Week1.md

Week 1 Notes

Terminal Emulators

• Terminal
• Konsole
• xterm
• guake

Command Prompt

• username@hostname:~$
  • ~$ is the path

Commands and Flags

• uname
  • prints the name, version and other details about the current machine and the operating system running on it
  • the -a displays hidden files that have a dot in front of them
• pwd
  • Present Working Directory
• ls - a : all . displays hidden files - l : use a long listing format - i : print index number of each file (inode) - s : shows blocks occupied by each file - 1 : each file name on a separate line
  • output of ls -l : drwxr-xr-x 5 ckg ckg 12288 Nov 25 10:00 Documents (d is file type ; rwxr-xr-x owner,group,others permissions ; 5 no of hard links ; ckg is owner ; ckg is group ; last modified time stamp ; filename)
  • ls F* gives a list of all files starting with F
• rm
  • remove a file
  • rm -i prompts before every removal (it can be set using alias rm="rm -i")
  • works only with write permission
  • use -d for removing directories
  • rm -r mydirectory
• mv
  • move , rename
  • mv file1 .. (moves file to parent dir)
  • mv file1 file1a (renames file1 to file1a)
• ps
  • currently running processes
• clear
  • or ctrl+l
• exit
  • or ctrl+d
• man
  • get help on any command in linux. eg : man ls
  • man sections (1 to 9) eg : man 1 ls
  • 1 - Executable programs or shell commands
  • 2 - System calls provided by Kernel
  • 3 - Library calls
  • 4 - Special files usually found in /dev
  • 5 - File formats and conversions
  • 6 - Games
  • 7 - Misc : macro packages and conventions
  • 8 - System admin commands
  • 9 - Kernel routines
• cd
  • change directory eg cd .. - goes to parent directory
  • cd without any arguments will take you to the home directory
  • cd / takes you to the root folder
  • cd - takes you to previous directory
  • cd ~ takes you to home directory
• cp
  • copy command : cp file1 file2
• date
  • date and time
  • date -R gives in RFC 5322 standard (used for email communications)
• cal
  • calendar of a month
  • eg : cal aug 1947
  • ncal gives calendar in flipped orientation
• free
  • memory statistics
  • use h flag to make it human readable
• groups
  • groups to which a user belongs
• file
  • what type of file
  • -f allows you to pass a file in which file names are separated by lines (ls -1 > files.txt; file -f files.txt)
  • file * will give a list of file name and types directly
• mkdir
  • create a directory
  • default permissions (umask)
• touch
  • used to change the last modified timestamp of a file
  • also used to create empty files
• chmod
  • chmod 777 file.txt
  • chmod g-w file.txt (removes write permissions from the group)
  • chmod o-x file.txt (removes executable permission from others)
  • chmod u-r file.txt (removes read permission from owner)
• whoami
  • prints username
• less
  • allows you to read a file page by page
• ln
  • used to create a hard link or a symbolic link (symlink) to an existing file or directory
  • s flag is used to create a soft link
  • usage : ln file1 file2 ; ln -s file1 file2
• cat
  • stands for concatinate
  • allows you to view the contents of a single file or multiple files (gets concatinated)

File types

• output of ls -l : drwxrwxrwx or lr-x--x--x (l indicates symbolic link and d indicates directory)
• - Regular file
• d Directory
• l Symbolic link
• c Character file (usually found in /dev ; typically the terminal)
• b Block file (usually found in /dev ; typically the hard disk)
• s Socket file
• p named pipe

Viewing and Adding to files

• cat - to view the contents of a file
• writing to a file : > eg : echo "Hello world" > test.txt
• appending to a file : >> eg : echo "Helo world" >> test.txt

Hard links and Soft links

• inode - An entry in the filesystem table about the location in the storage media
• hard link points to the same inode
• soft link points to a hard link
• hard link must be on the same partition while soft link can point to a file at a totally different geographical location.
• inode is metadata for the file . eg : size ,permissions,blocks etc.
• ls -i <name>
• ln and ln -s is used for creating hard links and soft links
• inode is unique for every file : if there are multiple entries of inode then it means that they are all hard links
  • if there is a dir level1 with inode = 18874686
  • when you cd into that dir . will also have inode = 18874686
  • if i make a dir level2 inside level1 and then cd into level2 .. will have inode = 18874686 (no of hard links will increase by 1)
  • as number of sub directories increases the number of hardlinks also keeps increasing
• users cannot create hard links for directories (level1 to level2 and level2 to level1 will create a back and forth)

Permissions

• Files and directories do not inherit the parent directory permissions
• rwxrwxrwx (777)
  • 7 rwx
  • 6 rw-
  • 5 r-x
  • 4 r--
  • 3 -wx
  • 2 -w-
  • 1 --x
• rwx rwx rwx : Owner Group Others
• only owners can change permissions of a file
• Execute permission is required on a directory to cd into it (Even ls and tocuh to a dir will not work)
• If you want to access a file, all its parent direcories should have x permission. This works even without r and w permissions if you know the path.
• r and w permissions along with x is required to ls a directory or touch a file into a directory
• Removing a file works only if it has write permission

Linux Virtual Machine

• ISO

  • image of Linux OS (Ubuntu 20.04 LTS for x86_64 platform)

• Hypervisor

  • (eg: Oracle VirtualBox or VMWare Workstation Player)
  • A Hypervisor creates and runs virtual machines
  • It allows running multiple operating systems while sharing hardware resources

Command Line Environments

• Cloud - replit and cocalc
• Phone - Termux by Fredrick Fornwall

File System of Linux OS

• Filesystem Hirearchy Standard FHS 3.0 (June 03, 2015) (refspecs.linuxfoundation.org/fhs.shtml)
• / is root directory and field separator or delimiter for sub-directories
• . references the current directory (. is a special file in every directory)
• .. references the parent directory (.. is a special file in every directory)
• Path for traversal can be absolute or relative
• boot directory is where the kernel is located
• /usr/bin contains commands that we will use
• /bin - essential command binaries
• /boot static files of the bootloader
• /dev device files (different character in long format of file listing 'c' instead of 'l' or 'd'. 'c' indicates character file - means you can read from it character by character. if first character is 'b' they are block devices typically hdds - the block devices are made available as files.)
• /etc Host specific system configuration (.conf files)
• /lib Essential shared libraries and kernel modules (Typically contain files with version number at the end)
• /media mount points for removable devices
• /mnt mount points
• /opt add on application software packages
• /run Data relevant to running processes
• /sbin essential system binaries
• /srv data for services
• /tmp temporary files (normally flushed when system is rebooted)
• /usr secondary hierarchy
  • /usr/bin : user commands
  • /usr/lib : libraries
  • /usr/local : local hierarchy
  • /usr/sbin : non vital system binaries
  • /usr/share : architecture dependent data
  • /usr/include : header files included by c programs
  • /usr/src : source code
• /var variable data (/var/log contains logs for various services)
  • /var/cache : Application cache data
  • /var/lib : Variable state informtion
  • /var/local : variable data for /usr/local
  • /var/lock : lock files
  • /var/log : log files and directories
  • /var/run : data relevant to running processes
  • /var/tmp : temporary files preserved between reboots

	Shareable	Unsharable
static	/usr and /opt	/etc and /boot
variable	/var/mail	/var/run and /var/lock