Category: Linux

The “1, 2, 3” cache clear method in Linux refers to clearing different types of caches managed by the operating system. Each cache type is managed differently and cleared by writing a specific value to the file /proc/sys/vm/drop_caches.

Cache Types:

1. PageCache (Value: 1):
   • PageCache stores file contents that are cached in memory to reduce disk I/O.
2. Dentries and Inodes (Value: 2):
   • Dentry is a cache of directory entries, and inodes store file metadata.
3. PageCache + Dentries + Inodes (Value: 3):
   • Clears both PageCache and Dentry/inode caches.

Steps to Clear Cache with Examples:

1. Check Current Memory Usage

Use the free -h command to view current memory and cached memory:

free -h

Look under the “buff/cache” column to see the cache usage.

2. Clear Cache

Use the following commands to clear caches. Root privileges are required, so prefix with sudo:

Clear PageCache:

sudo sh -c "echo 1 > /proc/sys/vm/drop_caches"

Clear Dentry and Inode Cache:

sudo sh -c "echo 2 > /proc/sys/vm/drop_caches"

Clear Both PageCache and Dentry/Inode Cache:

sudo sh -c "echo 3 > /proc/sys/vm/drop_caches"

3. Check Memory Usage Again

Run the free -h command again to verify that the cache has been cleared.

Example Workflow:

1. Check Memory Before Clearing: free -h Output: total used free shared buff/cache available Mem: 15Gi 4.0Gi 2.0Gi 0.5Gi 9Gi 10Gi Swap: 2Gi 0B 2Gi
2. Clear All Caches (PageCache, Dentries, and Inodes): sudo sh -c "echo 3 > /proc/sys/vm/drop_caches"
3. Check Memory After Clearing: free -h Output: total used free shared buff/cache available Mem: 15Gi 4.0Gi 7.0Gi 0.5Gi 4Gi 11Gi Swap: 2Gi 0B 2Gi

Notice how the “buff/cache” column decreases.

Best Practices

• Clearing caches isn’t typically necessary because Linux uses unused memory to cache files and automatically clears it when needed.
• Use this method only for debugging or specific needs (e.g., testing performance under no cache conditions).

Would you like to try this on your system now? Let me know if you need additional help!

Let’s break down PageCache, Dentries, and Inodes with simple explanations and commands you can use on your Ubuntu system to see how they work.

---

1. PageCache

• What it is:
  • PageCache stores file data read from disk in memory to speed up future reads.
  • If a file is accessed again, the system reads it from memory instead of the slower disk.
• Example:
  1. Create a Large File: dd if=/dev/zero of=testfile bs=1M count=100 This creates a file testfile of 100 MB size.
  2. Clear Cache: sudo sh -c "echo 3 > /proc/sys/vm/drop_caches"
  3. Read the File (Populates PageCache): time cat testfile > /dev/null The first read is slow because it loads data from disk.
  4. Read the File Again (From PageCache): time cat testfile > /dev/null The second read is faster as it fetches data from memory (PageCache).
  5. Verify Cache Usage: Check memory usage using: free -h

---

2. Dentries

• What it is:
  • A dentry (directory entry) cache stores metadata about directories, such as filenames and their locations in the filesystem.
  • This speeds up directory traversals and file lookups.
• Example:
  1. Clear Cache: sudo sh -c "echo 3 > /proc/sys/vm/drop_caches"
  2. List a Large Directory (Populates Dentry Cache): ls -R /usr > /dev/null
  3. List the Directory Again (From Dentry Cache): time ls -R /usr > /dev/null The second run will be faster because the directory structure is cached in memory.

---

3. Inodes

• What it is:
  • An inode is a data structure storing metadata about a file (e.g., permissions, owner, size).
  • Inode caching stores this metadata to reduce disk reads.
• Example:
  1. Find Inodes Used by the System: df -i This shows the inode usage for each mounted filesystem.
  2. Create Many Files (Populates Inode Cache): mkdir testdir for i in {1..10000}; do touch testdir/file$i; done
  3. Clear Cache: sudo sh -c "echo 2 > /proc/sys/vm/drop_caches"
  4. List Files (Rebuilds Inode Cache): time ls testdir > /dev/null
  5. List Files Again (From Inode Cache): time ls testdir > /dev/null The second run will be faster due to inode caching.

