but must give credit to the creator
  • Attribution ShareAlike (CC BY-SA)
    • Copyleft version of Attribution
    • Derived works must have same license
  • Attribution NoDerivs (CC BY-ND)
    • You can redistribute but cannot modify
  • Attribution NonCmercial (CC BY-NC)
    • CC BY but can't use it for commercial purposes
  • Attribution NonCommercial ShareAlike (CC BY-NC-SA)
    • CC BY-NC + changes must be shared under the same license
  • Attribution NonCommercial NoDerivs (CC BY-NC-ND)
    • Sharing can be done for non-commercial purposes, no modifying
  • No Rights Reserved (CC0)
    • Creative Commons version of public domain

Command line skills

Variables

• Local variables
  • Only exist in the current shell
    • When closing the shell, variables are lost
  • Can't affect other commands/applications
• Environment variables
  • Also called global variables
  • Examples: PATH, HOME, HISTSIZE
  • env outputs environment variables
• export turns a local variable into an environment variable
  • Remove exported variables with unset
• PATH variable
  • Contains a list of directories that the shell will look in to find commands

Command types

• type allows you to check a commands type
• Internal commands
  • Built-in to the shell
  • Eg. cd, echo
• External commands
  • Binary executables stored in directories
  • Eg. ls, cal
  • Using the which command will tell you where the shell is running that command from
• Aliases can be created with alias name=command
  • Shorten commands you commonly run
  • type will identify when a command is an alias

Quotes

• Double quotes
  • Prevent the shell from interpreting meta characters, including glob characters
  • Allow for command substitution and variable substitution
• Single quotes
  • Prevents the shell interpreting meta characters, glob characters, variables, and command substitution
• Backslash
  • Can be used to stop the shell from interpreting a character
• Backticks
  • Used to specify a command within a command (command substitution)

Control statements

• Semicolon ;
  • Used to run multiple commands, one after another
  • Each command runs regardless of the result of the previous command
• Double ampersand &&
  • Logical "and" operator
  • Only if the previous command succeeds will the next command run
• Double pipe ||
  • Logical "or" operator
  • Only if the previous command fails will the next command run

Helpful commands

• uname
  • Lists information about the current system
  • -n/--nodename will give the node name
• history
  • View your command history
  • Param is a number of how many history items you want to see (eg. history 5)

Getting help

Man pages

• Use man -f <command> to search man pages by name
  • Allows you to see when there are different man pages with the same name (eg. for passwd)
  • whatis often does the same thing
• Use man -k <command> to search man pages by name and description
  • apropos often does the same thing
• Sections
  • Name
    • Name of the command and a brief description
  • Synopsis
    • Examples and shows how to execute the command
    • Square brackets indicate something is optional
  • Description
    • Detailed description of the command
  • Options
    • All options for the command listed with descriptions
  • Files
    • Lists files associated with the command
  • Author
    • Name of the author
    • Sometimes includes contact info
  • Reporting bugs
    • Details how to report bugs
  • Copyright
    • Provides copyright information
  • See also
    • Helps find additional information related to the command
• Searching a man page
  • Use / to search through man pages
  • Press n to move to the next match
  • Press SHIFT+N to move to the previous match
• Category sections
  • Man pages are split into various sections
  • Next to the command you'll see a number (eg. CAL(1)) that identifies the category
  • Sections:
    1. General commands
    2. System calls
    3. Library calls
    4. Special files
    5. File formats and conventions
    6. Games
    7. Miscellaneous
    8. System administration commands
    9. Kernel routines

Finding stuff

• locate
  • Searches a database of all files/directories
    • The database is generated ~nightly
    • Can manually update the database with updatedb
  • -c shows how many files/directories match the search term
  • -b only includes listings that have the search term in the base name of the filename
  • \ in the search term only outputs filenames that exactly match
• info
  • Documentation with an organizational structure
  • More conducive for learning vs man pages which are more reference

Filesystem

• Directories are files
  • They store the names of the files within them and the inode number for where that file is on disk

Home directory

• /home
• When opening a shell, users are automatically placed in their home directory - They have full control to create/modify/delete files and directories here
• ~ is an alias for a user's home directory - ~<username> will take you to a specific user's home directory

