list of some common settings, numerical values and their meanings:

-rw------- (600) -- Only the user has read and write permissions.

-rw-r--r-- (644) -- Only user has read and write permissions; the group and others can read only.

-rwx------ (700) -- Only the user has read, write and execute permissions.

-rwxr-xr-x (755) -- The user has read, write and execute permissions; the group and others can only read and execute.

-rwx--x--x (711) -- The user has read, write and execute permissions; the group and others can only execute.

-rw-rw-rw- (666) -- Everyone can read and write to the file. Bad idea.

-rwxrwxrwx (777) -- Everyone can read, write and execute. Another bad idea.

syntax: <variable_name>=<variable_value>
eg: MYNAME=devops_user

How to print / check / substitute / call values of a variable?
--------------------------------------------------------------

$ (dollar) is mandatory while substituting varaible ==> $<variable_name>

echo $<variable_name>

ex: PLAYER=viratKohli
echo " my favorite sportsmen is $PLAYER "

Variable types:
---------------
1. Local variables / user defined variables.
variables created by user

2. system / predefined variables.
variables created by system
env ==> command to check system defined variables

user defined variables are only valid till your terminal(session) exists or your servers running.

to store variables irrespective of terminal running or restarts

How to set variable permanately in all shell sessions & also server restarts ?
------------------------------------------------------------------------------
Answer: By adding the variable inside startup script ( ~/.bashrc ).

*******IMPORTANT********************

EXIT CODE
-------------
every command /script generates an auomatic code which signifies, previous command execution is succesfull or not
syntax: echo $?

if output is 0 zero ==> command executed succesfully
if output is non-zero ==> Command not executed succesfully

SHELL:
------
1 Shell is responsible to read command provided by user.
2. Shell will check whether the command is valid or not.
3. Shell interpretes (converts) that command into kernel understandable form & Kernel execute that command with the help of hardware.

Shell + kernel = Linux OS.

Types of shell:
---------------
1. sh ==> basic shell, earlier / olden days is used in unix ==> /bin/sh
2. bash ==> adavnced shell, which is used in linux ==> /bin/bash
others: c-shell, k-shell etc

Note: echo $SHELL ==> Command to check which shell we are using

Steps to create & execute shell script:
----------------------------------------
step1. shell script will have extension called <script-name>.sh
vi <script-name>.sh
step2. in the script first line we are going to write is called as shebang #!/bin/bash

step3. you add your commands (save the script file)

step4. execute the script
sh <script-name>.sh (or) bash <script-name>.sh (or) ./<script-name>.sh

what is shebang line, why its sused?
------------------------------------
shebang line is used to specifies interpreter (or) shell to be used for execution of script

can we run script without shebang line?
------------------------------------------
yes, in that case whatever default shell ia there, that will get used.

script1. write a shell script to print date & time.
---------------------------------------------------
vi date-script.sh

#!/bin/bash
echo "Current date and time...."
date

save & quit

execute script
sh date-script.sh

script2. write a shell script to CHECK Storage(HDD) of server.
--------------------------------------------------------------

vi STORAGE-script.sh

#!/bin/bash
echo "Current storage used is ...."
df -kh | head -5 | tail -1 | awk -F " " '{print$5}'

save & quit

execute script
sh STORAGE-script.sh

script3: write a script to create a variable (sportsman=viratkohli) & verify the variable creation using echo command:
----------------------------------------------------------------------------------------------------------------------
[ec2-user@ip-172-31-6-171 ~]$ cat vars_check.sh
#!/bin/bash
sportsman=viratkohli
echo "my fav sportsamen is $sportsman"

script4: write a script to have employee data coming as variables & verify the variable creation using echo command:
----------------------------------------------------------------------------------------------------------------------
[ec2-user@ip-172-31-93-35 ~]$ cat employee_data.sh

#!/bin/bash
EMP_NAME=alice
EMP_ID=54234
EMP_LOC=bangalore
EMP_STATE=karnataka
echo "Display employee name $EMP_NAME "
echo "Display emloyee id $EMP_ID "
echo "Display employee location $EMP_LOC "
echo "Display employee state $EMP_STATE "

script5: script to check harware resources:
-------------------------------------------
[ec2-user@ip-172-31-6-171 ~]$ cat ram_usage_check.sh

#!/bin/bash
echo "Current free ram available is..."

free -m | grep Mem | awk -F " " '{print $4}'
#free -m

echo "current disk usage is..."
df -kh .

echo "number of cpu...."
nproc