Open-Source Licensing

• Purchasing Software:
  

  • Ownership - Who owns the intellectual property
  • Money Transfer - Does it cost anything? How do you pay?
  • Licensing - What do you get? What can you do with the software? How many computers? Can you share it?

• End User License Agreement (EULA) is a legal document you must accept before installing software
• GNU General Public License v2 (GPLv2) is a license that states the source code must be made available to anyone and that anyone can make changes. *Changes must be under the same license
• Free and Open-Source Software is software where anyone can view source code, modify it, and redistribute it

Open Source Philosophy

• Philosophy that users have the right to obtain the software source code and modify it for their own use.
• Software projects use source code; a human-reaadable set of computer instructions
• Unix source code language preceded Linux. Unix was created at AT&T Bell Labs in 1969
• Standards organizations like IEEE and POSIX ensure that code has the ability to be compatible with other programs and OS for collaboration
• GNU project built tools that are compatible with UNIX, which were used to create Linux and now make Linux a more complete package

Open-Source Business Models

If the software is free, how can a company monetize it?

• Offer products and services; RedHat and Ubuntu
• Create tools; Wireshark
• Package hardware and open-source software; Tivo, appliances

Command Line Interface

• The Linux community promotes the CLI due to its power, speed and ability to accomplish a vast array of tasks with a single command line instruction
• the CLI provides more precise control, greater speed and the ability to automate tasks more easily through scripting
• By learning the CLI, a user can easily be productive almost instantly on ANY flavor or distribution of Linux

The Shell

• The Bash shell also has many popular features, a few of which are listed below:

  
  

  • Command Line History
  • Inline editing
  • Scripting
    

    • The ability to place cmds in a file and then interpret (effectively use Bash to execute the contents of) the file, resulting in all of the cmds being executed

  • Aliases
    

    • The ability to create short nicknames for longer commands

  • Variables
    

    • Used to Store info for the Bash shell and for the usr
      

• When a terminal application is run, and a shell appears, displaying an important part of the interface - the prompt

• The prompt contains info about the user and the system
  

  • sysadmin@localhost:~$

• The prompt shown contains the following info:
  

  • Username (sysadmin)
  • System name (localhost)
  • Current Directory (~)
    

    • The ~ symbol refers to the usrs home dir

Arguments

• An argument can be used to specify something for the command to act upon
• If the ls cmd is given the name of a dir as an argument, it lists the contents of that dir
  

  • sysadmin@localhost:~$ ls /etc/pp
    ip-down.d ip-up.d

• Some cmds accept multiple argurments:
  

  • sysadmin@localhost:~$ ls /etc/ppp /etc/ssh

Options

• Options can be used with cmds to expand or modify the way a cmd behaves
• For examples, using the -l option of the ls cmd results in a long listing, providing additional info about the files that are listed
• Often the character is chosen to be mnemonic for its purpose, like choosing the letter l for long or r for reverse
• Options can be used in conjunction with other options:
  

  • sysadmin@localhost:~$ ls -lr

• Options are often single letters; however sometimes they are words or phrases as well
• Older commands use single letters while newer commands use complete words for options
  

  • Single-letter options are preceded by a single dash - character, like the -h option
  • Full-word options are preceded by two dash -- characters like the full-word from of the -h option, the --human-readable option

Commands

• Some cmds require additional input to run correctly.
• This additional input comes in two forms: options and arguments
  

  • Options are used to modify the core behavior of a command
  • Arguments are used to provide additional info (such as filename or a usrname)

• The typical format for a command is as follows:
  

  • command [option] [arguments]

Command History

• When a command is executed in the terminal, it is stored in a history list
• This makes it easy to execute the same command later eliminating the need to retype the entire command
• Pressing the Up Arrow ↑ key displays the previous command on the prompt line

• To view the entire history list of a terminal, use the history command:

  
  

  • sysadmin@localhost:~$ history
    

    1. date
    2. ls
    3. cal 5 2030
    4. history
       

• If the desired command is in the list that the history cmd generates, it can be executed by typing an exclamation point ! character and then the number next to the command (i.e. !3)

