106 Chronologically structured chapter that builds foundational Linux concepts step-by-step

 Here's a chronologically structured chapter that builds foundational Linux concepts step-by-step and prepares you for FAANG-level interviews with tools, commands, and real-time scenarios.

🧠 Chapter: Mastering Linux Internals for Interviews

1. The Big Picture: How Linux Works

• Overview: Role of the kernel, user space vs kernel space

• Key Concept: Linux as a multitasking, multiuser, monolithic kernel system

2. CPU: The Core Executor

• What: Executes instructions, context switching, scheduling

• Terms: User time, system time, idle time

• Command: top, htop, mpstat, uptime

• Interview Insight: Explain CPU-bound vs I/O-bound processes

3. Memory: RAM and Virtual Memory

• Concepts: Virtual memory, paging, swapping, buffers, cache

• Commands: free -h, vmstat, /proc/meminfo, top (RES/VIRT/SHR)

• Interview Tip: What causes high swap usage?

4. Processes and Threads

• Process: An independent executing program (PID, PPID)

• Thread: Lightweight process sharing the same address space

• Commands: ps -ef, pstree, top, htop

• System Calls: fork(), exec(), exit(), wait()

• Key Difference: fork() duplicates, exec() replaces, exit() terminates

5. Kernel: The Brain

• Components: Process scheduler, memory manager, I/O manager

• System Calls Interface: Bridge between user and kernel space

• Command: uname -a, dmesg

• Real World: Debugging kernel logs with dmesg

6. I/O and Disk Subsystems

• Concepts: Block vs character devices, buffered I/O, async I/O

• Commands: iostat, iotop, df -h, du -sh, lsblk, mount

• Use Case: Identify I/O bottlenecks with iotop, pidstat -d

7. System Calls Deep Dive

• What: Interface to kernel services (e.g., file ops, process control)

• Examples: read(), write(), open(), close(), kill()

• Tool: strace — trace system calls and signals

• Interview: How exec() works under the hood? Use strace to show.

8. Process States and Lifecycle

• States: Running, Sleeping, Zombie, Stopped, Orphan

• Monitoring Tools:

  • top, htop – for real-time process view

  • watch -n 1 'ps aux | grep <pid>'

  • pidstat – CPU, memory, I/O usage over time

  • lsof – list open files by a process

• Real-World Debug: A zombie process scenario

9. Signals in Depth

• Types: TERM, KILL, STOP, CONT, HUP, INT, etc.

• Commands:

  • kill -SIGTERM <pid> – graceful shutdown

  • kill -9 <pid> – force kill

  • trap in shell scripts

• Tools: strace -p <pid> to inspect signal handling

• Real-Time Example: Releasing stuck processes via SIGKILL

10. Background & Detached Execution

• Commands:

  • nohup command & – runs even after logout

  • disown %job_id – remove from job table

• Use Case: Run long jobs on remote systems safely

11. Advanced Performance Debugging

• renice – change process priority

• pidstat – profile specific PIDs

• strace – syscall tracing

• lsof – open file/socket tracking

• dmesg – kernel ring buffer

12. Process Monitoring in Production

• Monitor All States:

  • ps -eo pid,state,cmd

  • top -H – thread view

  • watch -n 1 'ps aux | grep <app>'

• Real-World Case: Memory leak or CPU spike in production

  • top → strace → lsof → kill or renice

$ time ls -l

real    0.003s
user    0.001s
sys     0.002s

ps -eo pid,comm,ni,pri,cls,stat --sort=pid

ps -eo pid,etime,time,comm --sort=-time | head

strace -T -p <pid>

105 Process states and Management

 Here’s a complete guide on Process Management – Process States, focused on FAANG-level interview preparation, with clear explanations, real-world examples, and 20 high-quality Q&A to test and reinforce your understanding.

🔧 1. Key Topics to Cover (Process Management & Process States)

🚦 2. Process States in Linux (FAANG-focused)

📚 3. FAANG-Ready Interview Questions + Answers

✅ Basic to Intermediate

Q1. What are the different states a process can be in? Explain.

Answer:

• Running (R): Actively on CPU or ready to run.

• Sleeping (S): Waiting for I/O; can be interrupted by signals.

