Since its an open source thing,you not only have the previledge of making changes as you like,but it also becomes a necessity.And this OS might look impossible at times.But to quote Walt Disney -

“Doing the impossible is kind of fun”

One of the major commands to use at the terminal(yeah,the command prompt of Linux) is chmod,and here I explain how to use it :

There are three types of access restrictions:

There are also three types of user restrictions:

Directories have directory permissions. The directory permissions restrict different actions than with files or device nodes.

• read restricts or allows viewing the directories contents, i.e. ls command
• write restricts or allows creating new files or deleting files in the directory. (Caution: write access for a directory allows deleting of files in the directory even if the user does not have write permissions for the file!)
• execute restricts or allows changing into the directory, i.e. cd command
  

  Permissions in Action

  user@host:/home/user$ ls -l /etc/hosts
  
  -rw-r--r--  1 root root 288 2005-11-13 19:24 /etc/hosts
  
  user@host:/home/user$

  Using the example above we have the file “/etc/hosts” which is owned by the user root and belongs to the root group.

  What are the permissions from the above /etc/hosts ls output?

  -rw-r--r--owner = Read & Write (rw-)
  
  group = Read (r--)
  
  other = Read (r--)

  Changing Permissions

  The command to use when modifying permissions is chmod. There are two ways to modify permissions, with numbers or with letters. Using letters is easier to understand for most people. When modifying permissions be careful not to create security problems. Some files are configured to have very restrictive permissions to prevent unauthorized access. For example, the /etc/shadow file (file that stores all local user passwords) does not have permissions for regular users to read or otherwise access.

  user@host:/home/user# ls -l /etc/shadow
  
  -rw-r-----  1 root shadow 869 2005-11-08 13:16 /etc/shadow
  
  user@host:/home/user#Permissions:
  
  owner = Read & Write (rw-)
  
  group = Read (r--)
  
  other = None (---)
  
  Ownership:
  
  owner = root
  
  group = shadow

  chmod with Letters

  Usage: chmod {options} filename

  Options	Definition
  u	owner
  g	group
  o	other
  x	execute
  w	write
  r	read
  +	add permission
  -	remove permission
  =	set permission

  Here are a few examples of chmod usage with letters (try these out on your system).

  First create some empty files:

  user@host:/home/user$ touch file1 file2 file3 file4
  
  user@host:/home/user$ ls -l
  
  total 0
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file1
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file2
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file3
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file4

  Add owner execute bit:

  user@host:/home/user$ chmod u+x file1
  
  user@host:/home/user$ ls -l file1
  
  -rwxr--r--  1 user user 0 Nov 19 20:13 file1

  Add other write & execute bit:

  user@host:/home/user$ chmod o+wx file2
  
  user@host:/home/user$ ls -l file2
  
  -rw-r--rwx  1 user user 0 Nov 19 20:13 file2

  Remove group read bit:

  user@host:/home/user$ chmod g-r file3
  
  user@host:/home/user$ ls -l file3
  
  -rw----r--  1 user user 0 Nov 19 20:13 file3

  Add read, write and execute to everyone:

  user@host:/home/user$ chmod ugo+rwx file4
  
  user@host:/home/user$ ls -l file4
  
  -rwxrwxrwx  1 user user 0 Nov 19 20:13 file4
  
  user@host:/home/user$

  chmod with Numbers

  Usage: chmod {options} filename

  Options	Definition
  #--	owner
  -#-	group
  --#	other
  1	execute
  2	write
  4	read

  Owner, Group and Other is represented by three numbers. To get the value for the options determine the type of access needed for the file then add.

  For example if you want a file that has -rw-rw-rwx permissions you will use the following:

  Owner	Group	Other
  read & write	read & write	read, write & execute
  4+2=6	4+2=6	4+2+1=7

  user@host:/home/user$ chmod 667 filename

  Another example if you want a file that has –w-r-x–x permissions you will use the following:

  Owner	Group	Other
  write	read & execute	execute
  2	4+1=5	1

  user@host:/home/user$ chmod 251 filename

  Here are a few examples of chmod usage with numbers .