---

Summary of Commands:

• free -h: Check memory usage (PageCache in “buff/cache”).
• df -i: Check inode usage for filesystems.
• echo [1|2|3] > /proc/sys/vm/drop_caches: Clear caches.

The command dd if=/dev/zero of=testfile bs=1M count=100 is a Linux command used to create a file filled with zeros. Here’s a breakdown of each part:

Command Breakdown:

1. dd:
   • A low-level command-line utility used for copying and converting data between files or devices.
2. if=/dev/zero:
   • if stands for “input file.”
   • /dev/zero is a special file in Linux that produces a continuous stream of null bytes (zeros).
3. of=testfile:
   • of stands for “output file.”
   • testfile is the name of the file where the data (zeros) will be written.
4. bs=1M:
   • bs stands for “block size.”
   • This sets the size of each block of data to 1 Megabyte (MB).
5. count=100:
   • Specifies the number of blocks to copy.
   • In this case, 100 blocks of 1 MB each.

What the Command Does:

• It writes 100 MB (1 MB × 100 blocks) of zero-filled data to a file named testfile.

Example Output:

When you run the command, you’ll see something like this:

100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.123456 s, 847 MB/s

This means:

• 100+0 records in: 100 blocks read from the input file (/dev/zero).
• 100+0 records out: 100 blocks written to the output file (testfile).
• 104857600 bytes: Total bytes written (100 MB).
• 0.123456 s: Time taken to execute the command.
• 847 MB/s: Write speed.

Verify the File:

• Check the file size: ls -lh testfile You should see the size as 100 MB.
• Inspect the file contents: hexdump -C testfile | head This will display the file’s contents in hexadecimal format, and you’ll see it filled with 00 (zeros).

---

Why clearing dentries does not affect memory a lot

The behavior you’re observing can be explained by how Linux handles PageCache and Dentries/Inodes caching.

Key Points:

1. PageCache (Cleared with echo 1 > /proc/sys/vm/drop_caches):
   • Clearing PageCache removes cached file data but does not clear dentries and inodes.
   • If you only clear the PageCache, Linux has to read file contents from disk, but dentry and inode information might still be available in memory.
2. Dentries and Inodes (Cleared with echo 2 > /proc/sys/vm/drop_caches):
   • Clearing dentry and inode caches removes directory structure and file metadata information from memory.
   • If dentry and inode caches are cleared, listing directories or performing file operations becomes slower because the system must rebuild this information by reading from disk.

---

Your Test Results:

1. Before Cache Clear:
   • The first ls -R command runs quickly (0.343s) because both PageCache and dentry/inode caches are populated.
2. After Clearing PageCache:
   • After running echo 1 > /proc/sys/vm/drop_caches, the second ls -R command takes significantly longer (2.756s) because:
     • PageCache is cleared, so file contents must be reloaded from disk.
     • However, dentries and inodes are not cleared, so some metadata is still available.

---

Why It Still Takes More Time:

The additional time comes from the fact that:

1. After clearing the PageCache, the system has to read the file contents (or parts of the directory structure) from disk, which is slower than fetching them from memory.
2. Even though dentries and inodes are still in memory, they depend on PageCache for efficient access to file content. So the system has to do extra work to populate the PageCache again.
3. Directory traversal involves a combination of file metadata (dentries/inodes) and directory file contents (loaded via PageCache). Clearing just the PageCache impacts this process.

---

Solution to Test Dentry Cache Impact:

To focus solely on dentry and inode caches, you need to clear them explicitly with:

sh -c "echo 2 > /proc/sys/vm/drop_caches"

Then rerun:

time ls -R /usr > /dev/null

You’ll notice that this run will also take longer compared to when the dentry cache is intact, but the pattern will differ from clearing the PageCache.

---

Suggested Workflow to Understand All Cache Impacts:

1. Baseline: time ls -R /usr > /dev/null
2. Clear PageCache: sh -c "echo 1 > /proc/sys/vm/drop_caches" time ls -R /usr > /dev/null
3. Clear Dentries and Inodes: sh -c "echo 2 > /proc/sys/vm/drop_caches" time ls -R /usr > /dev/null
4. Clear Both PageCache and Dentries/Inodes: sh -c "echo 3 > /proc/sys/vm/drop_caches" time ls -R /usr > /dev/null

This should give you a clear understanding of how each type of cache contributes to performance.

