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Linux (The file structure (Commands that allow you to move around the file…

- - - - a command that shows you which directory you are in stemming from the root directory.
    - - The cd command allows you to navigate through the file system.
      - Example: If you're in the documents folder and you want to navigate to the taxes folder you would type the command - cd taxes
      - cd command shortcuts
        
        . (current directory). Example: cd .
        
        .. (parent directory). Example: cd ..
        
        ~ (home directory). Example: cd ~
        
        - (previous directory). Example: cd -
    - - The ls command will list directories and files in the current directory by default. You can also specify which path you want to list the directories of. Example: ls /home/matt
      - Not all files in a directory will be visible. Filenames that start with . are hidden. You can view the hidden files with the ls command and pass the -a flag to it (a for all). Example: ls -a.
      - the -l flag shows a detailed list of files in a long format. This will show you detailed information, starting from the left: file permissions, number of links, owner name, owner group, file size, timestamp of last modification, file/ directory name.
    - - Find will allow you to find a specific file within a directory. One cool thing to note is that find doesn't stop at the directory you are searching, it will look inside any sub directories that directory may have as well.
      - With find you'll have to specify the directory you'll be searching in, what you're searching for, and in this case we are trying to find a file by the name of puppies.jpg. Example: find /home -name puppies.jpg
      - You can specify what type of file you are trying to find. Example: find /home -type d -name myfolder. You can see that the type of file that is being searched for is (d) for directory that goes by the name myfolder.
    - - grep allows you to search files for characters that match a certain pattern. This is useful if you want to know if a file exists in a certain directory or if you wanted to see if a string was found in a file.
      - Example: grep fox sample.txt (you should see that grep found fox in the sample.txt file)
      - To get even more flexible with grep you can combine it with other commands with |
        
        Example: ls /somedir | grep '.txt' (should return all files ending with .txt in somedir)
  - - - Touch allows you to create new empty files. Example: touch newfile.
    - - Allows you to make copies of a file. Example: cp mycoolfile home/matt (mycoolfile is the file you want to copy and home/matt is where you are copying the file to).
      - You can copy multiple files and directories as well as use wildcards.
      - You'll need to use the -r flag to copy a directory that contains files and other directories. The -r flag will recursively copy the files and directories with a directory. Example: cp -r pumpkin/ home/matt.
      - The -i flag will prompt you in case you have a file with the same file name, preventing you from overriding it. Example: cp -i file home/matt
    - - Used to moving files and also renaming them.
      - You can rename files like this: mv oldfile newfile. You can also rename directories: mv directory 1 directory 2
      - To move a file just be sure to include the directory /. Example: mv file2 /home/matt
        
        Like cp, if you mv a file or directory it will overwrite anything in the same directory. You can use the -i flag to prompt you before overwriting anything. Example: mv -i directory1 directory2
        
        If you do want a file to overwrite the previous one, you can make a backup of that file and will just rename the older version with a ~. Example: mv -b directory1 directory2
    - - mkdir will create a directory if it doesn't already exist. Example: mkdir books
      - You can even make multiple directories at the same time. Example: mkdir books paintings.
      - You can also create subdirectories at the same time with the -p (parent flag). Example: mkdir -p books/hemingway
    - - To remove files and/or directories you can use the rm command. Example: rm file1
      - You can't just rm a directory by default, you'll need to add the -r flag (recursive) to remove all the files and any sub directories it may have. Example: rm -r directory. You can also remove a directory with the rmdir command.
      - Once the files are gone however, they are gone for good. There are some safety measures that can be put into place though, write-protected files will prompt you for confirmation before deleting them.
      - Adding the -i flag will give you a prompt on whether you want to actually remove the files or directories. Example: rm -i directory.
      - -f (force option) tells rm to remove all files, whether they are write protected or not, without prompting the user (as long as you have the appropriate permissions). Example: rm -f file1
  - - - gzip is a program used to compress files in Linux, they end in a .gz extension
      - To compress a file: gzip mycoolfile
      - To decompress a file: gunzip mycoolfile.gz
      - gzip can't add multiple files into one archive
    - - Tar allows you to add multiple files into one archive. When you create an archive using tar, it will have a .tar extension.
      - Example: tar cvf mycoolfile.tar mycoolfile1 mycoolfile2
        
        c - create
        
        v - tell the program to be verbose and let us see what it's doing
        
        f - the filename of the tar file has to come after this option, if you are creating a tar file you'll have to come up with a name.
      - To extract the contents of a tar file use: tar xvf mytarfile.tar
        
        x - extract
        
        v - tell the program to be verbose and let us see what it's doing
        
        f - the file you want to extract
      - Compressing and uncompressing archives with tar and gzip
        
        create a compressed tar file: tar czf myfile.tar.gz
        
        to uncompress a compressed tar file all you need to do is work outside in. First remove the compression with gunzip and then you can unpack the tar file.
        