• Uninterruptible Sleep (D): Waiting on resources like disk; cannot be interrupted.

• Stopped (T): Halted by a signal (e.g., SIGSTOP).

• Zombie (Z): Terminated but not reaped by parent.

• Dead (X): Process terminated and removed from system.

Q2. How can you identify zombie processes on Linux?

Answer:

ps aux | grep 'Z'

Or:

ps -eo pid,ppid,state,cmd | grep Z

Zombie processes show as state Z and can only be cleared if the parent process is terminated or calls wait().

Q3. What is the difference between zombie and orphan process?

Q4. How does a process move from running to waiting (sleeping)?

Answer:
A process transitions from running to sleeping when it requests I/O or is waiting for a resource (disk, network, user input). The scheduler then switches to another process until I/O is ready.

Q5. Explain the lifecycle of a Linux process.

Answer:

1. Created – via fork() or exec()

2. Ready – added to scheduler queue

3. Running – executes instructions

4. Waiting – for I/O or other resource

5. Terminated – exits using exit()

6. Zombie – if not reaped by parent

✅ Advanced FAANG-Level

Q6. How do you handle a zombie process in a production server?

Answer:

1. Identify with ps aux | grep Z

2. Check PPID

3. Kill parent process to allow init (PID 1) to adopt and reap the zombie.

Q7. How to find which syscall a process is waiting on?

Answer:
Use strace:

strace -p <pid>

This shows the system calls being waited on.

Q8. What does the D state (Uninterruptible sleep) mean and why is it dangerous?

Answer:
It indicates the process is waiting for I/O and cannot be killed or interrupted (even with kill -9). This can be a sign of hardware issues, NFS hang, or I/O lock.

Q9. How does the Linux kernel schedule processes?

Answer:

• Linux uses the CFS (Completely Fair Scheduler).

• It maintains a red-black tree to assign CPU time.

• Each process gets a fair share of time based on niceness and priority.

Q10. What happens when you run kill -9 <pid> on a zombie?

Answer:
Nothing. Zombie processes are already terminated. They can’t be killed. You need to terminate the parent process so the kernel can clean up the zombie.

✅ Real-World + Debugging

Q11. How do you debug high CPU usage from a process?

Answer:

top or htop
pidstat -p <pid> 1
strace -p <pid>
lsof -p <pid>

These tools help track what the process is doing, system calls, and open files.

Q12. How do you change the priority of a running process?

Answer:

renice -n 10 -p <pid>

Niceness range: -20 (highest priority) to 19 (lowest priority).

Q13. What’s the use of the pstree command?

Answer:
Shows process hierarchy, useful for visualizing parent-child relationships:

pstree -p

Q14. What signal is sent by default using kill command?

Answer:
SIGTERM (15) – allows process to gracefully shut down.

Q15. When would you use nohup?

Answer:
To run a process after logout/session close:

nohup ./my_script.sh &

✅ Behavioral + Situational

Q16. You found a process in D state for hours. What do you do?

Answer:

• Use strace or dmesg to check blocked I/O.

• Check if NFS, disk, or DB is hung.

• Consider restarting the service or rebooting if stuck.

Q17. A user reports background jobs disappearing after logout. Why?

Answer:

• Use nohup or disown to detach processes from terminal session.

nohup script.sh &
disown

Q18. How to monitor all states of processes?

Answer:

ps -eo pid,user,state,cmd | sort

Also use top, htop, watch.

Q19. What causes a process to become a zombie?

Answer:
Child process exits, but parent hasn’t called wait() to read its exit status. Kernel keeps it to return info when parent requests.

Q20. How can you force the cleanup of multiple zombie processes?

Answer:
Find the parent with:

ps -eo ppid,pid,state,cmd | grep Z

Then kill or restart the parent to allow init to reap the child processes.

📌 Summary Cheat Sheet

104 Basic Commands

The commands cpu, top, kill, and ps are essential tools in Linux for monitoring and managing processes and system resources. Let’s go over each command in detail, including:

• Purpose

• Syntax

• Common use-cases

• Example output

• How to analyze the output

🔹 1. top — Real-time System Monitoring

📌 Purpose:

Displays real-time information about system processes, CPU, memory usage, and load average.