You can watch realtime operations happing in directory using inotifywatch and inotifywait command

Install inotify tool

sudo apt install inotify-tools

Create directory

mkdir -p /tmp/test

Watch action

inotifywatch -v -e modify,create,delete /tmp/test

Ctrl + C

#Then check statistics

Real time watch action

inotifywait -m -e modify,create,delete /tmp/test/

inotifywait -m -r -e modify,create,delete /tmp/test/

Now try to create modify and delete the file

touch /tmp/test/file1.txt
echo "Hello" > /tmp/test/file1.txt
rm /tmp/test/file1.txt

Ctrl + C

To monitor multiple paths at a time run following commands

inotifywatch -e access,modify /home/user/myfolder /home/user/another_folder /home/user/myfile.txt
OR
inotifywait -m -e access,modify /home/user/myfolder /home/user/another_folder /home/user/myfile.txt

Install required tools

rpm -qa | grep rpmdevtools
yum list | grep rpmdevtools
yum install rpmdevtools

Create directory structure required for rpm build if not exist

rpmdev-setuptree

rpmbuild/
├── BUILD
├── RPMS
├── SOURCES
├── SPECS
└── SRPMS

Create source package under SOURCES

mkdir hello-0.0.1
cd hello-0.0.1

hello.sh

#!/bin/sh
echo "Hello world"

tar -cvzf hello-0.0.1.tar.gz hello-0.0.1

Create spec file and make necessary changes

cd ~/rpmbuild/SPECS
rpmdev-newspec hello.spec

Name:           hello
Version:        0.0.1
Release:        1%{?dist}
Summary:        A simple hello world script
BuildArch:      noarch

License:        GPL
Source0:        %{name}-%{version}.tar.gz

Requires:       bash

%description
A demo RPM build

%prep
%setup -q

%install
rm -rf $RPM_BUILD_ROOT
mkdir -p $RPM_BUILD_ROOT/opt/hello
cp * $RPM_BUILD_ROOT/opt/hello

%clean
rm -rf $RPM_BUILD_ROOT

%files
/opt/hello/*

%postun
rm -vrf /opt/hello

%changelog
* Sun Feb 04 2024 
- First version being packaged

Now directory structure should look like this

/home/tux/rpmbuild/
├── BUILD
├── BUILDROOT
├── RPMS
├── SOURCES
│   └── hello-0.0.1.tar.gz
├── SPECS
│   └── hello.spec
└── SRPMS

Build rpm using following command

rpmbuild -ba SPECS/hello.spec

where -b is build a is for both source and binary
if you want only binary then put -bb flag
if you want only source then put -bs flag

Install the rpm using following command

sudo rpm -ivh ~/rpmbuild/RPMS/noarch/hello-0.0.1-1.el8.noarch.rpm

Verify rpm installed successfully using following command

rpm -qi hello
tree /opt/hello

The %changelog entry of a package can be viewed, too:

rpm -q hello –changelog

Uninstall or erase installed rpm

sudo rpm -q | grep hello
sudo rpm -ve hello

Troubleshoot

To extract files and directories from rpm

rpm2cpio hello-0.0.1-1.el8.noarch.rpm | cpio -idmv

Day 1: Introduction to Linux

Topics:

1. Linux operating system
   Explanation: Learn about the Linux operating system, its features, and its popularity among developers and system administrators.
2. Basic Linux commands
   Explanation: Understand essential Linux commands such as ls (list files and directories), cd (change directory), pwd (print working directory), and mkdir (make directory).
3. File and directory management
   Explanation: Learn how to create, delete, move, and rename files and directories using commands like touch, rm, mv, and cp.
4. File permissions and ownership
   Explanation: Understand the concept of file permissions and how to set permissions using chmod, as well as changing file ownership using chown.
5. File manipulation and text editing
   Explanation: Explore commands like cat (concatenate and display file contents), less (view file contents page by page), and nano (a basic text editor).

Assignments:

1. Create a directory named “my_files” and navigate into it.
   Answer: mkdir my_files (to create the directory) and cd my_files (to navigate into it).
2. Create a file named “my_text.txt” and write some text into it.
   Answer: touch my_text.txt (to create the file) and nano my_text.txt (to open the file in the nano text editor and write the text).
3. List all the files and directories in the current directory.
   Answer: ls (to list files and directories).
4. Rename the file “my_text.txt” to “new_text.txt”.
   Answer: mv my_text.txt new_text.txt (to rename the file).
5. Delete the directory “my_files” and all its contents.
   Answer: rm -r my_files (to remove the directory and its contents).

