meeting-minutes-sh-posix-sell-session-day-4-july-2020

2020-07-09 16:58:39	+mohan43u-we	#ilugc channel is now in restricted mode
2020-07-09 16:59:51	+mohan43u-we	type '/msg mohan43u-we message' if you want to say something important
2020-07-09 17:00:46	+mohan43u-we	good evening to all
2020-07-09 17:01:06	+mohan43u-we	welcome to Day 4 of sh Posix shell session
2020-07-09 17:01:46	+mohan43u-we	we will continue from where we left yesterday
2020-07-09 17:02:09	+mohan43u-we	just a rewind, yesterday we looked into redirection, pipes, normal and environment variables
2020-07-09 17:03:10	+mohan43u-we	our next topic is control structures, like 'if' elif etc.,
2020-07-09 17:04:08	+mohan43u-we	before going into the topic, just want to let you guys how to disable that joined/left messages in the irc chat
2020-07-09 17:05:00	+mohan43u-we	if you are joining #ilugc through webchat.freenode.net, follow https://pasteboard.co/JgGXsrf.gif this link to disable join/part messages
2020-07-09 17:05:09	+mohan43u-we	now, coming to the topic
2020-07-09 17:05:31	+mohan43u-we	control structures
2020-07-09 17:06:17	+mohan43u-we	remember when I said that every command in linux returns a exitcode
2020-07-09 17:06:47	+mohan43u-we	we can take steps in our commandline based on this returncode
2020-07-09 17:07:34	+mohan43u-we	see, in presenter terminal, I executed two commands in single line by seperating them using ';'
2020-07-09 17:07:45	+mohan43u-we	the first command is a simple 'hi'
2020-07-09 17:07:57	+mohan43u-we	the second command uses '$?'
2020-07-09 17:08:34	+mohan43u-we	this $? is a special shell variable (will take about these kind of variables briefly later) which returns the last executed command's exitcode
2020-07-09 17:09:47	+mohan43u-we	so the two commands executed and returned 'hi' and '0' respectively
2020-07-09 17:10:14	+mohan43u-we	let see what happens when grep command cannot able to find our given string in a file
2020-07-09 17:10:29	+mohan43u-we	we already have a file falled output.txt filled with text
2020-07-09 17:10:39	+mohan43u-we	now, I'm going to use this file for our purpose
2020-07-09 17:11:08	+mohan43u-we	just executed grep command to search for string hi
2020-07-09 17:11:39	+mohan43u-we	lets see what exitcode the previous grep command returned
2020-07-09 17:12:08	+mohan43u-we	the last grep command returned 0 which means success or true in shell language
2020-07-09 17:12:32	+mohan43u-we	again, I executed another grep command, this time I asked it to search for string 'stupid'
2020-07-09 17:12:36	+mohan43u-we	in output.txt file
2020-07-09 17:13:07	+mohan43u-we	this time, grep didn't returned anything as output, because there is no matching line in output.txt which contains string stupid
2020-07-09 17:13:22	+mohan43u-we	now see the return code of the last executed grep command
2020-07-09 17:14:04	+mohan43u-we	see, instread of returning success (or true), it returned 1
2020-07-09 17:14:13	+mohan43u-we	which means, the last grep command failed
2020-07-09 17:14:39	+mohan43u-we	now we are going to use this returncode to decide what to do next using 'if'
2020-07-09 17:16:08	+mohan43u-we	see, we just used if command to check the returncod of 'grep stupid output.txt', if the grep command returned success, we asked if command to execute 'echo success' or else we asked it to execute 'echo failure'
2020-07-09 17:17:08	+mohan43u-we	this is how we control execution of commands using 'if'-'fi', 'if'-'else'-'fi' and 'if'-'elif'-'fi' commands
2020-07-09 17:17:49	+mohan43u-we	this time I only used 'if'-'fi'
2020-07-09 17:17:57	+mohan43u-we	and chcked for only success
2020-07-09 17:18:24	+mohan43u-we	you can learn the syntex of this 'if' command through 'man sh'
2020-07-09 17:19:25	+mohan43u-we	usually it is like this
2020-07-09 17:20:48	+mohan43u-we	if <check-condition-command>; then <true-execution-command>; [elif <another-check-command>] [; else <false-command>; fi
2020-07-09 17:22:24	+mohan43u-we	see, I gave you example of 'if'-elif-else-fi
2020-07-09 17:23:03	+mohan43u-we	you can also use switch command to use multiple branching
2020-07-09 17:24:30	+mohan43u-we	apologies it is called case command
2020-07-09 17:26:52	+mohan43u-we	ok, just trying to show the case-esac through command line itself
2020-07-09 17:27:19	+mohan43u-we	but forgot the exact syntex, but you can get those kind of SYNOPSIS through manual pages
