Special permissions (setuid, setgid, sticky bit) in Linux CLI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the command to set the setuid bit on the file myapp.

Linux CLI

chmod [1] myapp

Drag options to blanks, or click blank then click option'

Ao+t

Bu+s

C+x

Dg+s

Attempts:

3 left

💡 Hint

Common Mistakes

Using g+s sets the setgid bit, not setuid.

Using +x only sets execute permission.

Using o+t sets the sticky bit, not setuid.

The setuid bit is set using u+s in the chmod command.

2fill in blank

medium

Complete the command to set the setgid bit on the directory shared.

Linux CLI

chmod [1] shared

Drag options to blanks, or click blank then click option'

Au+s

Bo+t

Cg+s

D+x

Attempts:

3 left

💡 Hint

Common Mistakes

Using u+s sets the setuid bit, not setgid.

Using o+t sets the sticky bit, not setgid.

Using +x only sets execute permission.

The setgid bit is set using g+s on directories to make new files inherit the group.

3fill in blank

hard

Fix the error in the command to set the sticky bit on the directory temp.

Linux CLI

chmod [1] temp

Drag options to blanks, or click blank then click option'

A+x

Bg+s

Cu+s

Do+t

Attempts:

3 left

💡 Hint

Common Mistakes

Using u+s or g+s sets setuid or setgid, not sticky bit.

Using +x only sets execute permission.

The sticky bit is set using o+t on directories to restrict file deletion.

4fill in blank

hard

Fill both blanks to create a directory project with setgid and sticky bits set.

Linux CLI

mkdir project && chmod [1][2] project

Drag options to blanks, or click blank then click option'

Ag+s

Bo+t

Cu+s

D+x

Attempts:

3 left

💡 Hint

Common Mistakes

Using u+s sets setuid, not setgid.

Using +x only sets execute permission.

Setgid is g+s and sticky bit is o+t. Both can be combined in chmod.

5fill in blank

hard

Fill all three blanks to create a file runme with setuid, setgid, and sticky bits set.

Linux CLI

touch runme && chmod [1][2][3] runme

Drag options to blanks, or click blank then click option'

Au+s

Bg+s

Co+t

D+x

Attempts:

3 left

💡 Hint

Common Mistakes

Using +x only sets execute permission.

Mixing up setuid and setgid bits.

Setuid is u+s, setgid is g+s, and sticky bit is o+t. All can be combined in chmod.

Practice

1. What does the setuid permission do on an executable file?

easy

A. Runs the program with the file owner's permissions

B. Allows anyone to delete the file

C. Makes new files inherit the folder's group

D. Prevents the file from being executed

Solution

1. Step 1: Understand setuid purpose

   The setuid permission allows a program to run with the permissions of the file owner, not the user running it.

2. Step 2: Compare options with setuid behavior

   Only Runs the program with the file owner's permissions correctly describes this behavior; others describe different permissions or incorrect effects.

3. Final Answer:

   Runs the program with the file owner's permissions -> Option A

4. Quick Check:

   setuid = runs as owner [OK]

Hint: Setuid means run as file owner, not current user [OK]

Common Mistakes:

• Confusing setuid with sticky bit
• Thinking setuid changes file deletion rights
• Mixing setuid with setgid effects

2. Which command correctly sets the setgid bit on a directory named shared?

easy

A. chmod g+s shared

B. chmod +s shared

C. chmod u+s shared

D. chmod o+s shared

Solution

1. Step 1: Identify correct chmod syntax for setgid

   The setgid bit is set using g+s with chmod on directories.

2. Step 2: Verify options

   chmod g+s shared uses chmod g+s shared, which is correct. Other options use incorrect flags or users.

3. Final Answer:

   chmod g+s shared -> Option A

4. Quick Check:

   Setgid = chmod g+s [OK]

Hint: Setgid uses 'g+s' with chmod on directories [OK]

Common Mistakes:

• Using '+s' without user class
• Confusing setuid (u+s) with setgid
• Trying to set setgid on files with wrong syntax

3. Given a directory /tmp/shared with permissions drwxrwxrwt, what does the t at the end signify?

medium

A. Setuid bit is set

B. Sticky bit is set

C. Setgid bit is set

D. No special permissions set

Solution

1. Step 1: Interpret permission string ending with 't'

   The 't' at the end of directory permissions indicates the sticky bit is set.

2. Step 2: Match 't' with sticky bit meaning

   The sticky bit prevents users from deleting files they don't own in that directory.

3. Final Answer:

   Sticky bit is set -> Option B

4. Quick Check:

   't' at end = sticky bit [OK]

Hint: 't' at end of permissions means sticky bit set [OK]

Common Mistakes:

• Confusing 't' with setuid or setgid
• Thinking 't' means no special permission
• Misreading permissions string

4. You set the setuid bit on a script using chmod u+s script.sh, but it doesn't run with owner permissions. What is the likely problem?

medium

A. You need to set the sticky bit instead

B. The file owner must be root for setuid to work

C. The script is a shell script and setuid does not work on scripts

D. You forgot to set execute permission

Solution

1. Step 1: Understand setuid behavior on scripts

   Most Linux systems ignore setuid on shell scripts for security reasons.

2. Step 2: Evaluate other options

   Sticky bit is unrelated, owner need not be root, and execute permission is usually set if script runs.

3. Final Answer:

   The script is a shell script and setuid does not work on scripts -> Option C

4. Quick Check:

   Setuid ignored on scripts [OK]

Hint: Setuid rarely works on scripts, only on binaries [OK]

Common Mistakes:

• Assuming setuid works on all files
• Confusing sticky bit with setuid
• Not checking execute permission

5. You want to create a shared directory /data/shared where all files created inside inherit the directory's group and users cannot delete others' files. Which commands set this up correctly?

hard

A. chmod 2777 /data/shared && chmod g+s /data/shared

B. chmod 777 /data/shared && chmod g+s /data/shared

C. chmod 2777 /data/shared

D. chmod 2777 /data/shared && chmod +t /data/shared

Solution

1. Step 1: Set setgid and sticky bit on directory

   Setgid (2) makes new files inherit group; sticky bit (1) prevents deleting others' files.

2. Step 2: Analyze commands

   chmod 2777 /data/shared && chmod +t /data/shared sets permissions to 2777 (setgid + rwxrwxrwx) and adds sticky bit with chmod +t. This correctly applies both.

3. Final Answer:

   chmod 2777 /data/shared && chmod +t /data/shared -> Option D

4. Quick Check:

   Setgid + sticky bit = chmod 2777 + t [OK]

Hint: Setgid=2xxx, sticky=1xxx; combine for shared dir [OK]

Common Mistakes:

• Using sticky bit without setgid
• Confusing octal codes for permissions
• Setting wrong permission order