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Users, permissions, and processes / Stage 2 of 13

File and directory permissions

Not running Starting usually ~20 s
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Step 1 of 6 · on a machine · 5 min

Your file, someone else's read

What you will learn

  • Read the owner, the group, and the three permission triples in ls -l output.
  • Tell what r, w, and x do on a file and on a directory.
  • Explain a denial by the process's account and the directories on the path to the file.

A colleague moved the weekly report into the shared directory /srv/permissions-lab and writes: "The report is yours now, and I opened it for everyone to read." Check who can actually read it.

Start prepares the directory and takes you there. The last command reads the report as www-data, the web server's account.

Exercise · look

Who can read the report

You are in /srv/permissions-lab. Run the three commands one by one: look at the file's line, read it yourself, and read it as www-data.

First, predict

ls -l will show you as the owner of report.txt. What will your cat report.txt print?

Now start the exercise, run the command and compare your prediction with the output.

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Check yourself 1 question

Why was the owner of report.txt refused while www-data read the file?

  • www-data has UID 33: an ordinary account, not root. sudo -u www-data cat /etc/shadow gets Permission denied.

  • The directory is open: ls -l report.txt read the file's details, and cat reached the file and was refused the read.

  • sudo -u www-data runs the command as www-data: sudo -u www-data id prints uid=33(www-data). Without -u it would run as root.

The line -------r-- gives reading to others, and not to the owner student.

Found an inaccuracy? Report an error in the lesson

Ubuntu 24.04
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