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Linux Essentials Mods 10 - 18 - Coggle Diagram

- - - - The if statement is the basic operator to implement branching
        
        A basic if statement looks like this:
        `if somecommand; then` `# do this if somecommand has an exit code of 0` `fi`
        
        If the exit code is 0 then the contents up until the closing fi will be run
        
        The test command gives you easy access to comparison and file test operators
        
        i.e., The -f operator to the test command checks if the file exists:
        
        test -f /dev/ttyS0 - Will test if the file exists
        
        test -d /tmp - Will test if directory exists
        if test -f /tmp/foo; then if [ -f /tmp/foo]; then
        
        The if statement has a final form that lets you do multiple comparisons at one time using elif (short or else if)
        
        If the first argument passed to the script is hello, the first block is executed. If not, the script checks to see if it is goodbye and echos a different message if so. Otherwise, a third message is sent.
      - The case statement provides a different way of making multiple tests easier
        #!/bin/bash case "$1" in echo "hello yourself" ;; goodbye) echo "nice to have met you" echo "I hope to see you again" ;; *) echo "I didn't understand that" esac
        
        the case statement starts off with a description of the expression being tested: case EXPRESSION in.
        
        Next, each set of tests are executed as a pattern match terminated by a closing parenthesis
        
        Following that are the commands to be executed if the pattern returns true, which are terminated by two semicolons
        
        The * pattern is the same as an else because it matches anything
- - - - Kernel Processes
        
        A key function of the Linux kernel is to manage processes
        
        The kernel accepts commands and manages processes that carry out those commands
        
        The kernel gives commands access to devices like memory, disks, network interfaces, keyboard, mice, monitors, and more
        
        The kernel also provides access to info about active processes through a pseudo filesystem is visible under the /proc directory. Other pseudo filesystem include /dev/ and /sys, which give info about hardware devices
        
        Pseudo filesystems are ones that appear to be real files on disk, but exist only in memory
      - The /proc Directory
        
        The /proc directory not only contains info about processes (as name "proc" suggests), but also info about system hardware and current kernel configuration
        
        The output shows a variety of named and numbered directories
        
        Some of the commands that read form /proc directory such as:
        
        /proc/cmdline - Contains info passed to kernel during boot
        
        /proc/meminfo - Contains info about kernel memory usage
        
        /proc/modules - Contain list of modules loaded into the kernel
      - Process Hierarchy
        
        when the kernel finishes loading during boot, it starts the init process and assigns it a PID of 1
        
        The process then starts other system processes and assigns a PID in sequential order
        
        When one process starts another process, the first process is called a parent process, the second process is called a child process
        
        Process can be mapped into a "tree" which can be viewed with the pstree command
      - Viewing Process Snapshot
        
        Another way of viewing processes is with the ps command
        
        The ps command can also be used with the head and grep commands to filter processes displayed:
        ps -e | grep firefox
      - Viewing Process Snapshot in Real Time
        
        the top command has a dynamic, screen-based interface that will regularly update the output of running processes
        top
      - Viewing Memory
        
        To view a snapshot of the memory used at that moment, use the free command
        
        Mem: is the statistics for physical memory on the system
        
        -/+ buffers/cache: is the physical memory minus memory used by the kernel
        
        Swap: is virtual memory
      - Log Files
        
        Processes running on a system produce output that describes what the process is doing
        
        Some output goes to the terminal, however other output is not seen in the terminal and gets written to files as log messages ( or log data) instead.
        
        Some process log data by default, while others use a daemon to log data
        
        Examples of daemons include; syslogd, klogd, rsyslogd, journald
        
        Log files are placed under the /var/log directory
        
        To view log files:
        
        use cat or less command
        
        use journalctl command
        
        Log files are rotated, meaning older log files are renamed and replaced with newer log files.
        
        Most log files contain text, which can be viewed safely with many tools. Other files such as the /var/log/btmp and /var/log/wtmp files contain binary.
        Use the file command to view binary log files
      - Kernel Messages
        
        Kernel messages can be found in the following files:
        
        /var/log/dmesg - contains the kernel messages that were produced during system startup
        
        /var/log/messages - will contain kernel messages that are produced as the system is running
        
        To view messages generated by the kernel, use the dmesg command. To filter the output, use a pipe with the less or grep command
        dmesg | grep -i usb
    - - Organization Within the Filesystem Hierarchy
        
        User and Home Directories: The /home directory will typically have a directory underneath it for each user account
        
        Binary Directories: Contains the programs that users and administrators execute to start processes or applications running on the system
        
        Includes /bin, /usr/bin, /usr/local/bin and other non-user specific directories
        
        Root Restricted Binaries: the sbin directories are primarily intended to be used by the system administrator (the root user) and include:
        
        /sbin,/usr/sbin, and /usr/local/sbin
        
        Software Applications Directories:
        
        Mircrosoft Windows - Applications files are installed in a single subdirectory under the C:\Program Files directory
        
        Linux - Applications may have files in multiple directories spread out throughout the Linux filesystem
        
        To view list of application files, use dpkg -L packagename (Debian) and rpm -ql packagename (Red Hat)
        
        Library Directories: Files which contain code that is shared between mulitple programs
        
        Commonly use file extension of.so
        
        Examples include: /lib, /lib64, /usr/lib, /usr/lib64, /usr/local/lib
        
        Variable Data Directories: The /var directory and many of its subdirectories can contain data that will change frequently
        
        Examples include: /var/mail, /var/spool/mail, /var/spool/cups
- - - - User Config
        
        The /etc/login.defs file also contains values will be applied by default to new users you create with the useradd command.
        
