Week 1 Notes

Terminal Emulators

Terminal
Konsole
xterm
guake

Command Prompt

username@hostname:~$

~$ is the path

Commands and Flags

username@hostname:~$ 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

username@hostname:~$  pwd

Present Working Directory

username@hostname:~$ 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

username@hostname:~$ 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

username@hostname:~$ mv

move , rename
mv file1 .. (moves file to parent dir)
mv file1 file1a (renames file1 to file1a)

username@hostname:~$ ps

currently running processes

username@hostname:~$ clear

or ctrl+l

username@hostname:~$ exit

or ctrl+d

username@hostname:~$ 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

username@hostname:~$ 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

username@hostname:~$ cp

copy command : cp file1 file2

username@hostname:~$ date

date and time
date -R gives in RFC 5322 standard (used for email communications)

username@hostname:~$ cal

calendar of a month
eg : cal aug 1947
ncal gives calendar in flipped orientation

username@hostname:~$ free

memory statistics
use h flag to make it human readable

username@hostname:~$ groups

- groups to which a user belongs

username@hostname:~$ 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

username@hostname:~$ mkdir

create a directory
default permissions (umask)

username@hostname:~$ touch

used to change the last modified timestamp of a file
also used to create empty files

username@hostname:~$  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)

username@hostname:~$whoami

prints username

username@hostname:~$ less

allows you to read a file page by page

username@hostname:~$  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

username@hostname:~$ 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`