meeting-minutes-sh-posix-sell-session-day-1-july-2020

2020-07-06 17:40:11	+mohan43u-we	the first thing you do in a shell is to see where you are
2020-07-06 17:40:50	@mohan43u	type pwd command to see in which place you are currently present.
2020-07-06 17:41:37	+mohan43u-we	the path /home/trainer is the current working directory
2020-07-06 17:41:54	+mohan43u-we	in unix, everything is a file
2020-07-06 17:42:07	+mohan43u-we	a directory is also a file
2020-07-06 17:42:35	+mohan43u-we	a file in unix is stored in one particular tree
2020-07-06 17:42:56	+mohan43u-we	unix filesytem starts from root
2020-07-06 17:43:40	+mohan43u-we	as you see in the presenter terminal, 'ls /' command will show the directories inside the root
2020-07-06 17:43:51	+mohan43u-we	the command 'ls' means list
2020-07-06 17:44:19	+mohan43u-we	here '/' represents the root of the file system
2020-07-06 17:45:02	+mohan43u-we	every users will have one directory allocated to himself whenever his/her userid created in the unix system
2020-07-06 17:45:09	+mohan43u-we	that is called home directory
2020-07-06 17:45:29	+mohan43u-we	the home directory of all users will be under /home
2020-07-06 17:46:02	+mohan43u-we	as you see in the presenter terminal, I showed user specific directories which are stored inside /home
2020-07-06 17:46:12	+mohan43u-we	my userid is trainer
2020-07-06 17:46:40	+mohan43u-we	so trainer userid have his home directory as /home/trainer
2020-07-06 17:47:21	+mohan43u-we	as you see, ls command will list directory contents of whatever path you give
2020-07-06 17:48:01	+mohan43u-we	'ls' is a basic command which everyone should know
2020-07-06 17:48:19	+mohan43u-we	there are few more options to 'ls' command to change its display behavious
2020-07-06 17:48:27	+mohan43u-we	*behavious*
2020-07-06 17:49:46	+mohan43u-we	as you see, '-l' option to ls make it show the listing in a different way
2020-07-06 17:50:16	+mohan43u-we	when I do 'ls -l /' it show detailed listing of '/' root directory
2020-07-06 17:50:42	+mohan43u-we	so every command in unix have *options* and *argument*
2020-07-06 17:50:57	+mohan43u-we	to read more about what all options 'ls' command have,
2020-07-06 17:51:06	+mohan43u-we	you have to read its manual page
2020-07-06 17:51:36	+mohan43u-we	this is the manual page. I'll explain more in detail about manual page later
2020-07-06 17:52:06	+mohan43u-we	as I previously told, everything in unix is a file
2020-07-06 17:52:25	+mohan43u-we	even a directory is also a file
2020-07-06 17:52:38	+mohan43u-we	to get more details about file, we use 'stat' command
2020-07-06 17:53:13	+mohan43u-we	this stat command is very useful to know what exactly a file represents
2020-07-06 17:53:38	+mohan43u-we	'stat' command can provide you more details about a file or directory like when it created
2020-07-06 17:53:48	+mohan43u-we	when a file got modified last
2020-07-06 17:53:53	+mohan43u-we	etc.