        To view contents of /etc/login.defs file without comments and blank lines, use the grep command
        grep -Ev `^#|^$` /etc/login.defs
        
        Values of /etc/login.defs file include:
        
        Mail Directory - The dir in which the usr's mail spool file will be created
        MAIL_DIR /var/mail/spool
        
        Password Max Days - The maximum number of days that a user can continue to use the same password
        PASS_MAX_DAYS 99999
        
        Password Min Days - The shortest time that a user is required to keep a password
        PASS_MIN_DAYS 0
        
        Password Minimum Length - The minimum number of characters that a password must contain
        PASS_WARN_AGE 7
        
        UID Minimum - Although it can go up to four billion, for maximum compatibility it's recommended to leave it at its default value of 60000
        UID_MAX 60000
        
        GID Minimum - Determines the first GID that will be assigned to an ordinary group
        GID_MIN 500
        
        GID Maximum - The Max number of days that a user can continue to use the same password
        GID_MAX 60000
        
        Home Directory - Determines whether or not a new directory will be created for the user when their account is created
        CREATE_HOME yes
- - - - Write
        
        File
        
        Can be written to by the process. The w permission requires r permission to work
        
        Directory
        
        Files can be added to or removed from the directory. The w permission requires the x permission to work
      - Execute
        
        File
        
        A file can be executed or run as a process
        
        Directory
        
        User can use the cd command to get into directory and use pathname to access files in directory
      - Read
        
        File
        
        Allows process to read contents of the file, which means contents can be viewed and copied.
        
        Directory
        
        Names of directory are listed, but no other details are available
- - - - Creating Hard Links
        
        Every file on a partition has a unique ID number called an Inode number
        
        To display the inode number of a file, use the ls -i cmd
        ls -i /tmp/file.txt 215220874 /tmp/file.txt
        
        Hard links are two files names that point to the same inode. Take the passwd and mypasswd file names
        
        passwd = 123
        mypasswd = 123
        
        You can access the file data using either name because they have the same inode number
        
        You can view the link count number of a file by executing the ls -l command:
        ls -li file.*
        
        To create a hard link, use the ln cmd with two arguments:
        ln target link_name
        
        When a hard link is created, the link count will increase by one:
        ln file.original file.hard.1 ls -li file.* <value moves from 1 to 2>
      - Creating a Symbolic Link
        
        A symbolic link, aka a soft link, is a file that points to another file. Take the /etc directory for example:
        lrwxrwxrwx 1 root root 22 Feb 15 2011 /etc/grub.conf -> ../boot/grub/grub.conf
        
        In the above example, the /etc/grub.conf "points to" the ../boot/grub/grub.conf file.
        
        To create a symbolic link, use the -s option with the ln command:
        ln -s /etc/passwd mypasswd ls -l mypasswd lrwxrwxrwx 1 sysadmin sysadmin 11 Oct 31 13:17 mypasswd -> /etc/passwd
      - Comparing Hard and Symbolic Links
        
        Although they have the same result, each produces different results and have advantages and disadvantages
        
        Hard Links vs Soft Links Advantages
        
        Hard Link Advantage:
        
        If there are mulitple files with the same hard link, deleting any four of these files would not result in deleting the actual file contents. With a soft link; if the original file was removed, then any files linked to it, will fail.
        
        Soft Link Advantage:
        
        Soft links are easier to see
        
        Soft links can link to any file because it uses a pathname. Hard links cannot be created that attempt to cross file systems because each file system has a unique set of inodes
        
        Soft links can link to a directory
    - - Filesystem Hierarchy Standard
        
        The Filesystem Hierarchy Standard (FHS) is a set of guidelines for Linux directories and their contents
        
        The FHS standard categorizes each system directory in a couple of ways
        
        a dir can be categorized as either shareable or not, meaning if the dir could be shared on a network and used by multiple machines.
        
        The dir is put into a category of having either static files (file contents won't change) or variable files (file contents can change)
      - Organization Within the Filesystem
        
        The following provides a generalized description of the layout of directories as they actually exist on a typical Linux distribution
        
        User Home Directory:
        
        The /home directory will typically have a directory underneath it for each user account (i.e., /home/bob)
        
        Binary Directories:
        
        Contain programs that users and admins execute to start processes or applications
        
        Software Application Directories:
        
        Application in Linux may have their files in multiple directories spread throughout the Linux Filesystem
        
        Library Directories:
        
        Libraries are files which contain code that is shared between multiple programs. Most library file names will end in a file extension of .so, which means shared object
        
        Variable Data Directories:
        
        The /var directories and many of its subdirectories can contain data that will change frequently (i.e., /var/mail and /var/log)