Day 2: Working with Files and Permissions

Topics:

1. File manipulation commands
   Explanation: Explore more file manipulation commands such as head (display the beginning of a file), tail (display the end of a file), and wc (word count).
2. File permissions and access modes
   Explanation: Understand the three levels of file permissions (owner, group, others) and how to set permissions using symbolic notation (rwx) and numeric notation (777).
3. Changing file ownership and group
   Explanation: Learn how to change the owner and group of a file using the chown and chgrp commands.
4. File compression and archiving
   Explanation: Discover commands like tar (archive files) and gzip (compress files) to manage large file collections efficiently.
5. File searching and filtering
   Explanation: Learn about commands like find (search for files and directories) and grep (search for text patterns in files).

Assignments:

1. Create a file named “sample.txt” and write some content into it.
   Answer: touch sample.txt (to create the file) and nano sample.txt (to open the file in the nano text editor and write the content).
2. Display the first 5 lines of the file “sample.txt”.
   Answer: head -n 5 sample.txt (to display the first 5 lines).
3. Set the file permissions of “sample.txt” to read and write for the owner only.
   Answer: chmod 600 sample.txt (to set the permissions).
4. Change the owner of “sample.txt

” to a different user.
Answer: chown username sample.txt (to change the owner to the specified username).

5. Archive the file “sample.txt” and compress it into a single file.
   Answer: tar -czvf sample.tar.gz sample.txt (to create the archive and compress it).

Day 3: Introduction to Vim Editor

Topics:

1. Introduction to Vim
   Explanation: Understand the basics of the Vim editor, including its modes (Normal, Insert, Visual), navigation, and command execution.
2. Opening and saving files
   Explanation: Learn how to open files in Vim, make changes, and save them using commands like :e (open), :w (save), and :q (quit).
3. Moving and navigating within files
   Explanation: Explore various movement commands like h (left), j (down), k (up), l (right), and using line numbers to jump to specific locations.
4. Editing and modifying text
   Explanation: Understand text editing commands like i (insert), o (open a new line), x (delete a character), and yy (copy a line).
5. Undo and redo operations
   Explanation: Learn how to undo and redo changes made in Vim using the u (undo) and Ctrl+R (redo) commands.

Assignments:

1. Open the file “my_file.txt” in Vim and navigate to the end of the file.
   Answer: vim my_file.txt (to open the file) and G (to move to the end of the file).
2. Insert a new line after the current line and write some text into it.
   Answer: Press o (to open a new line) and start typing the desired text.
3. Delete the current line in Vim.
   Answer: Press dd (to delete the current line).
4. Copy the current line in Vim and paste it below the current line.
   Answer: Press yy (to copy the current line) and then p (to paste it below).
5. Save the changes made to the file and exit Vim.
   Answer: Press :wq (to save and quit).

Day 4: Advanced Vim Editing

Topics:

1. Visual mode in Vim
   Explanation: Learn how to select and manipulate blocks of text using Visual mode, including commands like v (characterwise), V (linewise), and Ctrl+V (blockwise).
2. Searching and replacing text
   Explanation: Discover how to search for specific text patterns using / (forward search) and ? (backward search), as well as replacing text using :s (substitute).
3. Advanced editing commands
   Explanation: Explore advanced editing commands like x (delete a character), r (replace a character), J (join lines), and . (repeat the last command).
4. Advanced movement and navigation
   Explanation: Learn advanced movement commands like w (jump to the beginning of the next word), b (jump to the beginning of the previous word), and gg (jump to the first line).
5. Split windows and tabs in Vim
   Explanation: Understand how to split the Vim editor window vertically and horizontally using commands like :split and :vsplit, and navigate between tabs.

Assignments:

1. Open the file “my_notes.txt” in Vim and search for the word “important”.
   Answer: vim my_notes.txt (to open the file) and /important (to search for the word “important”).
2. Replace all occurrences of the word “old” with “new” in the current line.
   Answer: :s/old/new/ (to perform the substitution).
3. Join the current line with the line below it.
   Answer: Press J (to join the lines).
4. Split the Vim editor window vertically.
   Answer: :vsplit (to split the window vertically).
5. Move the cursor to the beginning of the next word in Vim.
   Answer: Press w (to jump to the beginning of the next word).