2020-07-06 17:54:07	+mohan43u-we	to give an example, I'm switching to my home directory
2020-07-06 17:55:12	+mohan43u-we	as I explained before, we need to know which directory we are currently using in the entire filesystem before we begin changing the directories
2020-07-06 17:55:33	+mohan43u-we	'pwd' command is useful to see where we are currently in the filesystem
2020-07-06 17:55:58	+mohan43u-we	the prsenter terminal currently show that I'm currently in /home/trainer directory
2020-07-06 17:56:02	+mohan43u-we	which is my home directory
2020-07-06 17:56:13	+mohan43u-we	to switch to '/' directory, we use 'cd' command
2020-07-06 17:56:59	+mohan43u-we	as you can see, pwd command now shows '/' is my current directory
2020-07-06 17:57:43	+mohan43u-we	'cd' command is a shell build-in command
2020-07-06 17:57:52	+mohan43u-we	there are three types of commands in unix
2020-07-06 17:57:57	+mohan43u-we	one is regular commands
2020-07-06 17:58:07	+mohan43u-we	second one is build-in commands
2020-07-06 17:58:18	+mohan43u-we	and third one is 'aliases'
2020-07-06 17:59:05	+mohan43u-we	as you see in presenter terminal, I just created one alias
2020-07-06 17:59:14	+mohan43u-we	for 'ls' command called 'll'
2020-07-06 17:59:35	+mohan43u-we	instead of typing 'ls -ltr' command, we can use 'll' to do the same function
2020-07-06 17:59:54	+mohan43u-we	see, both commands did the same thing
2020-07-06 18:00:23	+mohan43u-we	so this is how you created aliases in unix
2020-07-06 18:00:35	+mohan43u-we	the other type of command is called 'build-in' command
2020-07-06 18:01:06	+mohan43u-we	before explaining 'cd', just want to mention 'clear' command
2020-07-06 18:01:21	+mohan43u-we	'clear' command is used to clear the screen and put prompt in the top line
2020-07-06 18:01:58	+mohan43u-we	as you see, I justtyped 'll' then followed by 'clear' which clears the terminal and put the prompt at the start of the line
2020-07-06 18:02:51	+mohan43u-we	now, one more command to explain before explaining build-in command
2020-07-06 18:02:55	+mohan43u-we	'which' command
2020-07-06 18:03:15	+mohan43u-we	'which' command will show the full filename of a command
2020-07-06 18:04:07	+mohan43u-we	as you see in the presenter terminal, I typed 'which which', here I'm asking which command to find out full filename of 'which' command (that is itself)
2020-07-06 18:04:33	+mohan43u-we	so the which command says, its filename is '/usr/bin/which'
2020-07-06 18:05:13	+mohan43u-we	'which' command takes one argument and show the full filename of that command
2020-07-06 18:05:34	+mohan43u-we	as you see, when I type 'which ls', it shows me full filename of ls command
2020-07-06 18:05:58	+mohan43u-we	but, when I typed 'which cd' it doesn't show any output
2020-07-06 18:06:24	+mohan43u-we	this is because 'cd' is a shell built-in command
2020-07-06 18:07:06	+mohan43u-we	as you see, the manual page of 'sh' will explain what 'cd' command is doing
2020-07-06 18:07:35	+mohan43u-we	a builtin command will not be present in the filesystem like the normal commands like 'ls' or 'which'
2020-07-06 18:07:45	+mohan43u-we	they are built inside the 'sh' shell itself
2020-07-06 18:09:05	+mohan43u-we	whenever you execute a built-in command, shell itself will execute it instead of searching for the command in filesystem and then start executing
2020-07-06 18:09:58	+mohan43u-we	as you see in the presenter terminal, there are lot of build-in command each shell provides, it will be documented inside the manual page of that particular shell
2020-07-06 18:10:28	+mohan43u-we	'echo' is another important shell built-in command
2020-07-06 18:11:53	+mohan43u-we	we already know 'which' command can provide location of a command
2020-07-06 18:12:18	+mohan43u-we	but 'which' command uses 'PATH' variable to know the locations of the commands
2020-07-06 18:13:17	+mohan43u-we	as you see in presenter terminal, PATH variable consists of paths like /usr/local/sbin, /usr/local/bin, seperated by ':'
2020-07-06 18:13:56	+mohan43u-we	so everytime we execute a command, 'sh' shell will check whether the command is a built-in command first, if yes, then it will execute automatically
2020-07-06 18:14:34	+mohan43u-we	if the given command is not built-in, then it will look for 'alias' list, if an alias matches with the given command, 'sh' shell will execute the alilas
2020-07-06 18:14:43	+mohan43u-we	*alias*
2020-07-06 18:15:12	+mohan43u-we	if the given command is not a built-in as well as alias, then it will search these paths one by one
2020-07-06 18:15:28	+mohan43u-we	for example, when I give 'ls' comamnd to shell
