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+# homework 1
+  `$ ls -l 'which passwd'`  
+  `-rwsr-xr-x 1 root root 54256 พ.ค.  17 06:37 /usr/bin/passwd`  
+  //passwd được cài đặt setuid('s' trong quyền trên), nên nó chạy như owner của tệp chứ không phải user gọi nó.  
+  //do đó khi dùng passwd , setuid khiến cho chương trình có quyền root nên có quyền wr với tệp /etc/shadow  
+# homework 2
+  `$ mkdir sharing-directory`  
+  `$ groupadd framgia`  
+  `$ sudo chgrp framgia sharing-directory` //chuyển group thành framgia  
+  `$ chmod o= sharing-dicrectory` // cài lại quyền other user  
+  `$ sudo chmod g+s sharing-directory` //các tệp và thư mục con tạo ra sẽ kế thừa id group của thư mục cha, không ảnh hưởng đến owner ID  
+
+# homework 3
+### display line number:
+* set number  
+* set num  
+### add support elixir language:
+* clone vim-elixir: `git clone https://github.com/elixir-editors/vim-elixir`
+* Copy the contents of each directory in the respective directories inside ~/.vim.  
+### set file encodings:
+* set fileencodings=utf-8,shift-js,sjis  
+### disable arrow keys:
+* noremap <Up> <Nop>  
+* noremap <Down> <Nop>  
+* noremap <Left> <Nop>  
+* noremap <Right> <Nop>  
+### replace tab with 4 spaces:
+* set tabstop=4  
+* set shiftwidth=4  
+* set expandtab   
+
+# homework 4
+* create public-private key, named it dev_rsa:  
+`ssh-keygen -N '' -f dev_rsa -t rsa`  
+
+* File `~/.ssh/config`  
+```
+Host dev  
+  HostName `128.45.53.123`  
+  Port 2222  
+  IdentityFile ~/.ssh/dev_rsa`
+  User username  
+```
+# homework 5
+`file: home_work_5.sh`
diff --git a/Homeworks/home_work_5.sh b/Homeworks/home_work_5.sh
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+#!/bin/bash
+
+echo "Username: $1";
+echo "Password: $2";
+echo "Path: $3";
+
+#check author
+if [ $UID -ne 0 ]; then
+  echo "Please run this script as root or sudo command"
+fi
+
+#add Account
+useradd $1 -p $2;
+
+#change group
+usermod -a -G vmt $1;
+
+#check folder exist
+if [[ !(-d ~/.ssh) ]]; then
+  mkdir ~/.ssh;
+fi
+
+#check file authorized_keys exist
+if [[ !(-d ~/.ssh/authorized_keys) ]]; then
+  #create file authorized_keys
+  touch ~/.ssh/authorized_keys;
+  #change permission
+  chmod 700 ~/.ssh/authorized_keys;
+fi
+
+#append pubkey
+cat $3 >> ~/.ssh/authorized_keys;
+
+echo "Done!";