📌 Syntax:

top

📌 Example Output (Partial):

top - 08:45:26 up  2:34,  2 users,  load average: 0.15, 0.20, 0.25
Tasks: 138 total,   1 running, 137 sleeping,   0 stopped,   0 zombie
%Cpu(s):  3.0 us,  1.0 sy,  0.0 ni, 95.0 id,  1.0 wa,  0.0 hi,  0.0 si,  0.0 st
MiB Mem :   7850.2 total,  2200.5 free,  3400.6 used,  2249.1 buff/cache
PID  USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
1324 root      20   0  123456  45678   1234 R  23.4  0.6   0:10.53 chrome

📌 How to Analyze:

• Load average: First line shows 1, 5, and 15-minute system load. Rule of thumb: if load > number of CPUs, the system is overloaded.

• %CPU: High %us (user) means CPU is working on your tasks, %sy (system) is kernel, %id is idle time.

• %MEM: Memory usage per process.

• PID/COMMAND: Helps identify the exact process consuming resources.

🔹 2. ps — Snapshot of Current Processes

📌 Purpose:

Displays a snapshot of current processes (unlike top which is real-time).

📌 Syntax:

ps aux       # All processes
ps -ef       # Also shows all, but with different format

📌 Example Output:

USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.1  8960  2340 ?        Ss   08:00   0:01 /sbin/init
prakash    1324  5.5  1.2 123456 45678 ?       Sl   08:45   0:10 chrome

📌 Key Fields to Analyze:

• PID: Process ID.

• %CPU, %MEM: CPU and memory usage.

• VSZ/RSS: Virtual and resident memory size.

• STAT: Process state (R running, S sleeping, Z zombie).

• COMMAND: The command/process name.

🔹 3. kill — Send Signal to a Process

📌 Purpose:

Terminates (or sends other signals) to a process using its PID.

📌 Syntax:

kill <PID>               # Send SIGTERM (default)
kill -9 <PID>            # Send SIGKILL (force kill)
kill -l                  # List all signals

📌 Example:

ps aux | grep chrome
# prakash   1324  5.5  1.2 ... chrome
kill -9 1324

📌 How to Analyze:

• If a process is unresponsive, use kill -9.

• Use ps or top to find the PID of the problem process before killing.

🔹 4. cpu — (Note: There's no native cpu command in Linux)

📌 Possible Meanings:

• You might be referring to:

  • Checking CPU usage via top, htop, or mpstat.

  • lscpu — to get CPU architecture info.

✅ Example 1: View CPU architecture

lscpu

Output:

Architecture:        x86_64
CPU(s):              8
Model name:          Intel(R) Core(TM) i7-8565U
CPU MHz:             1800.000

✅ Example 2: CPU usage

mpstat -P ALL 1

Output:

11:28:01 AM  CPU    %usr   %sys   %idle
11:28:02 AM  all     5.00   1.00   94.00

🔍 Summary Comparison Table

📌 Real-world Interview Tip (FAANG-ready):

Be prepared to:

• Find high CPU processes using top or ps.

• Kill a zombie or runaway process with kill -9.

• Monitor memory leaks or CPU bottlenecks.

• Explain CPU load and how to scale (e.g., vertical scaling, multithreading).

ps aux --sort=-%mem | head -n 6

mpstat -P ALL 1

ps aux | awk '$8=="Z" { print $2, $11 }'

ps -eo pid,ppid,stat,cmd | grep Z

pkill -f java

ps aux | grep java | awk '{print $2}' | xargs kill -9

watch -n 2 "ps -eo pid,comm,%cpu,%mem --sort=-%cpu | head -n 11"

ps -u <username>

pkill -P <parent_pid>

ps -eo pid,ppid,cmd

lsof | awk '{ print $2 }' | sort | uniq -c | sort -nr | head

lscpu

sar -u 1 10

lsof -p <pid>

top

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
12345 user1     20   0   10240    920    860 R 100.0  0.0   1:23.45 python

sudo strace -p 12345

clock_gettime(CLOCK_MONOTONIC, {tv_sec=12345, tv_nsec=678900000}) = 0
read(3, "", 4096)                         = 0
epoll_wait(4, [], 128, 0)                = 0
clock_gettime(CLOCK_MONOTONIC, {tv_sec=12345, tv_nsec=679000000}) = 0