Day 5: Vim Customization and Advanced Topics

Topics:

1. Vim configuration files
   Explanation: Understand the .vimrc file and how to customize Vim settings, key mappings, and plugins.
2. Customizing Vim colorschemes
   Explanation: Learn how to change the colorscheme in Vim to enhance the visual appearance and readability of your code.
3. Advanced Vim features and plugins
   Explanation: Explore advanced Vim features like macros, multiple cursors (using plugins), and code completion (using plugins).
4. Vim navigation shortcuts
   Explanation: Discover useful navigation shortcuts like Ctrl+U (scroll half a page up), Ctrl+D (scroll half a page down), and gg (jump to the first line).
5. Vim documentation and help
   Explanation: Learn how to access Vim documentation and help resources, including built-in help pages and online resources.

Assignments:

1. Customize the Vim settings by adding the following lines to your .vimrc file:

• Set the tab width to 4 spaces.
• Enable line numbers.
  Answer: Open the .vimrc file in Vim (vim ~/.vimrc) and add the desired configurations.

1. Change the colorscheme in Vim to a different one.
   Answer: In Vim, type :colorscheme <colorscheme_name> to change the colorscheme.
2. Create a macro in Vim that inserts a specific code snippet.
   Answer: Record the macro using q<register> and replay it using @<register>.
3. Install a plugin in Vim for code completion or any other desired functionality.
   Answer: Install the desired plugin using a plugin manager like Vundle or Pathogen.
4. Access the Vim built-in help and find information on a specific Vim command or feature.
   Answer: In Vim, type :help <command_or_feature> to access the built-in help pages.

Shell Scripting

Apologies for the oversight. Here are the shell scripting assignments along with sample answers for each day:

Day 1:

1. Write a shell script that takes two numbers as input and prints their sum.

#!/bin/bash

echo "Enter the first number: "
read num1

echo "Enter the second number: "
read num2

sum=$((num1 + num2))
echo "The sum is: $sum"

2. Create a shell script that reads a filename from the user and checks if it exists in the current directory. If the file exists, display a message confirming its existence; otherwise, display an error message.

#!/bin/bash

echo "Enter a filename: "
read filename

if [ -e "$filename" ]; then
    echo "The file '$filename' exists in the current directory."
else
    echo "Error: The file '$filename' does not exist in the current directory."
fi

3. Write a shell script that reads a string from the user and prints it in reverse order.

#!/bin/bash

echo "Enter a string: "
read input_string

reversed_string=$(echo "$input_string" | rev)
echo "Reversed string: $reversed_string"

4. Create a script that takes a directory name as input and lists all the files in that directory.

#!/bin/bash

echo "Enter a directory name: "
read directory

if [ -d "$directory" ]; then
    echo "Files in $directory:"
    ls "$directory"
else
    echo "Error: '$directory' is not a valid directory."
fi

5. Write a shell script that generates a random number between 1 and 100 and asks the user to guess the number. Provide appropriate feedback based on the user’s guess.

#!/bin/bash

random_number=$((RANDOM % 100 + 1))

echo "Guess the number between 1 and 100: "
read user_guess

if [ "$user_guess" -eq "$random_number" ]; then
    echo "Congratulations! You guessed the correct number."
elif [ "$user_guess" -lt "$random_number" ]; then
    echo "Try again. The number is higher than your guess."
else
    echo "Try again. The number is lower than your guess."
fi

Day 2:

1. Write a shell script that takes a filename as input and checks if it is a regular file or a directory.

#!/bin/bash

echo "Enter a filename: "
read filename

if [ -f "$filename" ]; then
    echo "'$filename' is a regular file."
elif [ -d "$filename" ]; then
    echo "'$filename' is a directory."
else
    echo "Error: '$filename' is neither a regular file nor a directory."
fi

2. Create a script that takes a file containing a list of numbers and calculates their sum.

#!/bin/bash

sum=0
while read -r num; do
    sum=$((sum + num))
done < "$1"

echo "Sum of numbers in the file: $sum"

3. Write a shell script that renames all files in a directory with a specific extension to have a prefix “backup_” followed by the original filename.