• If the history command is passed a number as an argument, it outputs that number of previous commands from the history list

  
  

  • sysadmin@localhost:~$ history 3
    

    6. date
    7. ls /home
    8. history 3
       

• to execute the most recent command type !! and hit Enter

• to execute the most recent iteration of a specific command, type !command and hit Enter

Local Variables

• Local or shell, variables exist only in the current shell. When the user closes a terminal window or shell, all of the variables are lost
• To set the value of a variable, use the following assignments expression
  

  • variable=value

• the following example creates a local variable named variable1 and assigns it a value of something

  
  

  • sysadmin@localhost:~$ variable1=ssomething
    

• to display the value of the variable, use a dollar sign $ charcter followed by the variable name as an argument to the echo cmd:

  
  

  • echo $variable1
    Something

Environment Variables

• Environment variables, also called global variables, are available system-wide
• Examples include the PATH, HOME, and HISTSIZE variables

• the cmd in the example below displays the value of the HISTSIZE variable:

  
  

  • sysadmin@localhost:~$ echo $HISTSIZE
    (output)
    

• The env cmd outputs a list of the environment variables.

• the export cmd is used to turn a local variable into an environment variable

  
  

  • sysadmin@localhost:~$ export variable1
  • sysadmin@localhost:~$ env | grep variable1
    variable1=Something
    

• Exported variables can be removed using the unset command

Path Variables

• One of the most important Bash shell variables to understand is the PATH variable
• The PATH variable lists all the places that the system can look for programs to execute

• the following command displays the path of the current shell:

  
  

  • sysadmin@localhost:~$ echo $PATH
    /home/sysadmin/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/sbin:/bin:/usr/
    

• If the command is not found in any directory listed in the PATH variable, then the shell returns a cmd not found error

External Commands

• External commands are stored in files that are searched by the shell
• It can be beneficial to know where the shell is finding the command or which version it is using

• the which cmd searches for the location of a command by searching the PATH variable

  
  

  • sysadmin@localhost:~$ which ls
    /bin/ls
    sysadmin@localhost:~$ which cal
    /usr/bin/cal
    

• External commands can be executed by typing the complete path to the command

  
  

  • sysadmin@localhost:~$ /bin/ls
    Desktop Docs Downloads Music (etc)

• for external cmds, the type cmd displays the location of the cmd:
  

  -sysadmin@localhost:~$ type cal

  
  

  cal is /usr/bin/cal

• To display all locations that contain the command name, us the -a option to the type cmd
  

  • sysadmin@localhost:~$ type -a echo

Aliases

• An alias can be used to map longer commands to shorter key sequences
• i.e. the cmd ls -l is commonly aliased to l or ll
• to determine what aliases are set on the current shell use the alias cmd:
  

  • sysadmin@localhost:~$ alias

• The type command can ID aliases to other commands
  

  • sysadmin@localhost:~$ type ll
    ll is aliased to 'ls - alF'

Internal Commands

• Also called built-in commands, these commands are a part of the shell itself
  

  • i.e. the cd cmd is a part of the bash shell

• the type command IDs the cd command as an internal cmd
  

  • sysadmin@localhost:~$ type cd
    cd is a shell builtin

Functions

• Functions can also be built using existing commands to:

  
  

  • Create new commands
  • Override commands built-in to the shell or commands stored in files
    

• Aliases and Functions are normally loaded from the initialization files when the shell first starts

Control Statements

• Control statements allow you to use multiple commands at once or run additional commands
• Control statements include:
  

  • Semicolon - ;
  • Double ampersand - &&
  • Double Pipe - ||

• This semicolon can be used to run multiple commands, one after the other:
  

  • sysadmin@localhost:~$ cal 1 2015; cal 2 2015; cal 3 2015

• The double ampersand && acts as a logical "and" if the first cmd is successful, then the second command (to the right of the &&) will also run:
  