sudo strace -tt -T -p 12345 -o strace.log

sudo lsof -p 12345

COMMAND   PID   USER   FD   TYPE DEVICE SIZE/OFF     NODE NAME
python  12345  user1  cwd    DIR  8,1     4096      123456 /home/user1/myapp
python  12345  user1  txt    REG  8,1   123456      654321 /usr/bin/python3.8
python  12345  user1  mem    REG  8,1    45678      987654 /lib/x86_64-linux-gnu/libc.so.6
python  12345  user1    3u   REG  8,1   345678      456789 /home/user1/myapp/log.txt
python  12345  user1    4u  IPv4 23456      0t0        TCP 127.0.0.1:45678->127.0.0.1:3306 (ESTABLISHED)

sudo pstack 12345

#0  0x00007f3f5dbae430 in __libc_recv () from /lib64/libc.so.6
#1  0x000055b6d2e5a891 in socket_read ()
#2  0x000055b6d2e4ff3a in main_loop ()

kill [-SIGNAL] PID

top

kill 12345

kill -9 12345

pkill [-SIGNAL] pattern

pkill python

pkill -9 nginx

pkill -u prakash java

pkill -t pts/1 bash

pstree

systemd─┬─NetworkManager───2*[{NetworkManager}]
        ├─sshd───sshd───bash───pstree
        ├─cron
        ├─dbus-daemon
        ├─gnome-shell───2*[{gnome-shell}]
        ├─firefox───9*[{firefox}]
        ├─pulseaudio───{pulseaudio}
        └─cupsd

pstree -p -a

systemd(1)─┬─cron(585)
           ├─sshd(980)───sshd(1032)───bash(1033)───pstree(1080) -p -a
           ├─nginx(850) -g daemon off;───nginx(851)

+-----------------+        +----------------+       +------------------+
|     CPU         |<-----> |    Processes   |<----->|      Memory       |
+-----------------+        +----------------+       +------------------+
      ^  |                          ^   |                      |
      |  |                          |   |                      v
      |  |                    +-----+   +------+        +-------------+
      |  |                    | High CPU usage |        |   Swapping  |
      |  +------------------->| Infinite loops |        |   Thrashing |
      |                       | Busy-waiting   |        +-------------+
      |
      |          +----------------+    
      |          | Interrupts     |     
      +----------| Context Switch |    
                 +----------------+   

        |
        v
+----------------+
|     Load Avg   |
|  (uptime, top) |
+----------------+

+-------------------+        +-------------------+
|     Disk I/O      |<-----> |     Processes     |
+-------------------+        +-------------------+
        |                              ^
        |                              |
        v                              |
+-------------------+        +--------------------------+
|  High I/O Wait    |<-------|  Log-heavy apps, DBs     |
|  (iostat, iotop)  |        |  Misconfigured writes    |
+-------------------+        +--------------------------+

+-------------------+        +-------------------+
|    Network I/O    |<-----> |     Processes     |
+-------------------+        +-------------------+
        |                              ^
        |                              |
        v                              |
+---------------------+       +-------------------------+
| High RX/TX, Drops   |<------| Chatty apps, poor MTU   |
| (iftop, netstat)    |       | Packet loss, congestion |
+---------------------+       +-------------------------+

103 Sudo vs Su and Principle of least privileges

1. Difference Between su and sudo

Basic Definition

• su (Substitute User or Switch User):
  Allows you to switch to another user account, typically the root user. You’ll be prompted to enter the target user’s password.

• sudo (Superuser Do):
  Allows you to run a single command with elevated privileges, but you use your own password (not root's). Access is controlled through the /etc/sudoers file.

Key Differences

Real-World Usage

• Use su when:

  • You need to maintain a full root shell session.

  • You're working in an environment where sudo isn’t configured.

• Use sudo when:

  • You want to minimize risk by limiting access to specific commands.

  • You're working in a multi-user environment and need an audit trail.

  • You want to adhere to best security practices.

Example

# Using su to become root
su -
# (enter root password)
apt update

# Using sudo to run a command as root
sudo apt update
# (enter your password)

2. Principle of Least Privilege (PoLP)

What It Means

The Principle of Least Privilege is a security best practice stating that users and processes should be granted only the minimum level of access needed to perform their tasks — no more, no less.