2020-07-06 18:15:56	+mohan43u-we	'sh' will take PATH variable then go to the first path
2020-07-06 18:16:04	+mohan43u-we	and see any filename matching 'ls'
2020-07-06 18:16:58	+mohan43u-we	as you see, the 'stat' command tells that there is no filename called 'ls' in the first path mentioned in PATH variable
2020-07-06 18:17:06	+mohan43u-we	then 'sh' shell takes the second path
2020-07-06 18:17:37	+mohan43u-we	so there also, 'ls' filename is not available, then it checks /usr/sbin
2020-07-06 18:17:48	+mohan43u-we	no, the command is also not there
2020-07-06 18:18:21	+mohan43u-we	see, the last command 'stat /usr/bin/ls' show the details of the filename
2020-07-06 18:18:42	+mohan43u-we	that means, 'ls' command is inside '/usr/bin' directory, this directory is also mentioned in PATH variable
2020-07-06 18:18:59	+mohan43u-we	thats how 'sh' shell finds a command and executes that command
2020-07-06 18:20:04	+mohan43u-we	after identifying the location, 'sh' will start creating a process for that program with filename '/usr/bin/ls' and executes that program
2020-07-06 18:20:20	+mohan43u-we	so it is very important to know the purpose of PATH variable
2020-07-06 18:20:39	+mohan43u-we	this variable consists of multiple directory paths seperated by ':'
2020-07-06 18:21:20	+mohan43u-we	shell will for given command in each and every directory in left-to-right order
2020-07-06 18:21:48	+mohan43u-we	so /usr/local/bin takes priority than /usr/local/bin
2020-07-06 18:22:00	+mohan43u-we	/usr/sbin takes priority than /usr/bin
2020-07-06 18:22:03	+mohan43u-we	like that
2020-07-06 18:22:45	+mohan43u-we	I think I covered the topics for today
2020-07-06 18:23:25	+mohan43u-we	we will be releasing the restriction now, so anyone have doubt please ask
2020-07-06 18:23:41	+mohan43u-we	I hope everyone followed the commands I executed in the presenter terminal
2020-07-06 18:25:25	+mohan43u-we	here are few links to start learning shell
2020-07-06 18:25:28	+mohan43u-we	http://www.linuxtraining.co.uk/download/new_linux_course_modules.pdf
2020-07-06 18:25:38	+mohan43u-we	https://freetamilebooks.com/ebooks/learn-gnulinux-in-tamil-part1/
2020-07-06 18:27:20	--	Mode #ilugc [-m] by mohan43u
2020-07-06 18:27:23	aaryan7476	What's the difference between 'whereis' and 'which' command?
2020-07-06 18:27:35	aaryan7476	Also when will be the next session?
2020-07-06 18:27:45	+mohan43u-we	whereis need 'locate' database
2020-07-06 18:27:45	ravisankar	?
2020-07-06 18:27:51	+mohan43u-we	which uses PATH command
2020-07-06 18:27:57	+mohan43u-we	ravisankar: go ahead
2020-07-06 18:28:14	ravisankar	how to search in man
2020-07-06 18:28:39	ravisankar	i mean how to find a particular text in manual page
2020-07-06 18:28:43	+mohan43u-we	ravisankar: man -k <searnword>
2020-07-06 18:28:43	aaryan7476	How is sh different from my normal terminal screen?
2020-07-06 18:28:59	rajkumar	?
2020-07-06 18:29:01	+mohan43u-we	aaryan7476: there are lot of terminals available for unix/linux
2020-07-06 18:29:13	ravisankar	?
2020-07-06 18:29:14	aaryan7476	So sh is standard we used
2020-07-06 18:29:20	+mohan43u-we	aaryan7476: you typically land in 'bash' if you are using 'osx' and 'linux'
2020-07-06 18:29:34	+mohan43u-we	aaryan7476: no 'bash' is standard for mordern unix
2020-07-06 18:29:41	+mohan43u-we	ravisankar: go ahead
2020-07-06 18:30:22	ravisankar	how to delete a particular alias
2020-07-06 18:30:36	+mohan43u-we	ravisankar: unalias <alias>
2020-07-06 18:30:50	+mohan43u-we	see the presenter terminal
2020-07-06 18:31:02	rajkumar	?
2020-07-06 18:31:05	+mohan43u-we	the ll alias which I set now gone
2020-07-06 18:31:09	+mohan43u-we	rajkumar: go ahead
2020-07-06 18:31:15	rajkumar	Is 'sh' a file? So Linux boots with it?
2020-07-06 18:31:33	+mohan43u-we	'sh' is also a program
2020-07-06 18:31:50	+mohan43u-we	see presenter terminal 'sh' lives inside /usr/bin
2020-07-06 18:32:09	bharath	?
2020-07-06 18:32:14	aaryan7476	So if I add an alias in my 'sh' it won't interfere with my normal 'bash' or 'zsh'(in OS X)
2020-07-06 18:32:19	aaryan7476	*x)
2020-07-06 18:32:20	+mohan43u-we	but when you start a terminal, the terminal command typically will use /usr/bin/sh to show you one terminal
2020-07-06 18:32:25	+mohan43u-we	bharath: go ahead
2020-07-06 18:32:48	bharath	how to clear the whole terminal?
2020-07-06 18:32:50	+mohan43u-we	aaryan7476: sh is a different terminal than bash or zsh
2020-07-06 18:32:55	Aachman_m_	how to see the presenter terminal
2020-07-06 18:32:58	+mohan43u-we	bharath: 'clear' command
2020-07-06 18:33:10	rajkumar	?
