# Linux Essentials ## Filesystem Hierarchy Standard Is a Linux filesystem standard, presented in the following screen | / | Primary hierarchy root and root directory of the entire file system hierarchy. | | --------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | /bin | Essential command binaries that need to be available in single user mode; for all users, e.g., cat, ls, cp. | | /boot | Boot loader files, e.g., kernels, initrd. | | /dev | Device files, e.g., /dev/null, /dev/disk0, /dev/sda1, /dev/tty, /dev/random. | | /etc | Host-specific system-wide configuration files | | | There has been controversy over the meaning of the name itself. In early versions of the UNIX Implementation Document from Bell labs, /etc is referred to as the etcetera directory,\[3] as this directory historically held everything that did not belong elsewhere (however, the FHS restricts /etc to static configuration files and may not contain binaries).\[4] Since the publication of early documentation, the directory name has been re-explained in various ways. Recent interpretations include backronyms such as "Editable Text Configuration" or "Extended Tool Chest".\[5] | | /etc/opt | Configuration files for add-on packages that are stored in /opt. | | /etc/sgml | Configuration files, such as catalogs, for software that processes SGML. | | /etc/X11 | Configuration files for the X Window System, version 11. | | /etc/xml | Configuration files, such as catalogs, for software that processes XML. | | /home | Users' home directories, containing saved files, personal settings, etc. | | /lib | Libraries essential for the binaries in /bin and /sbin. | | /lib\ | Alternative format essential libraries. Such directories are optional, but if they exist, they have some requirements. | | /media | Mount points for removable media such as CD-ROMs (appeared in FHS-2.3 in 2004). | | /mnt | Temporarily mounted filesystems. | | /opt | Optional application software packages.\[6] | | /proc | Virtual filesystem providing process and kernel information as files. In Linux, corresponds to a procfs mount. Generally automatically generated and populated by the system, on the fly. | | /root | Home directory for the root user. | | /run | Run-time variable data: Information about the running system since last boot, e.g., currently logged-in users and running daemons. Files under this directory must be either removed or truncated at the beginning of the boot process; but this is not necessary on systems that provide this directory as a temporary filesystem (tmpfs). | | /sbin | Essential system binaries, e.g., fsck, init, route. | | /srv | Site-specific data served by this system, such as data and scripts for web servers, data offered by FTP servers, and repositories for version control systems (appeared in FHS-2.3 in 2004). | | /sys | Contains information about devices, drivers, and some kernel features.\[7] | | /tmp | Temporary files (see also /var/tmp). Often not preserved between system reboots, and may be severely size restricted. | | /usr | Secondary hierarchy for read-only user data; contains the majority of (multi-)user utilities and applications.\[8] | | /usr/bin | Non-essential command binaries (not needed in single user mode); for all users. | | /usr/include | Standard include files. | | /usr/lib | Libraries for the binaries in /usr/bin and /usr/sbin. | | /usr/lib\ | Alternative format libraries, e.g. /usr/lib32 for 32-bit libraries on a 64-bit machine (optional). | | /usr/local | Tertiary hierarchy for local data, specific to this host. Typically has further subdirectories, e.g., bin, lib, share.\[9] | | /usr/sbin | Non-essential system binaries, e.g., daemons for various network-services. | | /usr/share | Architecture-independent (shared) data. | | /usr/src | Source code, e.g., the kernel source code with its header files. | | /usr/X11R6 | X Window System, Version 11, Release 6 (up to FHS-2.3, optional). | | /var | Variable files—files whose content is expected to continually change during normal operation of the system—such as logs, spool files, and temporary e-mail files. | | /var/cache | Application cache data. Such data are locally generated as a result of time-consuming I/O or calculation. The application must be able to regenerate or restore the data. The cached files can be deleted without loss of data. | | /var/lib | State information. Persistent data modified by programs as they run, e.g., databases, packaging system metadata, etc. | | /var/lock | Lock files. Files keeping track of resources currently in use. | | /var/log | Log files. Various logs. | | /var/mail | Mailbox files. In some distributions, these files may be located in the deprecated /var/spool/mail. | | /var/opt | Variable data from add-on packages that are stored in /opt. | | /var/run | Run-time variable data. This directory contains system information data describing the system since it was booted.