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\newcommand*{\labTitle}{Tutorial: File Permissions}
\renewcommand*{\subject}{Operating Systems and Systems Integration}

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\begin{solution}
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\end{solution}

\begin{document}

\section{Background}
\label{sec:background}

For the background information you need to answer these questions,
please refer to Module~5, \emph{Basic Filesystems}, in the \emph{Linux
  Training} workshop notes.

\section{Questions}
\label{sec:questions}

\begin{enumerate}
\item \label{que:perms}A system administrator lists information about
  files in the current directory:
\begin{alltt}
$ \textbf{ls -l}    
-rw-rw-rw-    1 jimmy    students     5185 Feb 17 08:18 file1.txt
-rw-rw-r--    1 jenny    staff        5191 Oct  7 17:21 file2.txt
-rw-r--r--    1 jasmine  jasmine      2785 Oct 15 11:58 file3.txt
-rw-r-----    1 jacob    students    25920 Oct 15 11:58 file4.txt
-rw-------    1 julian   staff       13465 Oct 15 11:58 file5.txt
-r--------    1 jacky    students      736 Oct 15 11:58 file6.txt
----------    1 jill     students   179665 Oct 15 11:58 file7.txt
----r-----    1 juliet   students    13840 Feb 17 08:18 file8.txt
-------r--    1 jeremy   students     8418 Oct 12  2001 file9.txt
\end{alltt}%$
For each file, say which user(s) has(have):
\begin{enumerate}
\item read access?
\item write access?
\item no access?
\end{enumerate}
\begin{solution}
  \begin{tabularx}{\linewidth}{@{}>{\ttfamily}lYYY@{}}
    \toprule%
    &\textbf{read} & \textbf{write} & \textbf{no access}\\
    \midrule%
    file1.txt & all & all & none \\
    file2.txt & all & \texttt{jenny}, members of group \texttt{staff} & none \\
    file3.txt & all & \texttt{jasmine} & none \\
    file4.txt & \texttt{jacob} and members of group \texttt{students} &
    \texttt{jacob} & users who are not \texttt{jacob} and who are not
    members of the group students \\ 
    file5.txt & \texttt{julian} & \texttt{julian} & all users except \texttt{julian} \\
    file6.txt & \texttt{jacky} & none & all users except \texttt{jacky} \\
    file7.txt & none & none & all \\
    file8.txt & members of \texttt{students} who are not \texttt{juliet} & none &
    \texttt{juliet} and users who are not members of the group
    \texttt{students} \\
    file9.txt & users who are not \texttt{jeremy} and who are not members of the
    group \texttt{students} & none & \texttt{jeremy} and users who are
    members of the group \texttt{students}\\
    \bottomrule
  \end{tabularx}
\end{solution}

\item For each of the permissions (i.e., the first column only) in
  question~\ref{que:perms}, write a umask value that will result in
  all files created by the user having the default permission shown.

  \begin{solution}
    It may seem that \texttt{umask 111} would be suitable for setting
    permissions as for \texttt{file1.txt}, but it would have the
    horrible side effect that anyone with this umask will create
    directories that no one can change into, and will compile programs
    that are not executable.  So a better answer will be
    \texttt{umask~0}.
    \par\smallskip\par%
    \begin{tabular}[t]{@{}>{\ttfamily}l>{\ttfamily}lll>{\ttfamily}l>{\ttfamily}ll@{}}
      \toprule%
      \textbf{\textnormal{file}} & \textbf{\textnormal{permission}} &
      \textbf{umask} &&
      \textbf{\textnormal{file}} & \textbf{\textnormal{permission}} &
      \textbf{umask} \\
      \midrule%
      file1.txt & -rw-rw-rw- & 0   && file6.txt & -r-------- & 277 \\
      file2.txt & -rw-rw-r-- & 002 && file7.txt & ---------- & 777 \\
      file3.txt & -rw-r--r-- & 022 && file8.txt & ----r----- & 727 \\
      file4.txt & -rw-r----- & 027 && file9.txt & -------r-- & 772 \\
      file5.txt & -rw------- & 077 && & & \\
      \bottomrule
    \end{tabular}
    \par\medskip\par
  \end{solution}

\item For each of the permissions in question~\ref{que:perms}, write a
  \texttt{chmod} command to change the permission using~a:
  \begin{enumerate}
  \item numeric mode,
  \item symbolic mode.
  \end{enumerate}
  Your answer should work regardless of the permissions that existed
  previously. 
  \begin{solution}
    \par\bigskip\par%
    \begin{tabularx}{\linewidth}%
      {@{}>{\ttfamily}l>{\ttfamily}l>{\ttfamily}Y>{\ttfamily}Y@{}}
      \toprule%
      \textnormal{\textbf{file}} & \textnormal{\textbf{permission}}
      & \textnormal{\textbf{numeric}} & 
      \textnormal{\textbf{symbolic}} \\
      \midrule%
      file1.txt & -rw-rw-rw- & chmod 666 & chmod a=rw        \\
      file2.txt & -rw-rw-r-- & chmod 664 & chmod a=rw,o-w    \\
      file3.txt & -rw-r--r-- & chmod 644 & chmod a=r,u+w     \\
      file4.txt & -rw-r----- & chmod 640 & chmod u=rw,g=r,o= \\
      file5.txt & -rw------- & chmod 600 & chmod a=,u=rw     \\
      file6.txt & -r-------- & chmod 400 & chmod a=,u+r      \\
      file7.txt & ---------- & chmod 0   & chmod a=          \\
      file8.txt & ----r----- & chmod 040 & chmod a=,g+r      \\
      file9.txt & -------r-- & chmod 004 & chmod a=,o+r      \\
      \bottomrule
    \end{tabularx}
  \end{solution}
  