  • sysadmin@localhost:~$ ls /etc/xml && echo success
    catalog catalog.old xml-core.xml xml-core.xml.old
    success

• The double pipe || is a logical "or". it works similarly to &&; depending on the results of the first command, the second command will either run or be skipped:
  

  • sysadmin@localhost:~$ ls /etc/junk || echo failed
    ls: cannot access /etc/junk: No such file or directory
    failed

Backslash Character

• A technique to essentially single quote a single character is to use the backslash character \
• If the phrase below is placed in single quotes, $1 and $PATH are not variables:
  

  • sysadmin@localhost:~$ echo "The service costs $1 and the path is $PATH"

• What if you want to have $PATH treated as a variable and $1 not?
  

  • in this case. Use the backslash \ char in front of the dollar sign $ char to prevent the shell from interpreting it
    

    • sysadmin@localhost:~$ echo The service costs \$1 and the path is $PATH

Single Quotes

• Single quotes prevent the shell from doing any interpreting of special characters, including globs, variables, command substitution and other metacharacters
  

  • sysadmin@localhost:~$ echo The car cost $100
    The car cost 00
    sysadmin@localhost:~$ echo 'The car cost $100'
    That car cost $100

Backquotes

• Backquotes, or backticks, are used to specify a command within a command, a process called command subsitution

• Note the output of the echo Today is date command line:

  
  

  • sysadmin@localhost:~$ echo Today is date
    Today is date
    

• To execute the date cmd so the output of that cmd is sent to the echo cmd, put date cmd inside of two backquotes

  
  

  • sysadmin@localhost:~$ echo Today is date
    Today is Mon Nov 21 08:58:53 CST 2022

Double Quotes

• Double quotes stop the shell from interpreting some metacharacters, including glob characters
  

  • Glob char, aka Wild Cards, are symbols that have special meaning to the shell (i.e *, ?)

• This is useful when you want to display something on the screen that is normally a special character to the shell
• the Bash shell doesn't convert the glob pattern into filenames that match the pattern (like it normally would)
  

  • sysadmin@localhost:~$ echo "the glob character *, ? and [ ]"
    the glob characters are *, ? and [ ]

• Double quotes still allow for command substitution, variable substitution, and permit some other shell metacharacters (i.e. PATH variables)

Man Page Sections

• Man pages are broken into sections
• Each section is designed to provide specific info about the command
• The following describes some of the more common sections found in man pages:
  

  • NAME: provides the name of the command and very brief description
    ls - list directory contents
  • SYNOPSIS: Provides examples of how the command is executed
    ls [option]... [file]...
  • DESCRIPTION: Provides a more detailed description of the command
  • OPTIONS: Lists the options for the command as well as a description of how they are used
  • FILES: Provides are more detailed desc of the cmd
  • AUTHOR: Provides the name of the person who created the man page and (sometimes) how to contact the person
  • REPORTING BUGS: Provides details on how to report problems with the command
  • COPYRIGHT: Lists the options for the command as well as description of how they are used
  • SEE ALSO: Provides you with an idea of where you can find additional info. This often includes other commands that are related to this command

Searching Man Pages

• To search a man page for a term, press the / and type the term followed by the Enter key
• If the term is not found, or upon reaching the end of the matches, the programs will report Pattern not found (press Return).
• If a match is found, to move to the next match of the term, press n. To return to a previous match of the term, press N

Viewing Man Pages

• To view a man page for a command, use the man command
  

  • man command

• For example, the following displays the man page for the ls cmd:
  

  • sysadmin@localhost:~$ man ls

• Navigate the document using the arrow keys
• To exit viewing a man page, use the Q key
• The man cmd uses a pager to display documents, Usually, this pager is the less command, but on some distributions, it may be the more command. Both are very similar in how they perform

Man Page Categorized by Sections

• There are thousands of man pages on a typical Linux distribution

• To organize all of these man pages, they are categorized by sections.