\[10] | | | In FHS 3.0, /var/run is replaced by /run; a system should either continue to provide a /var/run directory, or provide a symbolic link from /var/run to /run, for backwards compatibility.\[11] | | /var/spool | Spool for tasks waiting to be processed, e.g., print queues and outgoing mail queue. | | /var/spool/mail | Deprecated location for users' mailboxes.\[12] | | /var/tmp | Temporary files to be preserved between reboots. | ## Linux Permission & Groups
Each file in Linux has Three(3) modes of access * Owner (section 2) * Group (section 3) * Others (section 4) Each "mode of access" or user can possess a combination of Three(3) available permissions * Read - R - 4 (octal representation of permission) * Write - W - 2 * Execute - X - 1 The Octal representation can benefit us when using tools like 'chmod' to modify the permissions * Section 1 reveals what type the object is: * d : directory * * : regular file * l : soft link * Section 2,3,4 are detailed permissions for the respective user * Section 5 details any alternative access methods * '-' : No other method * '.' : SELinux * '+' : FACLs * '+' : Any combination of methods * Section 6,7 are names of the Owner and Group respectively *** ## Programs manual - "man" command The manuals also document and present the type of "command" or tool as the following table displays: * 1 - User Commands * 2 - Programming interfaces for kernel system calls * 3 - Programming interfaces to the C library * 4 - Special files such as device nodes and drivers * 5 - File formats * 6 - Games and amusements such as screen-savers * 7 - Miscellaneous * 8 - System administration commands **Note:** Can search using keywords with the "-k" flag and also with Regular Expressions, Just apply it inline **Example:** ```bash man -k passwd ``` **Output:** ```bash chgpasswd (8) - update group passwords in batch mode chpasswd (8) - update passwords in batch mode gpasswd (1) - administer /etc/group and /etc/gshadow grub-mkpasswd-pbkdf2 (1) - generate hashed password for GRUB pam_localuser (8) - require users to be listed in /etc/passwd passwd (1) - change user password passwd (1ssl) - compute password hashes passwd (5) - the password file update-passwd (8) - safely update /etc/passwd, /etc/shadow and /etc/group ``` ## Descriptive tools search - "apropos" command A way to search for a given tool by searching keywords from it's description. **Example:** ```bash apropos partition ``` **Output:** ```bash addpart (8) - simple wrapper around the "add partition" ioctl all-swaps (7) - event signalling that all swap partitions have been activated cfdisk (8) - display or manipulate disk partition table cgdisk (8) - Curses-based GUID partition table (GPT) manipulator delpart (8) - simple wrapper around the "del partition" ioctl fdisk (8) - manipulate disk partition table fixparts (8) - MBR partition table repair utility gdisk (8) - Interactive GUID partition table (GPT) manipulator growpart (1) - extend a partition in a partition table to fill available space partprobe (8) - inform the OS of partition table changes partx (8) - tell the Linux kernel about the presence and numbering of on-disk partitions resizepart (8) - simple wrapper around the "resize partition" ioctl sfdisk (8) - partition table manipulator for Linux sgdisk (8) - Command-line GUID partition table (GPT) manipulator for Linux and Unix ``` ## Find files in Linux * find * Common usage - ```bash # Find all the files whose name is tecmint.txt in a current working directory. find /home -name tecmint.txt # Find Files Using Name and Ignoring Case find /home -iname tecmint.txt # Find Directories Using Name find / -type d -name Tecmint # Find Files With 777 Permissions find . -type f -perm 0777 -print ``` * locate * which ## System Services management * systemctl * ss (viewing) ## Advanced Package Tools Kali linux is a Debian distribution that utilizes the Advanced package tool (APT) software to provide a package management software services in kali linux. Two main application exists as an interface to managing software packages: * apt > apt is a simplified and user-friendly version of apt-get Common command: ```bash list - list packages based on package names search - search in package descriptions show - show package details install - install packages reinstall - reinstall packages remove - remove packages autoremove - Remove automatically all unused packages update - update list of available packages upgrade - upgrade the system by installing/upgrading packages full-upgrade - upgrade the system by removing/installing/upgrading