  \begin{questionsonly}
  \begin{flushright}
    \textbf{Questions are continued on the next page\,\ldots}
  \end{flushright}
\clearpage
\end{questionsonly}
\item \label{que:directory}A directory has the following permissions:
\begin{alltt}
$ \textbf{ls -ld directory}
drwxrwxr-x    1 jimmy    students      376 Feb 17 08:18 file1.txt
\end{alltt}%$
The following users are members of the following groups:
\par\smallskip\par
\begin{tabular}[t]{@{}lll@{}}
\toprule%
\textbf{user} & \textbf{primary group} & \textbf{secondary groups}\\
\midrule%
jimmy & jimmy & project year2 csa \\
nicku & nicku & staff laboratory \\
andy & andy & students year2 csa \\
jenny & jenny & students year3 csa \\
\bottomrule
\end{tabular}
\par\medskip\par
For each of the four users,
\begin{enumerate}
\item Can he/she create a file in the directory?
\item If the user can create a file,
  \begin{enumerate}
  \item who is the owner of the file?
  \item which group owns the file?
  \item If the \texttt{umask} is 002, what are the file permissions on
    a file created in that directory?
  \item Which of the other users above can:
    \begin{enumerate}
    \item read the file?
    \item write to the file?
    \item delete the file?
    \end{enumerate}
  \end{enumerate}
\end{enumerate}
\begin{solution}
  \begin{tabularx}{\linewidth}{@{}lYYYY@{}}
    \toprule%
    &\textbf{jimmy} & \textbf{nicku} & \textbf{andy} & \textbf{jenny}
    \\
    \midrule%
    can create file:  & yes    & no & yes & yes \\
    owner of file:    & jimmy  &    & andy & jenny \\
    group:            & jimmy  &    & andy & jenny \\
    permissions:      & \texttt{-rw-rx-r--} & & \texttt{-rw-rx-r--} &
    \texttt{-rw-rx-r--}\\ 
    users can read:   & all    &    & all & all \\
    users can write:  & none   &    & none & none \\
    users can delete: & andy, jenny & & jimmy, jenny & jimmy, andy \\
    \bottomrule%
  \end{tabularx}
\end{solution}

\item Now the \SGID (set group \ID) permission is added to the
  directory shown in question~\vref{que:directory}.
  \begin{enumerate}
  \item Write a command to add this permission, leaving other
    permissions unchanged.
    \begin{solution}
\begin{alltt}
$ \textbf{chmod g+s directory}
\end{alltt}%$
    \end{solution}
  \item Write the permissions on the directory as they would be shown
    as the first field of the output of \texttt{ls -ld directory}
    \begin{solution}
\begin{alltt}
$ \textbf{ls -ld directory}
drwxrwsr-x    1 jimmy    students      376 Feb 17 08:18 file1.txt
\end{alltt}%$
    \end{solution}
  \item Answer the questions from question~\vref{que:directory} for
    each user again, but write ``unchanged'' where the result is the
    same, but where the result is different, explain how.
  \end{enumerate}
\begin{solution}
  \begin{tabularx}{\linewidth}{@{}lYYYY@{}}
    \toprule%
    &\textbf{jimmy} & \textbf{nicku} & \textbf{andy} & \textbf{jenny}
    \\
    \midrule%
    can create file:  & yes    & no & yes & yes \\
    owner of file:    & jimmy  &    & andy & jenny \\
    group:            & students &    & students & students \\
    permissions:      & \texttt{-rw-rx-r--} & & \texttt{-rw-rx-r--} &
    \texttt{-rw-rx-r--}\\ 
    users can read:   & all    &    & all & all \\
    users can write:  & andy, jenny &    & jenny & andy \\
    users can delete: & andy, jenny & & jimmy, jenny & jimmy, andy \\
    \bottomrule%
  \end{tabularx}
\end{solution}


\item Now the \emph{restricted deletion flag} (or ``sticky bit'')
  permission is added to the directory shown in
  question~\vref{que:directory}, \emph{as well as} the \SGID
  permission.
  \begin{enumerate}
  \item Write a command to add this permission, leaving other
    permissions unchanged.
    \begin{solution}
\begin{alltt}
$ \textbf{chmod o+t directory}
\end{alltt}%$
    \end{solution}
  \item Write the permissions on the directory as they would be shown
    as the first field of the output of \texttt{ls -ld directory}
    \begin{solution}
\begin{alltt}
$ \textbf{ls -ld directory}
drwxrwsr-t    1 jimmy    students      376 Feb 17 08:18 file1.txt
\end{alltt}%$
    \end{solution}
  \item Answer the questions from question~\vref{que:directory} for
    each user again, but write ``unchanged'' where the result is the
    same, but where the result is different, explain how.
  \end{enumerate}
\end{enumerate}
\begin{solution}
  \begin{tabularx}{\linewidth}{@{}lYYYY@{}}
    \toprule%
    &\textbf{jimmy} & \textbf{nicku} & \textbf{andy} & \textbf{jenny}
    \\
    \midrule%
    can create file:  & yes    & no & yes & yes \\
    owner of file:    & jimmy  &    & andy & jenny \\
    group:            & students &    & students & students \\
    permissions:      & \texttt{-rw-rx-r--} & & \texttt{-rw-rx-r--} &
    \texttt{-rw-rx-r--}\\ 
    users can read:   & all    &    & all & all \\
    users can write:  & andy, jenny &    & jenny & andy \\
    users can delete: & only jimmy & & andy and jimmy &  jenny and
    jimmy \\
    \bottomrule%
  \end{tabularx}
\end{solution}
\end{document}