Merge pull request Tikam02#152 from sundaramawasthi/＜sundram＞

＜sundram＞

LINUX/Basic_Command

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+## LINUX COMMAND
+
+##DIRECTORY
+
+    sudo - super user do
+---
+    pwd --> displays present working directory
+---
+    mkdir (directory name)  --> This command create a directory
+---
+    cd (directory/folder where you want to navigate) -->This command change directory.
+---
+    ls --> lists contents of current directory
+---
+    ls -l --> list contents of current directory with detailed output
+---
+    ll --> list contents of current directory with detailed output
+---
+    cd <folder>
+---
+    cd .. --> to come out of a directory
+---
+    rm -r testdir --> to remove a directory
+---
+    ~ --> Home directory
+---
+    mkdir -p folder/subfolder/subfolder2 --> create parent and child directory
+
+---
+---
+
+##FILE
+
+    touch test.txt(file name) --> creates a empty file
+---
+    ls -l --> lists the contents in current directory
+---
+    echo  Print Output
+
+    echo "This is a text" > test.txt
+---   
+    cat test.txt --> writes the content of file into terminal & exits
+---
+    more test.txt --> writes the content of file on terminal page by page ( to move to next page need to hit space bar on keyboard )
+---
+    less test.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
+---
+    vi test.txt --> Visual Editor
+    		-This opens the file in read only mode
+    		-To edit the file, press 'i' on keyboard for INSERT mode & then you can write anything
+    		-once the text is entered, press 'ESC' on keyboard to return back on 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
+---
+    rm test.txt --> To remove a file
+---
+COPY/RENAME/MOVE/OPERATION
+
+    While performing these operations, we can always use absolute paths or relative paths accrodingly.
+    These commands expects source & dest values
+
+    cp A.txt B.txt  --> makes a copy of A.txt names it to B.txt in current directory
+
+    cp /home/A.txt /tmp/A.txt --> make a copy of A.txt to /tmp
+
+    cp -r testdir/ newdir/ --> makes a copy of testdir & names it to newdir in current dirctory
+
+    cp -r /home/testdir /tmp/testdir --> makes a copy of testdir to /tmp
+
+    mv A.txt new.txt ---> renames A.txt to new.txt in current path
+
+    mv A.txt /tmp ---> moves to A.txt to /tmp
+
+    mv testdir newdir --> renames testdir to newdir in crrent path
+
+    mv newdir /tmp ---> moves newdir to /tmp path
+
+---
+
+##PERMISSION
+
+Observe the output of ls:
+
+drwxrwxr-x 2 manifoldailearning manifoldailearning 4096 Nov 7 23:38 testdir
+-rw-rw-r-- 1 manifoldailearning manifoldailearning 126 Nov 7 23:37 abc.txt
+-rw-rw-r-- 1 manifoldailearning manifoldailearning 126 Nov 7 23:38 one.txt
+
+    drwxrwxr-x or -rw-rw-r--   --> Read/Write/Execute Permission for a file or a directory in linux
+
+    (1st value)manifoldailearning --> owner of the file/directory
+    (2nd value)manifoldailearning --> group who owns file/directory
+
+---
+
+How to update the owner & group for a file/dir -
+
+    chown ( change owner ) is the command to update the owners of dir/file
+    note: note that you need to have previliges to update the permissions for a file/directory
+
+    Syntax: chown owner:group filename/dirname
+
+---
+
+How to update the read,write & execute permissions for a file/dir
+
+    	How to decode the Read/Write/Execute Permission terminology:
+
+    	-/d    ---> denotes if it is a file or directory ( - means file / d means directory )
+    	rwx    ---> means read,write & execute permissions for super user ( root )
+    	rw-    ---> means read,write permissions for the owner of the file ( ex: manifoldailearning 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, in the case of a directory, search it.
+
+    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
+    	If super user needs to have read,write & exeute give 7
+    	If the owner need to read & write give 6
+    	If other need to have only read give 4
+
+
+    	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
+---