packages edit-sources - edit the source information file ``` * apt-get **Note:** apt installs software package and fulfils those software's dependencies recursively ### dpkg dpkg is a core tool used to install packages on linux directly or indirectly through "apt". dpkg those not resolve and install dependencies recursively according to the software's needs so that's something to take into account * dpkg can also install packages offline from a .deb file by using the -i or —install flag *** ## Bash - Tips & Tricks ### Environment Variables Example: ```jsx echo $PATH /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin ``` View environment variable: * env Add environment variable: * export ### Piping and Redirection The are 3 streams of information in linux CLI * STDIN(0) - program input * STDOUT(1) - program output / result output * STRERR(2) - error messages stream the information present in those streams can be redirected in multiple ways: * redirect information to be written as a file ( >, >> ) * redirect information and input to a program ( | ) * reverse redirection, etc. ( < ) **Example:** ```bash command: cat some_file.txt ### some_file.txt # STDIN 'some_file.txt' "this is the text of 'some_file.txt'" # STDOUT "file doesn't exist" #STDERR ### Redirection and piping ### cat some_file.txt > new_file.txt "appended text" >> new_file.txt cat some_file.txt | grep -i grep 'something' < file_to_grep.txt ``` ## Bash history & CLI tricks Bash have a history file named ".bash\_history" that can be displayed by the command "history" ```bash kali@kali:~$ history ... 140 sudo apt upgrade 141 apt -g 142 cat some_file.txt ``` Shortened CLI command execution: * ! - will execute the command in the specified numerical row in the history file * !! - will execute the last executed command * CTRL + R - reverse-i-search will prompt a search bar for search commands in the history file and then executing them. ### Text Searching and Manipulation Common and powerful tools are: * grep - searches for regex occurrences in the files * awk - like cut but more complex and verbose * cut - used to cut text from a line according to specific parameters * sed - edits text from a stream ## File Editing, Manipulation & Analysis ### Text Editors There are many text editor, some more capable and complicated than others * nano * vi * Good to know flags * *insert -* to begin typing * \~ - to enter command mode ```bash dd # Delete current line yy # Copy current line p # Pase clipboard contentcom ``` * :q! - Quit without saving changes * :w - Write file to disk * :wq! - Save changes and quit ### Comparing Files * comm - creates output of 3 columns in this order \[ A\_UNIQUE\_LINES(1), B\_UNIQUE\_LINES(2), AB\_SIMILLAR\_LINES] * -n \ - a flag to control visibility of columns > -n 12 (column 1 and 2) * diff - ' - ' precedes occurrence in 1st file where not in 2nd file ' + ' precedes occurrence in 2nd file where not in 1st file **Note:** When using the -c format flag, similar lines will be present in the output, when using the -u format flag, only different lines will be printed * vimdiff - compares two files against each other visually with color coding
## Managing Process Processes can work in the "background" or in the "foreground", the difference between the two is the terminal focus position. Also, there is a mechanism that combines multiple processes that work together as a "job" which can also be managed. * bg - sending a process / job to the background allowing you to continue working in the terminal * Another easy way to background a process is by appending "&" to the end of the execution command. * fg - Sending a process or a job to foreground * jobs - shows the current active jobs with an ID number for each, you can use bg and fg in combination with that number. * ps - shows active process on the machine * -ef - select all process with full format listing * -C - searches for the process execution command * kill - kills a process by getting a PID ## File Monitoring ### tail "tail" display the last lines of a given text file > "-n" enables you to choose number of lines to display ### watch "watch" is a utility that executes a command in a given (or default) time interval > "-n" will determine the interval ## File Download ### wget download webpages and files. > -O to choose a new name for the downloaded file ### curl download webpages and files. ### axel A download accelerator, downs a file in multiple streams *** ## Bash Environment Customazation ### .bashrc \~/.bashrc is the bash configuration file. ### export a utility to export environment variable for global terminal usage > export HISTTIMEFORMAT='%F %T ' ### alias A utility for creating aliases > alias .. ="cd .."