#!/bin/bash

echo "Enter the directory name: "
read directory

echo "Enter the file extension to rename: "
read ext

for file in "$directory"/*."$ext"; do
    filename=$(basename "$file")
    mv "$file" "$directory/backup_$filename"
done

4. Create a script that reads a file and counts the number of lines, words, and characters in it.

#!/bin/bash

echo "Enter a filename: "
read filename

if [ -f "$filename" ]; then
    line_count=$(wc -l < "$filename")
    word_count=$(wc -w < "$filename")
    char_count=$(wc -c < "$filename")

    echo "Number of lines: $line_count"
    echo "Number of words: $word_count"
    echo "Number of characters: $char_count"
else
    echo "Error: '$filename' is not a valid file."
fi

5. Write a shell script that takes a number as input and prints all the prime numbers less than or equal to that number.

#!/bin/bash

echo "Enter a number: "
read num

if [ "$num" -lt 2 ]; then
    echo "There are no prime numbers less than 2."
else
    echo "Prime numbers less than or equal to $num:"
    for ((i = 2; i <= num; i++)); do
        is_prime=1
        for ((j = 2; j <= i / 2; j++)); do
            if ((i % j == 0)); then
                is_prime=0
                break
            fi
        done
        if [ "$is_prime" -eq 1 ]; then
            echo "$i"
        fi
    done
fi

Day 3:

1. Create a shell script that takes a directory name as input and finds all the subdirectories within it.

#!/bin/bash

echo "Enter a directory name: "
read directory

if [ -d "$directory" ]; then
    echo "Subdirectories in '$directory':"
    find "$directory" -type d
else
    echo "Error: '$directory' is not a valid directory."
fi

2. Write a script that reads a file and removes all the empty lines from it.

#!/bin/bash

echo "Enter a filename: "
read filename

if [ -f "$filename" ]; then
    sed -i '/^[[:space:]]*$/d' "$filename"
    echo "Empty lines removed from '$filename'."
else
    echo "Error: '$filename' is not a valid file."
fi

3. Create a shell script that takes a string as input and checks if it is a palindrome.

#!/bin/bash

echo "Enter a string: "
read input_string

reverse_string=$(echo "$input_string" | rev)

if [ "$input_string" = "$reverse_string" ]; then
    echo "The string is a palindrome."
else
    echo "The string is not a palindrome."
fi

4. Write a shell script that reads a number as input and checks if it is even or odd.

#!/bin/bash

echo "Enter a number: "
read num

if ((num % 2 == 0)); then
    echo "$num is an even number."
else
    echo "$num is an odd number."
fi

5. Create a shell script that prints the current date and time.

#!/bin/bash

current_date=$(date +"%Y-%m-%d")
current_time=$(date +"%H:%M:%S")

echo "Current date: $current_date"
echo "Current time: $current_time"

Day 4:

1. Write a shell script that takes a directory name as input and deletes all the files in that directory with a “.tmp” extension.

#!/bin/bash

echo "Enter a directory name: "
read directory

if [ -d "$directory" ]; then
    find "$directory" -type f -name "*.tmp" -delete
    echo "All '.tmp' files in '$directory' deleted."
else
    echo "Error: '$directory' is not a valid directory."
fi

2. Create a script that reads a file and replaces all occurrences of a word with another word.

#!/bin/bash

echo "Enter a filename: "
read filename

if [ -f "$filename" ]; then
    echo "Enter the word to replace: "
    read old_word

    echo "Enter the new word: "
    read new_word

    sed -i "s/$old_word/$new_word/g" "$filename"
    echo "Occurrences of '$old_word' replaced with '$new_word' in '$filename'."
else
    echo "Error: '$filename' is not a valid file."
fi

3. Write a shell script that reads a number as input and checks if it is prime.

#!/bin/bash

echo "Enter a number: "
read num

if [ "$num" -lt 2 ]; then
    echo "The number must be greater than or equal to 2 to check for primality."
else
    is_prime=1
    for ((i = 2; i <= num / 2; i++)); do
        if ((num % i == 0)); then
            is_prime=0
            break
        fi
    done

    if [ "$is_prime" -eq 1 ]; then
        echo "$num is a prime number."
    else
        echo "$num is not a prime number."
    fi
fi

4. Create a shell script that takes a directory name as input and counts the number of files and subdirectories in it.

#!/bin/bash

echo "Enter a directory name: "
read directory

if [ -d "$directory" ]; then
    file_count=$(find "$directory" -type f | wc -l)
    dir_count=$(find "$directory" -type d | wc -l)

    echo "Number of files: $file_count"
    echo "Number of subdirectories: $dir_count"
else
    echo "Error: '$directory' is not a valid directory."
fi

