Basic Linux Commands

Directory Operation Commands:

        pwd -- displays prsent working direcotry
	mkdir testdir  --- create a directory 
	cd testdir -- change direcotry
	ls -- list contents of current directory
	ls -l -- list contents of current dirctory with detailed output
	ll -- list contents of current directory with detailed output
	cd .. -- to come out of a directory 
        rm -r testdir -- to remove a directory
	

File Operations:

touch abc.txt -- creates a empty file 
ls -l -- lists the contents in current directory
cat abc.txt -- writes the content of file into terminal & exits 
more abc.txt -- writes the content of file on terminal page by page ( to move to next page need to hit space bar on keyboard )
less abc.txt -- open the file on terminal & can be read line by line (use arrows to scroll up & down)     
                 to  comeout need to press 'q' on the key board
rm abc.txt -- to remove a file 
 
vi abc.txt -- opens the file in read only mode
      to edit the file, press 'i' on keyboard for INSERT mode & then you can write anything
      once required text is entered press 'ESC' on keyboard to back readonly mode
      press ':wq' on keyboard -- to save & come out of the file
      press ':q' on keyboard -- to come out of the file without saving
      press ':wq!' on keyboard -- to save forcefully & come out of the file
      press ':q!' on keyboard -- to come out of the file forcefully without saving
               ( note: vi is an file editor, you can use any editor you like by installing it on your machine )

Copy/Rename/Move Operations:

We can always use absolute or relative paths for these operations. these commands expect source & dest values

	
	cp one.txt two.txt  -- makes a copy of one.txt names it to two.txt in current directory
	cp /home/one.txt /tmp/one.txt -- makde a copy of one.txt to /tmp 
	cp -r abcdir/ newdir/ -- makes a copy of abcdir & names it to newdir in current dirctory 
	cp -r /home/abcdir /tmp/abcdir -- makes a copy of abcdir to /tmp
	
	mv one.txt new.txt --- renames one.txt to new.txt in current path
	mv one.txt /tmp --- moves to one.txt to /tmp
	mv testdir newdir -- renames testdir to newdir in crrent path 
	mv newdir /tmp --- moves newdir to /tmp path

File/Dir Permissions

drwxr-xr-x 2 naresh devops     4096 Jun 18 07:59 aws/
-rw-r--r-- 1 naresh devops   20276 Feb 20 04:17 ansible.cfg
-rw-r--r--  OR drwxr-xr-x   ---> are the read/write/execute permission for a file or directory in linux 
	
naresh -- is the owner of the file / directory 
devops -- group owns the the file / direcotry

To update the owner & group for a file/dir

chown ( change owner ) is the command to update the owners of dir/file

note that you need to have privileges to update the permissions for a file/directory

	
chown owner:group filename/dirname
chown ravi:dbadmin filename/dirname -- updates the owner & group of the file/directory to ravi & dbadmin

To update the read, write & execute permissions for a file/dir

we can see read, write, and execute permission below

-/d - -> denotes if it is a file or directory ( - means the file / d means directory )
rwx - -> means read, write & execute permissions for the super user ( root )
rw- - -> means read, write permissions for the owner of the file ( ex: naresh as above )
r - - -> means read-only permissions for others ( whoever login to the machine )
r - Permission to read the file.
w - Permission to write (or delete) the file.
x - Permission to execute the file, or search it in the case of a directory.

chmod ( change mode ) is the command to update the read/write/execute permissions for a file/directory

r = 4, w = 2, x = 1
- | rwx | rwx | rwx
- | 421 | 421 | 421
to update the permissions we can sum 421

means, if the superuser needs to have read, write & execute give 7 ( same applicable for owner & others ), if the owner needs to read & write give 6 ( same relevant for root & others ), if others need to have only read give 4 ( same applicable for root & owner ), so

chmod 777 file/dir - rwx for root, rwx for owner, rwx for others
chmod 764 file/dir - rwx for root, rw for owner, r for others
chmod 755 fire/dir - rwx for root, rw for owner, rw for others
chmod 400 firl/dir - r for root, for owner/other no permissions

likewise, we can update the permissions as needed