Listing files and directories

• ls
  • -a shows hidden files
  • -l lists metadata for the files/directories
  • -h shows the file size in a human-readable size
    • Must be used with -l
  • -d refers to the current directory
    • Not really useful without -l
  • -R performs recursive listing
  • -S sorts by file size
  • -t sorts by timestamp
  • --full-time will list complete timestamps
  • -r reverse the sort
• The ? can be used to match exactly 1 character in a name
  • eg. ls -d /etc/???? - list all files in /etc that are exactly 4 characters long
• [ ] can be used to match any character from a set
  • eg. ls -d /etc/[abcd]* - display all files in /etc that start with a, b, c, or d

Long listing

**sysadmin@localhost:~$** ls -l /var/log/
total 900                                                                       
-rw-r--r-- 1 root   root  15322 Dec 10 21:33 alternatives.log
drwxr-xr-x 1 root   root   4096 Jul 19 06:52 apt
-rw-r----- 1 syslog adm     371 Dec 15 16:38 auth.log
-rw-r--r-- 1 root   root  35330 May 26  2018 bootstrap.log
-rw-rw---- 1 root   utmp      0 May 26  2018 btmp
-rw-r----- 1 syslog adm     197 Dec 15 16:38 cron.log
-rw-r--r-- 1 root   adm   85083 Dec 10 21:33 dmesg
-rw-r--r-- 1 root   root 351960 Jul 19 06:52 dpkg.log
-rw-r--r-- 1 root   root  32064 Dec 10 21:33 faillog
drwxr-xr-x 2 root   root   4096 Jul 19 06:51 journal
-rw-rw-r-- 1 root   utmp 292584 Dec 15 16:38 lastlog
-rw-r----- 1 syslog adm   14185 Dec 15 16:38 syslog
-rw------- 1 root   root  64128 Dec 10 21:33 tallylog
-rw-rw-r-- 1 root   utmp    384 Dec 15 16:38 wtmp

• File type
  • First character of each line
  • Types of files:
    • d - directory
    • - - regular file
    • l - symbolic link
    • s - socket
      • Allows for communication between processes
    • p - pipe
      • Allows for communication between processes
    • b - block file
      • Used to communicate with hardware
    • c - character file
      • Used to communicate with hardware
• Permissions
  • Next nine characters
• Hard link count
  • How many hard links there are to the file
• User owner
  • Every file is owned by a user account
  • Owner can set permissions on the file
• Group owner
  • Which group owns the file
  • Any member of a group inherits the permissions to the file
• File size
  • Size of file in bytes
  • For directories, how many bytes are reserved to keep track of the filenames in the directory
    • Not useful for directories
• Timestamp
  • When the contents were last modified
  • For directories, when a file was added/removed
• File name

Archiving and compression

• Archiving
  • Combine multiple files into one
• Compression
  • Make files smaller by removing redundant information
  • Two types:
    • Lossless
    • Lossy
• gzip
  • Most common Linux tool used for compression
  • -l provides info about the compression
    • Compression ratio shows how much the file was reduced
  • Uses the Lempel-Ziv data compression algorithm
• gunzip
  • Used to decompress
  • Same as gzip -d
• tar
  • tar -c [-f ARCHIVE] [OPTIONS] [FILE...]
  • Used to archive files
  • Short for TApe aRchive
  • Has three modes:
    • Create
      • Make a new archive
    • Extract
      • Pull one or more files out of an archive
    • List
      • Show contents of an archive witout extracting
  • Options
    • -c (create mode)
      • Create an archive
    • -t (list mode)
      • List files in an archive
    • -x (extract mode)
      • Extract files from an archive
      • -v
        • Verbosely list processed files
    • -f ARCHIVE
      • Specify the archive file to operate on
    • -z
      • Compress/decompress using gzip
    • -j
      • Compress/decompress using bzip2

Working with Text

• cat
  • Useful for creating and display text files
• less
  • Default pager for commands like man
  • More advanced that cat
  • Search
    • Forward: / then Enter
    • Backward from current position: / then ?
    • Moving through matches:
      • Forward: n
      • Backward: Shift + N
• more
  • Less features than less but always available
• head/tail
  • Display the first/last ten lines of a file
• sort
  • Sorting
• wc
  • Prints file statistics
• cut
  • Extract columns from a file or STDIN
  • Usually used for delimited databases files (eg. csv)
• grep
  • Filter based on a specified pattern