💡 Why It’s Important

• Reduces attack surface: Limits what attackers can do if they gain access.

• Minimizes human error: Prevents accidental deletion or system changes.

• Improves auditability: Easier to track and understand permission usage.

• Supports compliance: Many regulations (e.g., HIPAA, GDPR) require it.

How It's Applied in Linux

1. User Permissions
   Users are placed into groups and given access to only necessary files or directories.

2. Sudo Configuration
   The /etc/sudoers file is used to allow users to run specific commands as root, without giving full root access.

   Example:

   john ALL=(ALL) NOPASSWD: /usr/bin/systemctl restart apache2
   

   This means john can restart Apache with sudo, but nothing else.

3. File Ownership and Permissions
   The chmod, chown, and chgrp commands are used to tightly control file access.

4. Service Accounts
   Processes like web servers or databases run under dedicated users (e.g., www-data) that only have access to their specific directories.

 Interview-Worthy Talking Points

• “Using sudo instead of su enforces least privilege by giving users temporary access to specific tasks.”

• “We implement PoLP by reviewing user permissions regularly and removing unnecessary sudo privileges.”

• “I once audited a system where developers had full root access — we moved them to role-based sudo rules, which enhanced security significantly.”

101 Linux ACL

 Here is the cleaned-up version of the Linux ACL (Access Control List) explanation without any special symbols:

What is ACL (Access Control List) in Linux?

Definition

ACL is a fine-grained permission system in Linux that allows you to grant different permissions to multiple users and groups on a single file or directory — something that traditional Unix permissions can't do.

Why Use ACL?

Traditional Linux file permissions only allow you to define:

• One owner (user)

• One group

• Permissions for others

So you can only have 3 sets of permissions: user, group, others

But what if:

• You want to give read access to another user

• Or let a different group have write access

• Without changing ownership or primary group

That's where ACL comes in.

ACL vs Traditional Permissions

Enabling and Using ACL

Check if filesystem supports ACL

Most modern Linux distros with ext4, xfs, or btrfs support it.

mount | grep acl

If not enabled, mount with:

mount -o remount,acl /mount/point

ACL Commands

1. Check current ACLs

getfacl filename

Example:

getfacl report.txt

Output:

file: report.txt
owner: prakash
group: devs
user::rw-
user:john:r--
group::r--
mask::r--
other::---

2. Add ACL for specific user

setfacl -m u:john:r file.txt

John gets read-only access, even if he is not the owner or in the group.

3. Add ACL for a group

setfacl -m g:designers:rw file.txt

Group 'designers' can read/write file.txt

4. Remove ACL entry

setfacl -x u:john file.txt

5. Set Default ACL on directory (for inheritance)

setfacl -d -m u:john:rw /project/folder

All new files inside /project/folder will automatically give john read/write access.

6. Remove all ACLs

setfacl -b file.txt

Example Scenario

You’re building a CI/CD pipeline and want:

• Dev team to have read/write on app.conf

• Ops team to have read-only access

• Jenkins user to have write-only access

Using ACL:

setfacl -m g:dev:rw app.conf
setfacl -m g:ops:r-- app.conf
setfacl -m u:jenkins:-w- app.conf

Output Explanation (getfacl)

file: file.txt
owner: prakash
group: devs
user::rw-
user:john:r--
group::r--
group:designers:rw-
mask::rw-
other::---

Mask defines the maximum permission limit for all ACL users and groups (excluding the owner).

Interview-Ready Q and A

Q1: Why use ACL when traditional permissions exist?

A: Traditional permissions only allow one user and one group. ACL allows multiple users and groups to have different access levels on the same file or directory — useful in collaborative or enterprise environments.

Q2: What does 'setfacl -m u:john:rw file.txt' do?

A: It gives read/write access to user john on file.txt without changing ownership or default permissions.

Q3: What does 'mask::r--' mean in 'getfacl'?

A: The mask defines the maximum permission limit for all ACL users and groups (excluding the owner). Even if ACL grants rw-, a mask of r-- will reduce it to read-only.

Q4: How do you make ACL changes persistent across reboots?

