Adds basic_cmd command informations

Linux/basic_cmd.md

@@ -84,6 +84,13 @@ back to previous folder
 
 > rm -rf filename
 
+Delete all of the files in the diectory including all subdirectories and tier contents
+> $ rm -r \* .\*  
+
+Remove all files with the .doc extension recursively in the current working directory.
+> $ rm \*\*/\*.doc
+
+
 - Mv: Move directory (can be used to rename a file)
 > mv file /opt/movedfile
 
@@ -185,7 +192,52 @@ total 4
 > -rw-r--**rw**- 1 marie marie 12 Apr  7 05:44 test
 
 
-Number alternatives: 
+##Using chmod in absolute mode
+
+In the absolute mode, permissions are represented in numeric form (octal system to be precise). In this system, each file permission is represented by a number.
+
+- r (read) = 4
+- w (write) = 2
+- x (execute) = 1
+- – (no permission) = 0
+
+With these numeric values, you can combine them and thus one number can be used to represent the entire permission set.
+
+| Number | Permission |
+|--------|------------|
+| 0 | — |
+| 1 | –x |
+| 2  | -w- |
+| 3 (i.e. 2+1) | -wx |
+| 4 | r– |
+| 5 (i.e. 4+1) | r-x |
+| 6 (i.e. 4+2) | rw- |
+| 7 (i.e. 4+2+1) | rwx |
+
+most commonly used:
+755 644 600 640
+
+Can you guess the file permission in numbers on agatha.txt file in our example so far? That’s right, it’s 764.
+
+Now that you know what number represents which permission, let’s see how to change file permission using this knowledge.
+
+Suppose you want to change the file permission on agatha.txt so that everyone can read and write but no one can execute it? In that case, you can use the chmod command like this:
+
+> $ chmod 666 agatha.txt
+
+## Danger :  if a folder has not the X (executable) right => you cannot open it.
+  
+-R for recursive on folder 
+
+If you list agatha.txt now, you’ll see that the permission has been changed.
+
+> -rw-rw-rw- 1 abhishek abhishek 457 Aug 10 11:55 agatha.txt 
+
+- Chown: change file owner and group
+> $ sudo chown marie:marie agatha.txt  
+
+> -rw-rw-rw- 1 marie marie 457 Aug 10 11:56 agatha.txt 
+
 ### Groups
 
 Groups: print the groups a user is in
@@ -206,9 +258,56 @@ marie steve
 
 ## Sysadmin tools 
 
+- How to create a symbolic link in Linux
+To create a symbolic link to target file from link name, 
+you can use the ln command with -s option like this:
+
+> ln -s target_file link_name
+
+The -s option is important here. It determines that the link is soft link. If you don’t use it, it will create a hard link. I’ll explain the difference between soft links and hard links in a different article.
+
 - Htop: Interactive processes viewer
 > $ htop
 
+- Changer default shell
+> $ vim /etc/passwd
+```bash
+steve:x:1002:1002:,,,:/home/steve:/bin/bash
+steve:x:1002:1002:,,,:/home/steve:/bin/fish
+
+```
+
+- |: pipe send result of the first command to the second
+> cat /etc/passwd **|** **grep** bash |**cut** -d ":" -f1 
+
+- Grep: print lines that match patterns
+> $ 
+
+Search for specific text with grep command
+> $ grep -l example document1.txt document2.txt  
+  
+> $ grep -l example \*.txt
+
+grep as long as you include the -r (recursive) option in the command.
+
+> $ grep -lr example /path/to/directory1/\*.txt /path/to/directory2
+
+Or, to search the current directory and all subdirectories, omit the path at the end of the command.
+
+> $ grep -lr example
+
+
+- Cut: remove sections from each line of files
+> $ 
+
+- Wc: print newline, word, and byte counts for each file
+> $ wc -l
+
+```bash
+$ realpath example.txt
+/home/username/example.txt
+```
+
 ## text editor
 - Nano: Nano's ANOther editor, an enhanced free Pico clone
 (simple text editor for noobies)