2020-07-06 18:33:15	+mohan43u-we	Aachman_m_: https://training.ilugc.in
2020-07-06 18:33:20	+mohan43u-we	rajkumar: go ahead
2020-07-06 18:33:34	bharath	but if we use clear command if we scroll up the terminal it show older commans
2020-07-06 18:33:40	rajkumar	How to visualize a file as directory? Please elaborate.
2020-07-06 18:33:54	bharath	i want to clear whole terminal any command?
2020-07-06 18:34:20	+mohan43u-we	bharath: you want to clear history, then use 'history -c'
2020-07-06 18:34:48	+mohan43u-we	rajkumar: not able to understand your question, visualize means?
2020-07-06 18:35:25	rajkumar	You said files are files and directories are also files. I'm trying to understand this.
2020-07-06 18:35:45	Aachman_m_	clear is the comand
2020-07-06 18:36:16	ravisankar	?
2020-07-06 18:36:49	+mohan43u-we	rajkumar: normally, we use to store data in file and files inside directory, when we say everything is a file, the directory is also like a file whose contents are details about files
2020-07-06 18:37:44	+mohan43u-we	rajkumar: when we do 'ls', that command basically read the a content of a file (but represented as directory) in the filesystem
2020-07-06 18:37:52	+mohan43u-we	ravisankar: go ahead
2020-07-06 18:38:01	ravisankar	once we get into shell using sh comman, tab key wont give results... why?
2020-07-06 18:38:32	+mohan43u-we	ravisankar: sh is not bash, it is oldest created for original unix
2020-07-06 18:39:11	+mohan43u-we	ravisankar: there are lot of shells available for unix based os, like csh, ksh, bash, ksh, fish, dash, ash, etc.,
2020-07-06 18:39:28	+mohan43u-we	ravisankar: each shell provides you more functionalities
2020-07-06 18:39:37	aaryan7476	Where are these terminals located?
2020-07-06 18:39:43	aaryan7476	in my fs
2020-07-06 18:39:44	+mohan43u-we	ravisankar: bash give you tab completion
2020-07-06 18:40:08	+mohan43u-we	ravisankar: sh is not that advanced, but it is simple
2020-07-06 18:40:42	+mohan43u-we	aaryan7476: mostly in /usr/bin, if the shell is not available, then you have to install through your distro's package manager
2020-07-06 18:41:02	ravisankar	?
2020-07-06 18:41:26	+mohan43u-we	ravisankar: go ahead
2020-07-06 18:41:30	aaryan7476	Thank you :)
2020-07-06 18:41:37	ravisankar	how do i know my default shell name? and also how do i know the available shells in my os?
2020-07-06 18:41:39	+mohan43u-we	aaryan7476: welcome
2020-07-06 18:41:52	+mohan43u-we	ravisankar: echo $0
2020-07-06 18:43:05	ravisankar	how do i know the available shells in my os?
2020-07-06 18:43:14	+mohan43u-we	ravisankar: there is no seperation between shells and other commands, everything lives inside /usr/bin directory nowadays
2020-07-06 18:43:39	+mohan43u-we	ravisankar: if you have installed that shell through your package manager, then it will be available in /usr/bin
2020-07-06 18:43:55	Aachman_m_	am I late?
2020-07-06 18:44:05	+mohan43u-we	Aachman_m_: when you joined?
2020-07-06 18:44:53	Aachman_m_	6:27
2020-07-06 18:44:56	ravisankar	what are all the variables that we could pass it as argument to echo?
2020-07-06 18:45:23	+mohan43u-we	Aachman_m_: session started properly at 5.40 and I completed around 6.30
2020-07-06 18:45:43	+mohan43u-we	ravisankar: I'll explain about variable in coming days
2020-07-06 18:45:56	ravisankar	ok, thank you
2020-07-06 18:46:09	+mohan43u-we	ravisankar: echo is a simple command, it doesn't know the different between 'hi' and '${hi}"
2020-07-06 18:46:41	--	Mode #ilugc [-v mohan43u-we] by mbuf
2020-07-06 18:46:48	mohan43u-we	ravisankar: the magic is done by shell, echo is just as its name says, echos whatever we give
2020-07-06 18:47:13	mohan43u-we	ravisankar: see presenter terminal
2020-07-06 18:48:14	ravisankar	ok
2020-07-06 18:48:29	ravisankar	(y)
2020-07-06 18:48:34	mohan43u-we	so 'echo' is just going to dump whatever we give it as its argument
2020-07-06 18:49:11	mohan43u-we	ravisankar: but the second echo I executed was specally processed by shell, it is called parameter expansion
2020-07-06 18:52:15	mohan43u-we	anyone have other doubts, feel free to ask here. today's session is now completed. see you tomorrow at 5.00 pm.