5. Write a shell script that reads a string as input and converts it to uppercase.

#!/bin/bash

echo "Enter a string: "
read input_string

upper_case_string=$(echo "$input_string" | tr '[:lower:]' '[:upper:]')

echo "Uppercase string: $upper_case_string"

Day 5:

1. Create a shell script that takes two numbers as input and swaps their values.

#!/bin/bash

echo "Enter the first number: "
read num1

echo "Enter the second number: "
read num2

echo "Before swapping: num1 = $num1, num2 = $num2"

# Swapping using a temporary variable
temp=$num1
num1=$num2
num2=$temp

echo "After swapping: num1 = $num1, num2 = $num2"

2. Write a script that takes a file and sorts its lines in ascending order.

#!/bin/bash

echo "Enter a filename: "
read filename

if [ -f "$filename" ]; then
    sort "$filename" > sorted_"$filename"
    echo "Lines in '$filename' sorted in ascending order and saved to 'sorted_$filename'."
else
    echo "Error: '$filename' is not a valid file."
fi

3. Create a shell script that reads a directory name as input and prints the names of the 10 largest files in it.

#!/bin/bash

echo "Enter a directory name: "
read directory

if [ -d "$directory" ]; then
    echo "The 10 largest files in '$directory':"
    du -ah "$directory" | sort -rh | head -n 10
else
    echo "Error: '$directory' is not a valid directory."
fi

4. Write a shell script that takes a list of filenames as arguments and checks if all of them exist in the current directory.

#!/bin/bash

for filename in "$@"; do
    if [ ! -e "$filename" ]; then
        echo "Error: '$filename' does not exist in the current directory."
    else
        echo "'$filename' exists in the current directory."
    fi
done

5. Create a shell script that calculates the factorial of a given number.

#!/bin/bash

echo "Enter a number: "
read num

factorial=1

for ((i = 1; i <= num; i++)); do
    factorial=$((factorial * i))
done

echo "Factorial of $num is: $factorial"

Location root vs alias

http {

	server {

		listen 8081;
		root /var/www/html/websites;

		#with root no need to specify in url as location automatically append to root
		location /sqr {
			root /var/www/html/websites;
		}

		#with alias need to specify in url as location does not append to alias
		location /test {
			alias /var/www/html/websites/vegetables/;
			try_files $uri veggies.html =404;
		}

		#with regex add try_files directive
		location ~* /foo$ {
			alias /var/www/html/websites/fruits/site;
			try_files $uri $uri/index.html =404;
		}

		#with regex add try_files directive above will serve only till foo but below code will serve after foo also e.g. http://site.com/foo/abc.html
		location ~* /foo {
			alias /var/www/html/websites/fruits/site;
			try_files $uri $uri/index.html =404;
		}

	}

}

events {}

nginx -s reload

Check using following url

http://yoursite/sqr

http://yoursite/test

http://yoursite/foo/

Redirect and Rewrite

#this will redirect to foo and also changes the URL
location /bar {
	return 307 /foo;
}

#this will rewrite the url i.e. URL would not change URL will be fizz and content will load from foo 
rewrite /fizz /foo;

Nginx as a Load Balancer

# Define an upstream block to group backend servers
upstream backend_servers {
    # List of backend servers
    server backend1.example.com:8080;
    server backend2.example.com:8080;
    server backend3.example.com:8080;
    server backend4.example.com:8080;
}



server {
    listen 80;  # Listen on port 80

    # Define the server name (domain or IP)
    server_name www.example.com;

    # Location block to define how requests should be handled
    location / {
        # Use the defined upstream group for proxying requests
        proxy_pass http://backend_servers;
}

Run from terminal

while true; do curl -s http://www.example.com; sleep 1; done;

Example

you can run 2 servers locally using php -S or using docker containers by mapping 2 ports

http {

	upstream backend_servers {
		server localhost:1234;
		server localhost:4567;
	}


	server {

		listen 8081;
		#root /var/www/html/websites;


		location / {
			proxy_pass http://backend_servers;
		}


	}

}

events {}

PHP FPM socket configuration

user www-data;

events {}

http {

    root /var/www/samosa;


    server {

        access_log /var/log/nginx/samosa-access.log;
        error_log /var/log/nginx/samosa-error.log;

        error_page 404 /404.html;

        location ~ \.php$ {
            include snippets/fastcgi-php.conf;
            fastcgi_pass unix:/run/php/php8.1-fpm.sock;
        }
    }
}

create project and verify with normal url

mkdir /var/www/html/myproject

vim index.html

<h1>Hello World</h1>

vim index.php

<?php

phpinfo();