2020-07-09 17:27:33	+mohan43u-we	here is one good manual page for 'sh' posix shell
2020-07-09 17:27:36	+mohan43u-we	https://www.freebsd.org/cgi/man.cgi?query=sh&apropos=0&sektion=0&manpath=FreeBSD+12.1-RELEASE+and+Ports&arch=default&format=html
2020-07-09 17:27:55	+mohan43u-we	you can search for 'esac' key to find the syntext of case-esac control structure
2020-07-09 17:28:46	+mohan43u-we	now, coming to loops
2020-07-09 17:29:09	+mohan43u-we	there are two commands to do looping
2020-07-09 17:29:13	+mohan43u-we	one is the 'for' command
2020-07-09 17:30:22	+mohan43u-we	the for command in shell script is not like you see in programing languages like c/c++ java etc., this for loop is different
2020-07-09 17:31:10	+mohan43u-we	the for command which I executed iterates over values 0 1 2 etc and assigns each value to variable 'n' in each iteration of the loop
2020-07-09 17:31:50	+mohan43u-we	so I asked the for loop to just execute 'echo ${n}", means print the value of $n in each iteration
2020-07-09 17:32:07	+mohan43u-we	thus, the for loop command did print number 0 to 5
2020-07-09 17:33:01	+mohan43u-we	ok, now instead of using numbers why dont we use something else
2020-07-09 17:34:01	+mohan43u-we	see, its just assigning some variable to some value for each iteration
2020-07-09 17:34:34	+mohan43u-we	now, I'm going to introduce you to something called command substitution which will going to help you tremendesly in shell scripting
2020-07-09 17:34:50	+mohan43u-we	I feel this is the right time to introduce command substitution
2020-07-09 17:35:24	+mohan43u-we	instead of just using static values in the for loop, I'm doing to dynamically generate the values for each iteration
2020-07-09 17:35:31	+mohan43u-we	using command substitution
2020-07-09 17:35:36	+mohan43u-we	watch this
2020-07-09 17:35:58	+mohan43u-we	see the 'ls /' command provides all the directory names form '/' directory
2020-07-09 17:37:21	+mohan43u-we	see instead of using static values for each iteration, I just generated the values for each iteration dynamically
2020-07-09 17:38:13	+mohan43u-we	same command, just modified little but to show it in the screen properly
2020-07-09 17:38:37	+mohan43u-we	see, in the for loop, I used $() command substitution method
2020-07-09 17:39:22	+mohan43u-we	this $() command substitution replaces the output of the inner command in the command line
2020-07-09 17:40:13	+mohan43u-we	so when shell executed 'for dir in $(ls /); do echo "${dir}"; done, it actually replaced '$(ls /)' with the actual output of 'ls /' command
2020-07-09 17:40:45	+mohan43u-we	so it become 'for dir in bin dev home lib32' etc..
2020-07-09 17:41:02	+mohan43u-we	so this is called command substitution
2020-07-09 17:41:26	+mohan43u-we	you can use this substitution to assign dynamic values to variables
2020-07-09 17:42:14	+mohan43u-we	see, I first assigned $(ls /) output to variable 'alldirs'
2020-07-09 17:42:22	+mohan43u-we	then I used that variable in the for loop
2020-07-09 17:43:04	+mohan43u-we	see, the output of '$(ls /)' is still available inside 'alldirs' variable, I can use this variable anywhere I want in the shell script
2020-07-09 17:43:22	+mohan43u-we	command substitution is one important aspect in shell scripting
2020-07-09 17:44:14	+mohan43u-we	the other loop command is 'while' command
2020-07-09 17:44:55	+mohan43u-we	the command which I typed in presenter terminal is a sleep forever loop
2020-07-09 17:45:52	+mohan43u-we	the while command checks for the return code  of 'true' command, if the 'true' command returns '0', then it executes 'sleep 1'; otherwise it exits from the loop
2020-07-09 17:46:30	+mohan43u-we	as I already told, 'true' command always returns '0' every time it executes, so the while loop will never exit from looping
2020-07-09 17:47:13	+mohan43u-we	so, we have 'for' and 'while' commands for loop
2020-07-09 17:47:29	+mohan43u-we	we have 'if' and 'case' commands for branching
2020-07-09 17:47:59	+mohan43u-we	command substitution is very important way to generate dynamic values
2020-07-09 17:48:14	+mohan43u-we	now, moving to next topic
2020-07-09 17:48:54	+mohan43u-we	variable expansion
2020-07-09 17:50:13	+mohan43u-we	when we execute 'echo $variablename'command
2020-07-09 17:50:45	+mohan43u-we	shell substitutes the value of variable 'variablename' in the commandline, so the actual command befome 'echo value' and then it execute the commandline