Pager movement commands

Input/Output redirection

• STDOUT 1> or >
• STDERR 2>
• Both STDOUT and STDERR &>
• STDIN <

Regular expressions

• .
  • Single character
• [ ]
  • A list or range of characters to match one character
  • If ^ is the first character then it means any character not in the list
• *
  • Previous character zero or more times
• ^
  • If it is the first character in the pattern --> pattern must be at the beginning of the line
• $
  • If the last character in the pattern --> pattern must be at the end of the line
• ?
  • Matches previous character zero or one time
• +
  • Matches previous character one or more times
• |
  • Logical "or"

Where data is stored

Processes

• Info provided by pseudo filesystem under /proc
  • Also contains info about system hardware and current kernel config
• Hardware devices through files under /dev
  • Information about those devices provided by pseudo filesystem under /sys
• Pseudo filesystems appear to be real files on disk but only exist in memory
• ps aux
  • View all running processes
  • a -- all processes, not just for the current user
  • u -- display detailed user-oriented info
  • x -- all processes (extension of a including ones without a controlling terminal

Network configuration

IP addresses

• IPv4
  • Four 8-bit numbers
  • Limited to ~4.3 billion addresses
• IPv6
  • 128-bit addresses
  • Much larger address pool that won't start running out like IPv4

Network config files

• /etc/hosts
  • Table of hostnames to IP addresses
  • Used to supplement a DNS server
• /etc/resolv.conf
  • IP addresses of name servers system uses to resolve names of IP addresses
• /etc/nsswitch.conf
  • Used to modify where hostname lookups occur

Network tools

• ifconfig
  • Interface configuration
  • Used to display network configuration information
  • Can be used to temporarily modify network settings
• ip
  • Replacing ifconfig in some distros
  • One-stop shop for configuration and control of networking
• route
  • Show what routing devices are available on the network
• ping
  • Used to determine if a machine can be reached
• netstat
  • Provides network information
  • Can display routing table similar to route
• ss
  • Shows socket statistics
  • Find what connections are currently established between local and remote machines
  • Similar to netstat
• dig
  • Queries DNS server to determine if info needed is available
    • eg. dig example.com --> can example.com get resolved?
  • Can use +trace to show entire resolution process
• ssh
  • Used to connect to another machine across the network
  • If you don't provide a username it will use the username you are currently logged in as

System and user security

• Users belong to at least one group
• Root account is disabled on Ubuntu
• sudo
  • Execute commands as another user
  • root is assumed by default
  • Prompts for a user password initially and then every time a command is run >5 minutes apart
• su
  • Switch user
• /etc/passwd
  • Defines account info for user accounts
  • Contains the following data for each user:
    • Name
    • Password placeholder
    • User ID
    • Primary group ID
    • Comment
    • Home directory
    • Shell
• /etc/shadow
  • Contains account info related to user's password
  • Passwords are encrypted and cannot be read
• /etc/group
  • Defines supplement group membership for users
  • Contains the following fields:
    • Group name
    • Password placeholder
    • GID
    • User list
• id
  • Print user and group info for a specified user
• who
  • Displays a list of users who are logged in
  • Contains the following fields:
    • Username
    • Terminal
    • Date
    • Host
• last
  • Reads entire login history from /var/log/wtmp
  • Displays all logins and reboot records by default