  
  
  • By defaults there are nine default sections of man pages:
    

    • General Commands
    • System Calls
    • Library Calls
    • Special Files
    • File Formats and Conventions
    • Games
    • Miscellaneous
    • Sys Admin Commands
    • Kernel Routines
      

• The man cmd searches each of these sections in order until it finds the first match

• For examples, the cal cmd belongs to the first section of man pages:
• Sometimes there are man pages with the same name in different sections:
  

  • sysadmin@localhost:~$ man -f passwd
    passwd (5) - the password file
    passwd (1) - change user password
    passwd (lssl) - compute password hashes

• It may necessary to specify the section of the correct man page. To specify a different section, provide the number of the section as the first argument of the man cmd.
  

  • sysadmin@localhost:~$ man 5 passwd

Find Any File or Directory

• To find any file or directory, use the locate cmd

• This cmd searches a database of all files and directories that were on the system when the database was created

  
  

  • However, any files created that day will not be searchable with the locate cmd because the database is updated nightly
  • It is possible to update the locate database manually by running the updatedb cmd as root.
    

• The output can be quite large so it may be helpful to use the following options:

  
  

  • The -c option to the locate command will list how many files match:
    

    • sysadmin@localhost:~$ locate -c passwd

  • The -b option only includes listings that have the search term in the basename of the filename. to limit the output even further, place a \ character in front of the search term:
    

    • sysadmin@localhost:~$ locate -b "\passwd"

Info Documentation

• To display the info documentation for a command, use the info cmd

  
  

  • sysadmin@localhost:~$ info ls
    

• You can navigate the document using the arrow keys

• This documentation is broken up into nodes. In the example below the line highlighted in white shows it's currently in the ls invocation node:

Navigating Info Documentation

• A listing of movement ommands is available by hitting the Shift-H key while reading the info documentation
• To quit the info documentation, use the Q key

Viewing Info Documentation

• Man pages are excellent sources of info, but they do tend to have a few disadvantages:

  
  

  • Each man page is a separate document, not related to any other man page
  • Man pages may can be difficult to read
    

• The info cmd also provides documentation on OS cmds and features

  
  

  • Info documentation as a logical organizational structure, making reading documentation easier.
  • Info is broken down into categories that work much like a table of contents in a book
    

• Consider man pages to be more of a reference resource and info documents to be more of a learning guide

Using the Help Option

• Many commands will provide basic info, very similar to the SYNOPSIS found in man pages, by simply using the --help option to the command

Additional System Documentation

• On Most systems, there is a directory where additional documentation is found, such as documentation files stored by third-party software vendors
• These Docs files are often called README files since the files typically have names such as README or readme.txt
• Typical locations for these files include /usr/share/doc and usr/doc

Current Directory

• The pwd cmd can be used to determine where the user is currently located within the filesystem
• The pwd command prints the working directory, which is the current location of the user within the filesystem
  

  • pwd [OPTIONS]
    sysadmin@localhost:~$ pwd
    /home/sysadmin

Changing Directories

• When a user opens a shell, they typically begin in their home directory

• To navigate the filesystem, use the cd cmd:

  
  

  • sysadmin@localhost:~$ cd Documents
    sysadmin@localhost:~/Documents$
    

• After changing directories, the new location can also be confirmed in the new prompt, again shown in blue

• When used with no arguments, the cd cmd will take the user to their home dir ~
• If the user tries to change to a dir that does not exist, the cmd returns an error message
  

  • no such file or directory

Home Directory

• The dir name is the same as the name of the user.
• So, a user named sysadmin would have a home dir called /home/sysadmin

Paths - Shortcuts: The .. Characters

• Two period .. characters always represent one directory higher relative to the current dir, sometimes referred to as the parent dir
• For example, to move from the Art dir back to the School dir:
  

  • sysadmin@localhost:~/Documents/School/Art$ cd ..
    sysadmin@localhost:~/Documents/School$

• The double dot can also be used in longer paths as well:
  

