1. Directory Access

What access does user bob have on abc.txt?

drwxr-xr-x. 17 root root 4096 23:38 /
drwxr-xr--. 10 root root 128  03:38 /data
-rwxr-xr--.  1 bob  bob  100  21:08 /data/abc.txt

Answer : None

Explanation

  • File permissions suggest bob has full access (rwx) to abc.txt.
  • But to access a file, the user must have execute (x) permission on all parent directories.
  • On /data, bob has others permissions: r--.

  • Without execute (x) on /data, bob:

    • Cannot cd into the directory
    • Cannot access, read, write, or execute the file

Lesson Learned

Always check parent directory permissions before file permissions. File permissions only apply after directory access is allowed.

2. Viewing Directory Contents

Who can use ls /data to display the contents of the /data directory?

drwxr-xr-x. 17 root root 4096 23:38 /
drwxr-xr--. 10 root root 128  03:38 /data
-rwxr-xr--.  1 bob  bob  100  21:08 /data/abc.txt

Answer : All users

Explanation

  • To list directory contents, a user needs read (r) permission on the directory.

  • All users have:

    • Execute (x) on / → can traverse to /data
    • Read (r) on /data → can run ls /data

  • Therefore, any user can view the directory contents, including hidden files (ls -a).

Lesson Learned

Read (r) permission allows listing directory contents. Execute (x) permission is required for detailed access.

3. Deleting Directory Contents

Who can delete the file /data/abc.txt?

drwxr-xr-x. 17 root root 4096 23:38 /
drwxrw-rw-. 10 root root 128  03:38 /data
-rwxr-xr--.  1 bob  bob  100  21:08 /data/abc.txt

Answer : Only the root user

Explanation

  • File permissions do NOT control deletion.

  • To delete a file, a user needs:

    • Write (w) permission on the directory
    • Execute (x) permission on the directory

  • Although everyone has write (w) on /data,

    • Only root has execute (x) on /data.

  • Without execute permission, users cannot access the directory to remove files.

Lesson Learned

Write (w) allows file deletion only when execute (x) is also present on the directory.

4. Accessing the Contents of a Directory

True or False: Can user bob successfully run more /data/abc.txt?

drwxr-xr-x. 17 root root 4096 23:38 /
dr-xr-x--x. 10 root root 128  03:38 /data
-rwxr-xr--.  1 bob  bob  100  21:08 /data/abc.txt

Answer : True

Explanation

  • To access a file, a user needs:

    • Execute (x) permission on all parent directories
    • Read (r) permission on the file

  • The read (r) permission on the directory is NOT required to access a known file.

  • User bob has:

    • x on /
    • x on /data
    • r on /data/abc.txt

  • Therefore, the command executes successfully.

Lesson Learned

Execute (x) allows entering a directory; read (r) on the directory is only needed to list files.

5. The Complexity of Users and Groups

True or False: Can user bob run more /data/abc.txt?

drwxr-xr-x. 17 root root    4096 23:38 /
dr-xr-x---. 10 sue  payroll 128  03:38 /data
-rwxr-xr--.  1 bob  bob     100  21:08 /data/abc.txt

Answer : Not enough information

Explanation

  • To access the file, user bob needs:

    • x on /
    • x on /data
    • r on /data/abc.txt

  • Bob already has required permissions on / and the file.

  • Access to /data depends on group membership:

    • If bob is in payroll group → permissions are r-x → command works
    • If not → permissions are --- → command fails

Lesson Learned

Always consider user, group, and group membership when evaluating permissions.

6. Permission Priority

True or False: Can user bob run more /data/abc.txt?

drwxr-xr-x. 17 root root 4096 23:38 /
dr-xr-x---. 10 bob  bob  128  03:38 /data
----rw-rwx.  1 bob  bob  100  21:08 /data/abc.txt

Answer : False

Explanation

  • The file owner is bob
  • Owner permissions on abc.txt are ---
  • Only owner permissions are checked for the owner
  • Group and others permissions are ignored for bob
  • Without r permission, more cannot read the file

Lesson Learned

File owner permissions override group and others permissions.