        You can alternatively use the Z option with tar, which just tells it to use the gzip or gunzip utility. Example: tar xzf file.tar
- - - - username
      - username's password - the password is not really stored in this file, it's usually stored in the /etc/shadow/ file. If you see an "x" that means the password is stored in the /etc/shadow file, a "a" means the user doesn't have login access, and if their is a blank field that means the user does not have a password.
      - The user ID
      - the group ID
      - GECOS feild - used to leave comments about the user account
      - user's home directory
      - user's shell - you'll probably see a lot of user's defaulting to bash for their shell.
  - - - username
      - Encrypted password
      - Date of last password change. This is expressed as number of days since Jan 1, 1970
      - Minimum password age (days that a user will have to wait before being able to change their password)
      - Maximum password age (maximum number of days before a user will have to change their password)
      - Password warning period (number of days before a user will have to change their password)
      - password inactivity period (number of days after a password has expired to allow login with their password)
      - Account expiration date (date that user will not be able to login)
      - reserved field for future use
  - - - group name
      - group password. There isn't a need to set a group password, using an elevated privilege like sudo is standard. A "*" will be put in place as the default value.
      - group id (GID)
      - List of users. You can manually specify users you want in a specific group
  - - - The adduser command contains more helpful features such as making a home directory and more. There are configuration files for adding new users that can be customized depending on what you want to allocate to a default user.
      - Example: sudo useradd bob. You'll see that the above command creates an entry in /etc/password for bob, sets up default groups and adds an entry to the /etc/shadow file
- - - - q - used to quit out of less and go back to the shell
      - page up, page down, up and down - navigate using the arrow keys and the page keys
      - g - moves to the beginning of the text file
      - G - moves to the end of the text file
      - /search - you can search for specific text inside the text document. Prefacing the words you want to search with.
      - h - if you need help using less
  - - - For executable programs, it's convention to have an option called --help or something similar. Example: echo --help
      - help is a built-in bash command that provides help for other bash commands. Example: help echo (this will give you a description and the options you can use when you want to run echo).
    - - you can see the manuals for a command with the man command.
      - man pages are manuals that are default built into most Linux operating systems. They provide documentation about commands and other aspects of the system.
    - - The whatis command provides a brief description of command line programs. The description gets sourced from the manual page of each command.
  - - - $HOME - see the path to the home directory
      - $USER - see your username
      - $ENV - outputs information about the environment variables you currently have set
      - $PATH - returns a list of paths separated by a colon that your system searches when it runs a command
    - - Lets say you download and install a package from the internet and put it in a non standard directory and want to run that command, you type $coolcommand and the prompt says command not found. What is happening is that $PATH variable doesn't check that directory for this binary so it's throwing an error.
      - Lets say you had tons of binaries you wanted to run out that directory, you can just modify the PATH variable to include that directory in your PATH environment variable
- - - - This shows you a quick snapshot of the current processes. The columns are:
        