A: ACLs are stored in extended attributes of files, and are persistent across reboots — provided the filesystem is mounted with acl support.

Q5: What’s the difference between default ACL and access ACL?

Summary Cheat Sheet

Let me know if you want a one-page printable PDF or a Notion-ready template with examples and command cheats.

101 Linux Permissions

 Here’s a comprehensive guide to Linux Permissions tailored for FAANG-level interviews—starting from beginner to advanced, along with 10 interview-style questions and answers for practice.

1. Linux Permissions: Beginner to Advanced

1.1 Basics of Linux Permissions

Each file/directory has:

[File Type][Owner][Group][Others]

Example:

-rwxr-xr-- 1 prakash devs 2345 Jun 6 12:00 script.sh

Breakdown:

• - = File (can be d for directory)

• rwx = Owner: read, write, execute

• r-x = Group: read, execute

• r-- = Others: read only

1.2 Types of Permissions

To get octal value:

chmod 754 filename
# => Owner: 7 (rwx), Group: 5 (r-x), Others: 4 (r--)

1.3 Managing Permissions

• View:

  ls -l
  

• Change using symbolic mode:

  chmod u+x file.sh     # Add execute for owner
  chmod g-w file.sh     # Remove write for group
  chmod o=r file.sh     # Set read-only for others
  

• Change using numeric mode:

  chmod 755 file.sh
  

1.4 Ownership

• Change owner:

  chown prakash file.txt
  

• Change group:

  chgrp devs file.txt
  

• Change both:

  chown prakash:devs file.txt
  

1.5 Special Permissions (Advanced)

1.5.1 SetUID (s) — Run as file owner

chmod u+s my_script
ls -l => -rwsr-xr-x

Used in programs like passwd.

1.5.2 SetGID (s) — Run with group permissions

chmod g+s my_script
ls -l => -rwxr-sr-x

1.5.3 Sticky Bit (t) — Protect deletion in shared dirs

chmod +t /tmp
ls -ld /tmp => drwxrwxrwt

1.6 Default Permissions – umask

Check umask:

umask        # e.g., 0022

Meaning:

• File default: 666 – 0022 = 644 (rw-r--r--)

• Dir default: 777 – 0022 = 755 (rwxr-xr-x)

1.7 Recursive Permission Change

chmod -R 755 /var/www
chown -R prakash:www-data /var/www

2. 10 Linux Permission Questions & Answers

Q1. What does chmod 755 file.sh do?

Answer:
Sets permissions to:

• Owner: rwx (7)

• Group: r-x (5)

• Others: r-x (5)

Q2. What is the use of chmod +x script.sh?

Answer:
Adds execute permission for the owner, allowing the script to be run directly.

Q3. What is the difference between chmod 777 and chmod 755?

Answer:

• 777: Everyone has full access (read/write/execute).

• 755: Only owner has write access, others can read/execute but not modify.

Q4. How do you give read & write to owner, read-only to others?

Answer:

chmod 644 file.txt

Q5. What does -rwsr-xr-x mean in ls -l output?

Answer:
SetUID is set:

• File will run with owner’s privileges, not current user’s.

Q6. What does the Sticky Bit do?

Answer:
Prevents users from deleting others’ files in a shared directory like /tmp.
drwxrwxrwt indicates sticky bit set.

Q7. How do you recursively change ownership of a directory and its contents?

Answer:

chown -R user:group /path/to/dir

Q8. Explain what umask 027 means?

Answer:
Default permissions mask:

• For files: 666 – 027 = 640 (rw-r-----)

• For dirs: 777 – 027 = 750 (rwxr-x---)

Q9. What’s the permission number for rw-rw-r--?

Answer:

rw- rw- r-- = 664

Q10. How to give only execute permission to others?

Answer:

chmod o=x file.sh

Bonus Practice Examples

Certainly! Let's break down umask (User Mask or User file creation MASK) in full detail, including concept, default values, calculation logic, and practical examples — especially useful for FAANG-level interviews.

What is umask?

umask defines the default permission bits to subtract from newly created files or directories.

• It doesn’t grant permissions — it masks/restricts them.

• When a user creates a file or directory, Linux applies a default permission first, then subtracts the umask.