  First create some empty files:

  user@host:/home/user$ touch file1 file2 file3 file4
  
  user@host:/home/user$ ls -l
  
  total 0
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file1
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file2
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file3
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file4

  Add owner execute bit:

  user@host:/home/user$ chmod 744 file1
  
  user@host:/home/user$ ls -l file1
  
  -rwxr--r--  1 user user 0 Nov 19 20:13 file1

  Add other write & execute bit:

  user@host:/home/user$ chmod 647 file2
  
  user@host:/home/user$ ls -l file2
  
  -rw-r--rwx  1 user user 0 Nov 19 20:13 file2

  Remove group read bit:

  user@host:/home/user$ chmod 604 file3
  
  user@host:/home/user$ ls -l file3
  
  -rw----r--  1 user user 0 Nov 19 20:13 file3

  Add read, write and execute to everyone:

  user@host:/home/user$ chmod 777 file4
  
  user@host:/home/user$ ls -l file4
  
  -rwxrwxrwx  1 user user 0 Nov 19 20:13 file4
  
  user@host:/home/user$

  chmod with sudo

  Changing permissions on files that you do not have ownership of: (Note that changing permissions the wrong way on the wrong files can quickly mess up your system a great deal! Please be careful when using sudo!)

  user@host:/home/user$ ls -l /usr/local/bin/somefile
  
  -rw-r--r--  1 root root 550 2005-11-13 19:45 /usr/local/bin/somefile
  
  user@host:/home/user$user@host:/home/user$ sudo chmod o+x /usr/local/bin/somefile
  
  user@host:/home/user$ ls -l /usr/local/bin/somefile
  
  -rw-r--r-x  1 root root 550 2005-11-13 19:45 /usr/local/bin/somefile
  
  user@host:/home/user$

  Recursive Permission Changes

  To change the permissions of multiple files and directories with one command. Please note the warning in the chmod with sudo section and the Warning with Recursive chmod section.

  Recursive chmod with -R and sudo

  To change all the permissions of each file and folder under a specified directory at once, use sudo chmod with -R

  user@host:/home/user$ sudo chmod 777 -R /path/to/someDirectory
  
  user@host:/home/user$ ls -l
  
  total 3
  
  -rwxrwxrwx  1 user user 0 Nov 19 20:13 file1
  
  drwxrwxrwx  2 user user 4096 Nov 19 20:13 folder
  
  -rwxrwxrwx  1 user user 0 Nov 19 20:13 file2

  Recursive chmod using find, pipemill, and sudo

  To assign reasonably secure permissions to files and folders/directories, it’s common to give files a permission of 644, and directories a 755 permission, since chmod -R assigns to both. Use the find command and a pipemill to chmod as in the following example.

  To change permission of only files under a specified directory.

  user@host:/home/user$ find /path/to/someDirectory -type f | while read var1; do sudo chmod 644 $var1; done
  
  user@host:/home/user$ ls -l
  
  total 3
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file1
  
  drwxrwxrwx  2 user user 4096 Nov 19 20:13 folder
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file2

  To change permission of only directories under a specified directory (including that directory):

  user@host:/home/user$ find /path/to/someDirectory -type d | while read var1; do sudo chmod 755 $var1; done
  
  user@host:/home/user$ ls -l
  
  total 3
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file1
  
  drwxr--r--  2 user user 4096 Nov 19 20:13 folder
  
  -rw-r--r--  1 user user 0 Nov 19 20:13 file2

  If you accidentally change the permissions and this command no longer works because find doesn’t have permission to read the directory you specified, put sudo before find.

  Recursive chmod

  To grant file permissions to all containing folders sub-folders and files,use Recursive chmod .Please note, Recursively deleting or chown-ing files are extremely dangerous. You will not be the first, nor the last, person to add one too many spaces into the command. This example will hose your system:

  user@host:/home/user$ sudo chmod -R / home/john/Desktop/tempfiles

  Note the space between the first / and home.I haven’t tried this one personally.

Long Live Ubuntu!