2020-07-09 17:51:53	+mohan43u-we	so, whenevery shell see '$' followed by a string it first tries to substiture the variable then it executes,
2020-07-09 17:52:00	+mohan43u-we	it is called variable expansion
2020-07-09 17:52:44	+mohan43u-we	there are various rules shell follows when expanding variables when it sees '$'
2020-07-09 17:53:03	+mohan43u-we	the man page link I shared with you have all the details
2020-07-09 17:53:13	+mohan43u-we	I'm giving one simple example
2020-07-09 17:54:16	+mohan43u-we	I used $() substitution to generate value for mydirs variable
2020-07-09 17:54:30	+mohan43u-we	so mydirs contains all the directory names from / directory
2020-07-09 17:54:46	+mohan43u-we	second I executed 'echo ${mydirs}' command
2020-07-09 17:54:53	+mohan43u-we	here I used ${mydirs}
2020-07-09 17:55:59	+mohan43u-we	so shell replaced 'echo ${mydirs}' commandline to 'echo bin boot dev etc' then it executed, we already know that echo just outputs whatever we give as its parameters
2020-07-09 17:56:08	+mohan43u-we	now, see this echo command
2020-07-09 17:56:22	+mohan43u-we	see, the difference
2020-07-09 17:57:08	+mohan43u-we	you see, there are different in how shell expands "${mydirs}" and ${mydirs}
2020-07-09 17:59:16	+mohan43u-we	when shell expands ${mydirs}, each line in mydirs variable become different parameters for 'echo' command
2020-07-09 17:59:55	+mohan43u-we	but when shell expands "${mydirs}", the show content of mydirs including '\n' end of line become an single parameter for echo command
2020-07-09 18:00:10	+mohan43u-we	thus the execution provides different output
2020-07-09 18:00:27	+mohan43u-we	ok. already 1 hour over :)
2020-07-09 18:00:51	+mohan43u-we	today also we will extend for another 30 mins
2020-07-09 18:01:09	+mohan43u-we	take a break for 5 mins and do join again
2020-07-09 18:01:16	 *	mohan43u-we taking 5 min break
2020-07-09 18:03:43	+shrini	if you are joining #ilugc through webchat.freenode.net, follow https://pasteboard.co/JgGXsrf.gif
2020-07-09 18:03:45	+shrini	this link to disable join/part messages
2020-07-09 18:04:13	+shrini	by doing this, you can avoid all the join/quit messages
2020-07-09 18:04:30	+shrini	and see only what we type in real
2020-07-09 18:04:47	+shrini	it will be very easy to follow mohan43u-we if you disable these join/quit messages
2020-07-09 18:04:59	+shrini	Hope you all disabled it already
2020-07-09 18:05:25	 *	mohan43u-we back from 5 min break
2020-07-09 18:05:44	+shrini	Another request
2020-07-09 18:05:53	+shrini	change all nicknames to your real names
2020-07-09 18:06:14	+shrini	no need to have your roll number/exam numbers as nick names in IRC
2020-07-09 18:06:28	+mohan43u-we	thanks shrini
2020-07-09 18:06:30	+shrini	it will be good to have real names or any other nick name you link
2020-07-09 18:06:37	+shrini	mohan43u-we: you continue
2020-07-09 18:06:44	+mohan43u-we	back to variable expansion
2020-07-09 18:07:41	+mohan43u-we	there are few more different things you can do when doing variable expansion
2020-07-09 18:10:00	+mohan43u-we	see, I used one expansion method called ':+'
2020-07-09 18:10:59	+mohan43u-we	the second line which assigns value to 'var2' first checks if var1 variable value is availabel or not, if var1 have value, then assign 'value2', thats what the second line is
2020-07-09 18:12:21	+mohan43u-we	see I used ':-' method, the line which assigns value to var3 basically check for variable 'var4', if var4 value not available, then assign 'value3' to 'var3'
2020-07-09 18:12:28	+mohan43u-we	thats why var3 have value3
2020-07-09 18:13:54	+mohan43u-we	see, I used ':-' again, this time, I check for 'var3' if var3 not have value, then assign 'value4', since 'var3' already available and have value, shell would have assigned 'value3' to 'var4'
2020-07-09 18:14:20	+mohan43u-we	this is how you can also do variable expansion
2020-07-09 18:15:13	+mohan43u-we	there are few more method available like ':+', ':-', one more is ':?' which throws error message when the variable is not assigned any value
2020-07-09 18:16:15	+mohan43u-we	see, it throws the error message which I provided, shell first check if there is any variable called 'var6', if it is not available, I asked shell to throw this error message
2020-07-09 18:16:28	+mohan43u-we	since var6 is not there, shell throwed error message