Default Permission Values

How umask Works (Step-by-Step)

Let's say:

• umask = 022

For files:

Default:   666
UMASK:     022
Result:    644 → rw-r--r--

For directories:

Default:   777
UMASK:     022
Result:    755 → rwxr-xr-x

So:

• Owner keeps full permissions

• Group & Others lose w permission

Common umask values & their effects

Examples in Shell

🔍 View current umask:

umask
# Output: 0022

 Set umask temporarily:

umask 0027
touch file.txt
mkdir dir1

ls -l
# file.txt => -rw-r-----
# dir1     => drwxr-x---

⚙️ Make umask permanent:

For bash shell, add this to:

vi ~/.bashrc
umask 027

Then:

source ~/.bashrc

Important Rules

1. umask removes bits, not adds them.

2. Files are never given execute (x) by default, even if umask allows it.

3. Directories do get x so you can cd into them.

Interview-ready Example

Q: If a user has a umask of 0077 and runs touch test.txt and mkdir demo, what are the resulting permissions?

Answer:

• Default for file: 666

• umask: 077

• Final: 600 (rw-------)

• Default for dir: 777

• umask: 077

• Final: 700 (rwx------)

So only the owner can read/write/execute. This is a secure configuration.

Visual Table: umask Logic

Absolutely! Let's dive deep into Linux Special Permissions — SetUID, SetGID, and the Sticky Bit — in a clear, detailed, and FAANG-level interview-ready manner with real-world use cases, diagrams (described), and practical examples.

Why Special Permissions Exist

Linux has standard permission bits for:

• User (Owner)

• Group

• Others

But what if you want:

• A user to run a program as root?

• Files in a shared directory to always inherit the same group?

• Prevent users from deleting others’ files in /tmp?

That’s where special permission bits come in.

1. SetUID (Set User ID)

What it Does:

When a binary file has SetUID, any user who executes it temporarily assumes the permissions of the file's owner (usually root).

Real-World Use Case:

• The passwd command allows any user to update their own password, but it needs to write to /etc/shadow, which is owned by root.

• So passwd runs as root, even when executed by a regular user.

Example:

ls -l /usr/bin/passwd
-rwsr-xr-x 1 root root 54256 Jan  1 00:00 /usr/bin/passwd
          ^
          └── 's' here means SetUID is ON for user

How to Set:

chmod u+s my_script

🔍 How to Verify:

ls -l my_script
-rwsr-xr-x 1 prakash devs 2345 Jun 6 12:00 my_script

2. SetGID (Set Group ID)

What it Does:

A) On Files:

• The process runs with group permissions of the file, not the executing user.

B) On Directories:

• New files/directories inside the directory will inherit the group ownership of the directory — not the creator's default group.

Real-World Use Case:

• In collaborative environments (e.g., /shared/projects), you want files created by any team member to have the same group, like devs.

Example on Directory:

mkdir shared
chgrp devs shared
chmod g+s shared

ls -ld shared
drwxr-sr-x 2 prakash devs 4096 Jun 6 12:00 shared
           ^
           └── 's' on group means SetGID is ON

Now, any file inside shared/ will automatically belong to group devs.

How to Set on File:

chmod g+s my_binary

How to Set on Directory:

chmod g+s /some/dir

3. Sticky Bit (t)

What it Does:

Only the owner of a file can delete or rename it, even if others have write access to the directory.

Real-World Use Case:

• /tmp directory: World-writable directory used by all users. Without sticky bit, users could delete each other’s temporary files.

Example:

ls -ld /tmp
drwxrwxrwt 10 root root 4096 Jun 6 12:00 /tmp
          ^
          └── 't' means Sticky Bit is set

How to Set:

chmod +t mydir

How to Remove:

chmod -t mydir

Summary Table

Interview-Style Example

Q: A file shows -rwsr-xr-x. What does it mean and why is it used?

Answer:

• The s in user field = SetUID.

• This means: when the file is executed, it runs with the owner's privileges, not the executor’s.

• Common use: commands like passwd which need to access /etc/shadow.

FAANG-Level Takeaway Tips

• Know how SetUID can be a security risk if misused (e.g., privilege escalation).

• Sticky bit is essential for shared directories to prevent accidental file deletion.

• SetGID on directories is useful in CI/CD pipelines and group collaboration.