Creating users and groups

• Some distributions automatically create a group account for the user (User Private Group)
  • Name of group matches username
  • User is the only one in the group
• If the distribution doesn't make a group for the user then they usually have the users group as their primary group
• groupadd
  • Used to create a group
  • -g allows you to set the group ID
    • Need to consider user IDs if setting the group ID
  • Group name considerations
    • Start with a-z or _
    • Ideally <16 characters, never more than 32
    • After the first character you can user alphanumerics, -, or _
    • Last character should not be a -
• groupmod
  • Change the name (-n) or GID (-g) of a group
  • Any files associated with a group when you change the GID will no longer be associated with any group name (only the old GID)
    • You can find orphaned files by searching with find / -nogroup
• groupdel
  • Delete a group
• useradd
  • Add users
  • -D view or change some default values (also stored in /etc/default/useradd)
    • Group
      • Default primary group
      • If not specified, usually 100 which is the users group
      • -g option on the CLI
    • Base
      • Default base directory for the user's home directory
      • -b option on the CLI
    • Inactive
      • Number of days after password expires that the account is disabled
      • -f option on the CLI
    • Expire
      • No default value
      • Expiration date of the user
      • -e option on the CLI
    • Shell
      • Default shell for a user
      • -s option on the CLI
    • Skeleton directory
      • Which skeleton directory has its contents copied into the new user's home directory
      • -k option on the CLI
    • Create mail spool
      • Where to place incoming mail
• usermod
  • Modify a user
  • Some commands fail while the user is logged in
  • Some commands don't apply until the next time a user logs in
• userdel
  • Delete a user
• /etc/login.defs
  • More user defaults
• passwd
  • Set a password for a user
• chage
  • Used to manage password aging info in /etc/shadow

Ownership and permissions

• Users own files they create by default
• Ownership is determined by the UID and GID associated with a file
  • Changing either of these cam orphan a file
• newgrp
  • Change your current primary group
  • Useful when you want to create a file that should belong to a different group than your current primary group
• chgrp
  • Change the group owner of an existing file
  • Without admin privileges a user can only change the group to one they belong to
• chown
  • Change user ownership of files and directories
  • Can only be used to change user ownership by the root user
  • Can also change group ownership

Permission groups

• [-]rw-r--r-- 1 root root 4135 May 27 21:08 /etc/passwd
  • Type of file:
    • -
      • Regular file
      • May be empty or contain text or binary data
    • -d
      • Directory
    • l
      • Symbolic link
      • Pointer to another file
    • b
      • Block file
      • Relates to a block hardware device
      • Data is read in blocks of data
    • c
      • Character file
      • Relates to a character hardware device
      • Data is read one byte at a time
    • p
      • Pipe file
      • Similar to the pipe symbol
      • Allows the output of one process to communicate to another process through the file
    • s
      • Socket file
      • Allows two processes to communicate
• -[rw-]r--r-- 1 root root 4135 May 27 21:08 /etc/passwd
  • User owner
  • If you are the owner then only the owner permissions are used to determine access
• -rw-[r--]r-- 1 root root 4135 May 27 21:08 /etc/passwd
  • Group owner
• -rw-r--[r--] 1 root root 4135 May 27 21:08 /etc/passwd
  • Other permissions ("world's permissions")
  • Permissions for anyone that is not the file owner or a member of the file's group

Permission types

• Read
• Write
• Execute

Numeric permission setting

• Based on the octal numbering system
• Each permission is assigned a numeric value
  • Read --> 4
  • Write --> 2
  • Execute --> 1
• Combination of numbers from 0 to 7 are used to set permissions
  • 7 --> rwx
  • 6 --> rw-
  • 5 --> r-x
  • 4 --> r--
  • 3 --> -wx
  • 2 --> -w-
  • 1 --> --x
  • 0 --> ---
• stat
  • Shows permissions in both symbolic and numeric methods
• umask
  • Determines default permissions set when a file or directory is created
  • The umask value is subtracted from the maximum allowable default permissions
  • Maximum default value permissions:
    • Files --> rw-rw-rw-
    • Directories --> rwxrwxrwx

Special directories and files

• setuid
  • Allows users to run an executable with permissions of the file owner vs the user running the executable
  • Add/subtract 4000 when using chmod to adjust permissions
• setgid
  • Used to make sure files/directories can inherit the group ID of their parent
• Sticky bit
  • Used to prevent users from deleting files they don't own in a shared directory
  • Add/subtract 1000 when using chmod to adjust permissions

Links

• Hard links are two files that point to the same inode
  • Use find -inum to find where files to an inode are
  • Don't break if one of the links is deleted
  • Can't be linked to directories
• Symbolic links are files that point to another file
  • Easier to see where they link to
  • Break if the original file is deleted
  • Can be linked to directories
• ln
  • Used to create hard links
  • -s creates symbolic links