2020-07-06 18:52:33	ravisankar	thank you, looking for the next session.. :]
2020-07-06 18:52:42	rajkumar	mohan43u-we: The session was simple and to the point. Thank you! Logistics can be improved in coming classes. Like preventing people from posting roll numbers. Good start!
2020-07-06 18:52:45	aaryan7476	Thank you so much mohan43u-we
2020-07-06 18:53:23	_181CS166	Thankyou
2020-07-06 18:53:47	Aachman_m	see you tommorow
2020-07-06 18:56:04	shrini	thanks mohan43u-we for the good start
2020-07-06 18:56:19	shrini	Did not expected this high number of participants
2020-07-06 18:56:28	gnurenga	Thank you mohan43u-we
2020-07-06 18:56:38	mohan43u-we	rajkumar: yes, we will make sure we put the channel into restricted mode before we start the session.
2020-07-06 18:57:04	mohan43u-we	gnurenga: the website broke in high load :)
2020-07-06 18:57:31	gnurenga	yes mohan I came to know why
2020-07-06 18:57:47	gnurenga	Mohan I have your no will call you
2020-07-06 18:57:57	gnurenga	in 30 mins
2020-07-06 18:58:03	mohan43u-we	gnurenga: sure
2020-07-06 19:15:51	SATHISHKUMARPVR	reply me anyone is there to chat
2020-07-06 19:16:07	shrini	yes SATHISHKUMARPVR
2020-07-06 19:16:08	shrini	tell
2020-07-06 19:16:12	mohan43u-we	yes
2020-07-06 19:16:30	mohan43u-we	SATHISHKUMARPVR: ask your question
2020-07-06 19:18:16	SATHISHKUMARPVR	sorry the connection with web is problematic here so i cane late i want to know
2020-07-06 19:19:34	SATHISHKUMARPVR	where do our commands actually saved and where they run
2020-07-06 19:21:25	SATHISHKUMARPVR	anyone here reply
2020-07-06 19:21:50	SATHISHKUMARPVR	Mr.mohan just quit
2020-07-06 19:24:21	mohan43u	SATHISHKUMARPVR: sorry, not able to understand your question, can you rephrase again? every shell command lives in one of the directories specified in PATH variable
2020-07-06 19:25:50	mohan43u	SATHISHKUMARPVR: "where they run"? wherever you invoke, from there commands run. there is something called 'present working directory' or 'current working directory'
2020-07-06 19:26:11	mohan43u	SATHISHKUMARPVR:'pwd' is the command to see where your current locaiton is in the shell
2020-07-06 19:26:32	mohan43u	SATHISHKUMARPVR: you invoke shell commands from the current working directory
2020-07-06 19:26:44	mohan43u	SATHISHKUMARPVR: did I answer your question?
2020-07-06 19:27:02	SATHISHKUMARPVR	i mean if i type whoami where did it execute
2020-07-06 19:28:01	SATHISHKUMARPVR	t/4
2020-07-06 19:28:02	mohan43u	SATHISHKUMARPVR: you execute 'whiami' from the current directory, the executable file of 'whoami' command resides in /usr/bin/whoami
2020-07-06 19:29:23	SATHISHKUMARPVR	it means it will go using path env vatiable and execute it right
2020-07-06 19:32:49	mohan43u	SATHISHKUMARPVR: yes, it uses PATH variable, I explained it briefly during the session. you might have missed the session. We will upload the session logs and provide you link to rewind
2020-07-06 19:36:16	jacke123	df
2020-07-06 19:40:53	mohan43u	jacke123: command not found
2020-07-06 19:42:54	jacke123	help
2020-07-06 19:46:13	mohan43u	jacke123: what kind of help you need?
2020-07-06 19:46:35	jacke123	I have log out
2020-07-06 19:46:58	mohan43u	jacke123: you want to quit from IRC? type /quit
2020-07-06 19:47:43	jacke123	do you provide the video link of today session
2020-07-06 19:49:41	Nero007	Hi folks
2020-07-06 19:56:33	shrini2	hi Nero007
2020-07-06 19:58:29	Nero007	I think many people here are from Linux durgapur group...
2020-07-06 20:12:34	shrini2	yes Nero007
2020-07-06 20:12:44	shrini2	many were from a engg college in tamilnadu
2020-07-06 20:13:30	Nero007	Ah I see