        TTY - controlling terminal associated with the process
        
        STAT - process status code
        
        TIME - total CPU usage time
        
        CMD - Name of executable / command
        
        PID - process ID
    - - a - displays all processes running, including the ones being run by other users
      - u - shows more details about the processes
      - x - lists all processes that don't have a TTY associated with it, these programs will show a ? in the TTY feild.
    - - USER - the effective user (the one whose access we are using)
      - PID - process ID
      - %CPU - CPU time used divided by the time the process has been running
      - %MEM - ratio of the process's resident set size to the physical memory on the machine
      - VSZ - virtual memory usage of the entire process
      - RSS - resident set size, the non-swapped physical memory that a task used
      - TTY - controlling terminal associated with the process
      - STAT - process status code
      - START - start time of the process
      - TIME - total CPU usage time
      - COMMAND - Name of executable/ command
    - - R - running or runnable, it is just waiting for the CPU to process it.
      - S - interruptible sleep, waiting for an event to complete, such as input from the terminal
      - D - uninterruptible sleep, processes that cannot be killed or interrupted with a signal, usually to make them go away you have to reboot or fix the issue
      - Z - Zombie, terminated processes that are waiting to have their statuses collected
      - T - stopped, a process that has been suspended / stopped
  - - - To free up the terminal and have the program run in the background you can use a &. Example: google-chrome & (when you do this the terminal will print out some info such as Job number and PID)
      - To bring a job back to the foreground use the following command: fg %5 (5 being the jobID)
    - - The kill command will close the program. Example: kill 2749 (2749 being the PID)
      - The kill -stop flag will freeze the program. Example: kill -stop 2749
  - - - A regular terminal device is a native terminal device that you can type into and send output to your system. This is not the terminal application you've been launching to get to your shell.
      - A pseudo terminal is what you've been working in (BASH), they emulate terminals with the shell terminal window and are denoted by PTS.
    - - These processes often start at system boot and usually get terminated when the system is shutdown. They run in the background and since we don't want these special processes to get terminated they are not bound to a controlling terminal
      - In the PS output, the TTY is listed as ? meaning it does not have a controlling terminal
  - - - sudo systemctl start foo - activates a service
      - sudo systemctl stop foo - deactivates a service
      - sudo systemctl restart foo - restarts a service
      - sudo systemctl status foo - tells if service is running
      - sudo systemctl enable foo - service started on bootup
      - sudo systemctl disable foo - service disabled on bootup
- - - - Debian style packages are used in distributions such as Debian, Ubuntu, LinuxMint, etc.
      - Red Hat style packages are seen in Red Hat enterprise Linux, Fedora, CentOS, etc.
    - - Install a package: rpm -i package.rpm
        
        the i stands for install. You can also use the longer format of --install.
      - Remove a package: rpm -e package.rpm
        
        e is for erase
      - List installed packages: rpm -qa
        
        q - for query
        
        a - all
  - - - Shared libraries are libraries of code that other programs want to use and don't want to have to rewrite for themselves.
- - - - The 4 parts for this example are d | rwx | r-x | r-x
      - The first part is the file type, which is denoted by the first character in the permissions
        
        "." indicates a file
        
        "d" indicates a directory
        
        ":" indicates a link
      - The next three parts of the file mode are the acutal permissions. The permissions are grouped into 3 bits each. The first 3 bits are user permissions, then group permissions, and then other permissions.
      - Each character represents a different permission
        
        r - readable
        
        w - writable
        
        x - executable (basically any executable program)
        
        - empty
  - - - Examples
        
        chmod +rwx myfile
        
        chmod -rwx mydir
    - - Examples
        
        chmod g+w workfolder
        
        chmod o-rwx testfile
    - - Examples
        
        chmod ugo-rwx workfolder
        
        chmod a=r workfoler
  - - - 0 - no permission
      - 1 - execute
      - 2 - write
      - 4 - read
    - - chmod 777 testfile
        
        This will give read, write, and execute permissions for everyone
- - - - If you take the command "echo Hello World > peanuts.txt" and checked the directory where that command was run you would see a file named peanuts that contains the text Hello World inside of it.
      - The > is a redirection operator that allows us to change where standard output goes. If the file does not already exist it will create it for us, but if the file does exist it will overwrite it.
      - The >> redirection operator will prevent you from overriding the file, instead it will append the input to the end of the file.
  - - - If you take the comand "cat < peanuts.txt > banana.txt" the output of cat peanuts.txt, which would be Hello World, gets redirected to another file called banana.txt.
      - We use > for stdout redirection and we use < for stdin redirection
      - Normally in the cat command, you send a file to it and that file becomes the stdin, in this case, we redirected peanuts.txt to be our stdin.
- - - - The shell is a program that takes your commands from the keyboard and sends them to the operating system to perform
- - - - Example: \(?\d{3}[-.) ](\d{3})[-. ]\d{4}
        
        This will find phone numbers that match (555) 555 5555 or 555-555-5555 or 555.555.5555 or any variation of those
    - - \d - any digit 0-9
      - . - any single character
      - \w - any character A-Z, a-z, or 0-9
      - \s - any whitespace
      - \W - anything that is not \w
      - \S - anything that is not \s
      - . - means literal dot (Note: anything with \ in front will mean literal. Example: \C will be literal C)
    - - * - 0 or more
      - + - 1 or more
      - ? - 0 or 1
      - {min, max)
      - {n} - n=number
    - - ^ - beginning of a line
      - $ - end of a line
      - \b - word boundary
    - - Character classes are things that appear in between square brackets [ ]
      - Examples:
        
        [abc] - will match a or b or c
        
        [-.] - will match - or literal dot
        
        [az ] - will match a or z or space
        
        [a-z0-9] - will match a through z or 0 through 9
      - Alternation allows you to use or statements. Example: (net|com) - will match net or com