Create server block conf file

vim /etc/nginx/conf.d/myproject.conf

server {
  listen 8082 default_server;
  server_name _;

  index index.php index.html;

  root /var/www/html/myproject;

  location ~ \.php$
  {
      include snippets/fastcgi-php.conf;
      fastcgi_pass unix:/var/run/php/php7.4-fpm.sock;
  }
}

Test nginx configuration

nginx -t

restart nginx

sudo service nginx restart
OR
sudo systemctl restart nginx

For Laravel Project
Specify path till public directory

Need to explore

worker process

log format

weight in upstream

upstream block ip:port down

nginx plus

directives – listen root index access_log error_log

access_log off;

error_log debug;
error_log warn;

$scheme variable

include /etc/nginx/mime.types;

nginx vs haproxy

User

List all users

cat /etc/passwd

List specific user

cat /etc/passwd | grep username
OR
grep username /etc/passwd

Create new user

useradd username

Delete existing user

userdel username

Set user password or update password

passwd username

You can use shortcut to add new user. Using following command you can create user set password and create home directory for newly added user

adduser username
ls -l /home/username

Rename existing user

usermod --login new-name old-name

Group

List all groups

cat /etc/group

List specific group

cat /etc/group | grep groupname
cat /etc/group | grep ^groupname
grep groupname /etc/group
grep ^groupname /etc/group

Add new group

groupadd group-name

Delete existing group

groupdel group-name

List all members of specific group

getent group group-name

Add new member in specific groups

usermod -a -G group-name1,group-name2,... user-name

Check in which groups user exist

groups user-name

Remove user from specific group

gpasswd -d user-name group-name

Permission

---

---

---

Permission List

• 7 – 111 => read write execute
• 6 – 110 => read write –
• 5 – 101 => read – execute
• 4 – 100 => read – –
• 3 – 011 => – write execute
• 2 – 010 => – write –
• 1 – 001 => – – execute
• 0 – 000 => no permission

How to change permission

chmod -Rf 777 path-to-file-or-directory

How to give only specific permission

#only executable
chmod -Rf +x path-to-file-or-directory

#only writable
chmod -Rf +w path-to-file-or-directory

#only readable
chmod -Rf +r path-to-file-or-directory

How to give permission to only specific role

#only owner
chmod -Rf u+x path-to-file-or-directory

#only group
chmod -Rf g+x path-to-file-or-directory

#only others
chmod -Rf o+x path-to-file-or-directory

Ownership

---

How to change ownership of any file or directory

chown -Rf user:group path-to-file-or-directory

Change attribute read only

chattr +a file.txt

• chattr: This is a command in Linux used to change file attributes on a file system.
• +a: This option sets the “append-only” attribute on the specified file. When this attribute is set, the file can only be opened in append mode for writing. Existing data in the file cannot be modified or removed. This attribute is often used to prevent accidental deletion or modification of important files.
• to revert use -a option for chattr

to open file

vim filename

copy/cut paste

yy => p
dd => p
v => select lines to copy => y => goto line where need to paste => p
v => select lines to cut => d => goto line where need to paste => p

to undo and redo

Esc
u u u u 

Esc
Ctrl + R Ctrl + R

to open file in read mode

vim -R filename

to open file on specific line number

vim filename +10

insert mode

i
o

escape / command mode

Esc

to write file

:w
:wq
:x

to minimize vim editor

Ctrl + Z

to see minimized files in vim

jobs

to see background job

bg

to open specific jobs or bring it to foreground

fg 1
fg +
fg -

search any word in vim editor

/word
/word\c
/word\C
#then press N or Shift + N

search and replace word

:%s/word/replacewith/i
:%s/word/replacewith/g

go to specific line

:10
:1
:$

show/hide line numbers in vim editor

:set number
:set nonumber

set tabstop

:set tabstop=4

set font color

:colorscheme murphy

vimdiff difference between 2 files

vimdiff file1 file2
#difference put
dp
#difference obtain
do