  • sysadmin@localhost:~/Documents/School$ cd ../../Downloads

Relative Paths

• Gives directions to a file relative to the current location in the filesystem
• The user must currently be in a directory that contains objects in the path
• Relative paths start with the name of the dir
  

  • sysadmin@localhost:~/Documents/$ cd School/Art
    sysadmin@localhost:~/Documents/School/Art$

Paths - Shortcuts: The . Characters

• the single period . char always represents the current directory
• For the cd this shortcut is not very useful, but it comes in handy for commands covered in subsequent sections

Absolute Paths

• Allows the user to specify the exact location of a dir
• absolute paths always starts at the root directory, and therefore it always begins with the / character
• The path /home/sysadmin is an absolute path; it tells the system to:
  

  • Begin at the root / directory > move into the home directory > then into the sysadmin dir

• If the path /home/sysadmin is used as an argument to the cd command, it moves the user into the home directory for the sysadmin user
  

  • sysadmin@localhost:~/Documents$ cd /home/sysadmin
    sysadmin@localhost:~$

Human Readable Sizes

• the -l option to the ls command displays file sizes in bytes
• for text files, a byte is 1 character so small files are easy to read, but for larger files, it is hard to comprehend how large the file is
• The file size is hard to determine in bytes. Is 1561400 a large file or small?
• To present the file size in a more human readable size, like megabytes or gigabytes, add the -h option (with the -l option) to the ls command

Listing Directories

• When the command ls -d is used, it refers to the current directory, and not the contents within it
• The . represents the current directory
• The ls -l command lists the contents of the dir
• To use the ls -d command in a meaningful way requires the addition of the -l option

Recursive Listing

• When you want to display all of the files in a dir as well as all of the files in subdir under a dir
• To perform a recursive listing, use the -R option to the ls command
• Note that in the example, the files in the /etc/ppp dir were listed first and the files in the /etc/ppp/peers dir were listed after

Listing Hidden Files

• a hidden file is any file (or directory) that begins with a dot . character
• The ls command omits hidden files by default
• To display all files, including hidden files, use the -a option to the ls cmd
• Most of the hidden files are customization files, designed to customize how Linux, your shell or programs work.
  

  • i.e., the .bashrc file in the home directory customizes features of the shell

Sort a Listing

• by default, the -ls command sorts files alphabetically by file name
• Sometimes it may be useful to sort files using different criteria
• To sort files by size, we can use the -S option (capital letter s)
• While the -S option works by itself, it is most useful when used with the -l option so the file size are visible
• It may also be useful to use the -h option to display human-readable file sizes
• The -t option sorts files based on the time they were modified
• For more detailed modification time info you can use the --full-time option to display the complete timestamp (including hours, minutes, seconds)
• It is possible to perform a reverse sort by using the -r option. it can be used alone or combined with either the -S or -t options

• The following describes each of the fields of data in the output of the ls -l command:
  

  • File Type:
    -rw-r--r-- 1 root 17869 Mar 14 17:48 alternative.log
drwxr-x--- 2 root adm 4096 Mar 14 17:48 apache2
    

    • The first character of each line indicates the type of file
      d - directory - a file used to store other files
      - - regular file - Includes readable files, images files, binary files, and compressed files
      l - symbolic link - points to another file
      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 comminate with hardware

• Permissions:

  
  

  *drwxr-xr-x* 1 root root 0 Apr 11 21:58 upstart

  
  

  • The next nine characters demonstrate the permissions of the file
  • Permissions indicate how certain users can access a file
    

• Hard Link Count:

  
  

  -rw-r----- *1* syslog adm 23621 Aug 23 15:17 auth.log

  
  

  • This number indicates how many hard links point to this file

• User Owner
  

  • Every file is owned by a user account
  • this is important because the owner has the rights to set perms on a file

• Group Owner
  

  • Indicates which group owns this file, this is important because any member of this group has a set of perms on the file

• File Size
  

  • Size of files in bytes

• Timestamps
  

  • Indicates when a file's contents were last modified

• File Name
  

  • The name of the file or dir