Question

Existing permission for file dept.txt is rw- --- --- . Retain the same permissions for user and other category. Add write permission to group using octal notation or symbolic notation in unix.

Answer 1

File Permissions in Unix

We are given the file $\texttt{dept.txt}$

Unix systems divide users into three categories:

• Owner/User - This is the owner of the file
• Group - Users belonging to predefined user group
• Other - All other users

These three are denoted by the letters u (user/owner), g (group) and o (other) respectively.

For each user type, there can be three types of permissions for any file:

• Read - r - User can read the file
• Write - w - User can edit the file
• Execute - x - User can execute the file (if it is an executable file like a program code or application)

The absence of a permission is denoted by a hyphen ( - ).

File permissions are mentioned in 3 letter codes. For example, a file permission of $\texttt{rwx rw- r--}$ means that:

• Owner can read, write and execute the file.
• Group can read and write the file, but not execute it
• Other can only read the file

The $\texttt{dept.txt}$ file has the permissions $\texttt{rw- --- ---}$

This means that:

• Owner can read and write the file, but not execute it
• Anyone else cannot do anything to the file, not even read it

File permissions can be edited using the chmod command.

Syntax:

$\texttt{chmod permissions filenam}\texttt{e}$

$\rule{300}{1}$

Symbolic Notation

In Symbolic Notation, we can change permissions using three characters:

$\begin{tabular}{c|l}\textbf{\textsf{Symbol}} & \textbf{\textsf{Description}}\\\cline{1-2}\sf + &\sf Add permission \\\sf - & \sf Remove permission \\\sf = &\sf Assign permission and override previous permissions \end{tabular}$

The symbols for the different user types are as follows:

$\begin{tabular}{c|l} \textbf{\textsf{Symbol}} & \textbf{\textsf{User Type}} \\ \cline{1-2} \sf u & \sf Owner/User \\ \sf g & \sf Group \\ \sf o & \sf Other \\ \sf a & \sf All user types \end{tabular}$

So, for example, $\texttt{dept.txt}$ currently has permissions of $\texttt{rw- --- ---}$

Then, doing $\texttt{chmod o=r-- dept.txt}$ will assign the permission of $\texttt{r--}$ to the Other user type for the file. The new permissions will be $\texttt{rw- --- r--}$

Now, if we do $\texttt{chmod a+x dept.txt}$ we are adding the execute permission to all the user types. The new permissions will look like $\texttt{rwx --x r-x}$

Answer:

We just want to add the Write permission to Group and retain everything else. So, the command using symbolic notation is

$\boxed{\texttt{chmod g+w dept.txt}}$

$\rule{300}{1}$

Octal Notation

In octal notation, we assign permissions using the digits 0-7 for each user type. So, we use a three digit code to assign permission for u, g and o. The three digits are in octal.

The reference table:

$\begin{tabular}{c|l|c} \textbf{\textsf{Digit}} & \textbf{\textsf{Permission}} & \textbf{\textsf{Symbol}} \\ \cline{1-3}\tt 0 &\sf No Permission &\tt --- \\ \tt 1 &\sf Execute &\tt --x \\ \tt 2 &\sf Write &\tt -w- \\ \tt 3 &\sf Write + Execute &\tt -wx \\ \tt 4 &\sf Read &\tt r-- \\\tt 5 &\sf Read + Execute &\tt r-x \\\tt 6 &\sf Read + Write &\tt rw- \\\tt 7 &\sf Read + Write + Execute &\tt rwx \\\end{tabular}$

This assigns absolute permissions and overrides previous permissions.

For example, doing $\texttt{chmod 754 dept.txt}$ will assign the following permissions:

• 7: $\texttt{rwx}$ permissions to user/owner
• 5: $\texttt{r-x}$ permissions to group
• 4: $\texttt{r--}$ permissions to other  

All previous permissions are overridden.

Answer:

The $\texttt{dept.txt}$ file has permissions $\texttt{rw- --- ---}$. These correspond to the digits 600.

We are retaining the permissions for user and other. So, 6 and third digit 0 will remain same. We want to add write permission to group. So, for group, we want the $\texttt{-w-}$ permissions, which corresponds to the digit 2.

So, our final digits are 620.

Hence, we use the command:

$\boxed{\texttt{chmod 620 dept.txt}}$

$\rule{300}{1}$

Summary

$\texttt{dept.txt}$ has the permissions $\texttt{rw- --- ---}$

We want the new permissions to be $\texttt{rw- -w- ---}$.

We can use either of the two commands:

Symbolic Notation: $\texttt{chmod g+w dept.txt}$

Octal Notation: $\texttt{chmod 620 g+w dept.txt}$

I have attached two screenshots showing these commands.

Answer 2

Read - r - User can read the file

Write - w - User can edit the file

Execute - x - User can execute the file (if it is an executable file like a program code or application