📚 Table of Contents

• 🐚 What Is a Shell?
• 📂 Filesystem Commands

What Is a Shell?

Your terminal is a program that lets you type text-based commands and see their output.

But what runs those commands?

That’s the shell.

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🛠️ What Does a Shell Do?

The shell’s main job is to:

• Interpret the commands you type
• Execute those commands

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🔄 Shells as REPLs

Shells are often called REPLs, which stands for:

• Read
• Eval (evaluate)
• Print
• Loop

In simple terms, a shell:

1. Reads the command you type
2. Evaluates it (runs the program or command)
3. Prints the output
4. Loops back, showing a new prompt for the next command

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💡 This cycle makes shells interactive and powerful command interpreters.

Filesystem Commands

These commands help you interact with your Linux filesystem — navigate, read, modify, and search files or directories.

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🔸 echo $variable

Displays the value of a variable.

NAME="Adarsh"
echo $NAME
# Output: Adarsh

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🔸 whoami

Prints the current logged-in username.

whoami
# Output: your_username

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🔸 pwd

Prints the current working directory (stands for “Print Working Directory”).

pwd
# Output: /home/user/projects

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🔸 cat

Displays the entire content of a file.

cat notes.txt

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🔸 head

Shows the first 10 lines of a file (default).

head log.txt

You can change line count:

head -n 5 log.txt

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🔸 tail

Shows the last 10 lines of a file (default).

tail log.txt

Watch file updates live:

tail -f log.txt

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🔸 mv

Moves or renames a file or directory.

mv old.txt new.txt       # Rename
mv file.txt /path/dir/   # Move

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🔸 cp

Copies files or directories.

cp a.txt b.txt           # Copy file
cp -r folder1 folder2    # Copy directory recursively

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🔸 grep

Searches for patterns inside files.

grep "error" logfile.txt

Case-insensitive search:

grep -i "error" logfile.txt

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🔸 find

Searches for files/directories in a path.

find . -name "*.py"

Find files modified in the last 1 day:

find . -mtime -1

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🔸 ls

Lists files and folders in a directory.

ls
ls -l        # Long format
ls -a        # Include hidden files

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🔸 rm

Deletes files or directories.

rm file.txt            # Delete file
rm -r folder/          # Delete folder and contents

⚠️ Use with caution. No undo.

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🔸 touch

Creates an empty file or updates timestamp.

touch hello.txt

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🔸 mkdir

Creates a new directory.

mkdir new_folder

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