2020-07-09 18:16:59	+mohan43u-we	you can read the man page of 'sh' to get all these special way of expanding shell variables
2020-07-09 18:17:20	+mohan43u-we	now the next topic is arithmetic expansion
2020-07-09 18:18:38	+mohan43u-we	see you can do integer arithmetic using $(()) expansion
2020-07-09 18:19:00	+mohan43u-we	$(()) cannot do floathing arithmetic, for that you need to use 'bc' or 'dc' commands
2020-07-09 18:19:39	+mohan43u-we	I dont have these commands installed, but you can use them for floating point arithmetic
2020-07-09 18:19:57	+mohan43u-we	for integer arithmetic, you can simply use $(())
2020-07-09 18:20:37	+mohan43u-we	see, this is the byte expansion of 5MB
2020-07-09 18:21:27	+mohan43u-we	thus the arithmetic expansion
2020-07-09 18:21:49	+mohan43u-we	now next topic
2020-07-09 18:21:52	+mohan43u-we	special variables
2020-07-09 18:22:22	+mohan43u-we	apart from normal vairables, environment variables, shell provides some built-in special variables
2020-07-09 18:23:03	+mohan43u-we	'$$' is a special variable, it provide the process id of current shell
2020-07-09 18:23:50	+mohan43u-we	'${?}' is also a special variable which shell provides, this variable provides the exitcode (or return code) of last executed command
2020-07-09 18:24:49	+mohan43u-we	"${!}" is also a special variable which provides pid of last executed background command
2020-07-09 18:25:22	+mohan43u-we	you may think that why we execute commands in background, there are time when you need to execute commands in paralled
2020-07-09 18:25:27	+mohan43u-we	*parrallel*
2020-07-09 18:25:46	+mohan43u-we	at that time, '&' and '${!}' comes handy
2020-07-09 18:26:27	+mohan43u-we	see, jobs command will also provide process id of background jobs apart from the jobid of background jobs
2020-07-09 18:27:12	+mohan43u-we	the forever while which I put in the background has '%1' as its job id, but its process id is 853133 which is the same we got from ${!}
2020-07-09 18:27:33	+mohan43u-we	'kill' command is used to kill processes
2020-07-09 18:29:28	+mohan43u-we	I'm stopping here for today. we will see 'functions', 'positional parameters', 'parameter expansion', 'shell patterns' and rest tomorrow
2020-07-09 18:30:00	--	Mode #ilugc [-m] by mohan43u
2020-07-09 18:30:00	_181CS262	tq sir
2020-07-09 18:30:07	suba	thank you
2020-07-09 18:30:07	LENOVO__	tq sir
2020-07-09 18:30:14	+mohan43u-we	#ilugc channel is open for questions
2020-07-09 18:30:16	MOHAN	thank you sir
2020-07-09 18:30:45	--	Mode #ilugc [-v mohan43u-we] by mohan43u
2020-07-09 18:30:51	--	Mode #ilugc [-v shrini] by mohan43u
2020-07-09 18:30:56	--	Mode #ilugc [-v mbuf] by mohan43u
2020-07-09 18:31:46	aaryan	Thank you !!!
2020-07-09 18:32:15	mohan43u-we	aaryan: useful today?
2020-07-09 18:32:29	aaryan	Yes learnt a lot!
2020-07-09 18:32:44	_172IT146	thank you sir
2020-07-09 18:32:53	aaryan	looking forward for tommorows session
2020-07-09 18:33:40	prit	thank you
2020-07-09 18:35:19	Ishwarya	Thank you mohan43u-we
2020-07-09 18:35:23	mohan43u-we	aaryan: day 5 was planned to do example shell scripting, but as I expected, the topics to cover itself pushed into day five. so I'm leaving the writing shell script part to users as excersise.
2020-07-09 18:35:37	ramswamy123	thank you, learned a lot today
2020-07-09 18:36:24	shrini	mohan43u-we: you can cover shell scripting in ilugc meet this saturtday, if possible
2020-07-09 18:37:01	shrini	For all with exam/roll numbers, you can join with your real names itself
2020-07-09 18:37:05	mohan43u-we	all, do read the manual page of 'sh' available here https://www.freebsd.org/cgi/man.cgi?query=sh&apropos=0&sektion=0&manpath=FreeBSD+12.1-RELEASE+and+Ports&arch=default&format=html, it is the official and definitive manual of 'sh' shell
2020-07-09 18:37:07	shrini	we dont take any attendance
2020-07-09 18:38:09	shrini	if you want to practise more, check here - https://www.webminal.org/
2020-07-09 18:38:24	shrini	register and check a web based terminal and more tutorials there
2020-07-09 18:38:39	shrini	learning commandline will give you super powers
2020-07-09 18:39:15	shrini	The floor is open
2020-07-09 18:39:22	shrini	any one can any questions
2020-07-09 18:39:32	shrini	looking for feebnack from you all
2020-07-09 18:39:35	mohan43u-we	if you want to practice **most**, install Linux :) and dont go back to windows if you got stuck :)