DOS & CLI: Commands, File Systems, Batch Scripts & Troubleshooting

TAKEAWAYS:

• DOS commands are categorized into file management, disk operations, networking, and system utilities, each with specific syntax and flags.
• The command-line interface (CLI) offers precise control over files, directories, and system processes, unlike GUI limitations.
• Batch scripting automates repetitive tasks using .bat files, combining commands with conditional logic (IF, FOR).
• Error handling in DOS relies on exit codes, redirection (>, >>, |), and logical operators (&&, ||).
• Modern CLI tools (e.g., PowerShell, Bash) extend DOS concepts with scripting, pipelines, and advanced text processing.
• Real-world applications include system administration (e.g., NTC network diagnostics), automation (e.g., Daraz inventory updates), and legacy system maintenance.

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1. Introduction to DOS and CLI

What is DOS?

DOS (Disk Operating System) is a non-graphical command-line interface for managing computer hardware and software. Developed by Microsoft (MS-DOS) and IBM (PC-DOS), it remains foundational for understanding:

• File systems (FAT16, FAT32, NTFS).
• System commands (e.g., DIR, COPY).
• Batch processing (automating tasks via scripts).

Why learn DOS?

• Legacy systems: Many industrial/embedded systems still use DOS.
• Networking tools: Commands like PING, TRACERT (from DOS roots) are used in NTC/Ncell diagnostics.
• Scripting: Batch files automate backups, log parsing, and system maintenance.

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CLI vs. GUI: Key Differences

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2. DOS File System and Directory Structure

How Files and Directories Work

• Root directory: The top-level folder (e.g., C:\).
• Subdirectories: Folders within folders (e.g., C:\Users\Documents).
• File paths: Specify location using backslashes (\). Example: C:\Data\Reports\Q1_Sales.txt.

Worked Example: Creating a Directory Structure Task: Replicate this structure using DOS commands:

E:\ROOT
├── dir_1/
│   ├── file_1.txt
│   └── file_2.txt
├── dir_2/
│   └── dir_3/
│       └── file_2.txt
└── dir_2.txt

Solution:

E:
MD ROOT
CD ROOT
MD dir_1
MD dir_2
CD dir_1
COPY CON file_1.txt
[Type text, press F6 then Enter to save]
COPY CON file_2.txt
[Type text, press F6 then Enter]
CD ..\dir_2
MD dir_3
CD dir_3
COPY CON file_2.txt
[Save file]
CD ..\..
COPY CON dir_2.txt
[Save file]

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File Naming Rules

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3. Essential DOS Commands

Categorized Command List

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Worked Example: Bank Transaction Log Analysis

Scenario: A bank (e.g., Nabil Bank) uses DOS batch scripts to log daily transactions. Write a script to:

1. Create a log file transactions.log.
2. Append today’s date and a sample transaction.
3. Display the log.

Solution:

@ECHO OFF
ECHO Today's Transactions - %DATE% > transactions.log
ECHO Account: 123456789, Amount: NPR 50,000, Type: Deposit >> transactions.log
ECHO Account: 987654321, Amount: NPR 20,000, Type: Withdrawal >> transactions.log
TYPE transactions.log
PAUSE

Output:

Today's Transactions - 05/10/2024
Account: 123456789, Amount: NPR 50,000, Type: Deposit
Account: 987654321, Amount: NPR 20,000, Type: Withdrawal

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4. Advanced DOS Features

Redirection and Piping

• Redirection (>): Overwrites a file with command output.

  DIR > filelist.txt
  

• Append (>>): Adds output to a file without overwriting.

  ECHO New line >> filelist.txt
  

• Piping (|): Sends output of one command to another.

  DIR | FIND "txt"
  

  Example: List only .txt files in the current directory.

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Batch Scripting Basics

Batch files (.bat) automate tasks using:

• Variables: %VAR% (e.g., %DATE%).
• Conditionals: IF statements.
• Loops: FOR loops.

Example: Backup Script

@ECHO OFF
SET BACKUP_DIR=C:\Backup\%DATE%
MKDIR %BACKUP_DIR%
COPY C:\Data\*.* %BACKUP_DIR%\ /Y
ECHO Backup completed at %TIME% >> %BACKUP_DIR%\backup_log.txt

Use Case: Daraz might use similar scripts to auto-backup inventory data nightly.

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Error Handling

• Exit Codes: 0 = Success, 1 = Error.

  IF ERRORLEVEL 1 ECHO Command failed!
  

• Logical Operators: && (AND), || (OR).

  DIR && ECHO Success || ECHO Failed
  

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5. DOS in Real-World Applications

In the Real World

1. NTC Network Diagnostics:

   • Tool: PING, TRACERT, IPCONFIG.
   • How: Technicians use these to troubleshoot internet outages in Kathmandu traffic routes (e.g., Thapathali to Balaju).
   • Example:

     PING 8.8.8.8
     TRACERT ntc.net.np
     

2. Khalti Payment Gateway:

   • Tool: Batch scripts for transaction logs.
   • How: Khalti’s backend systems use DOS-like scripts to generate daily reports of successful/failed transactions.
   • Example Script:

     ECHO Transaction Report - %DATE% > khalti_log.txt
     FOR /F "tokens=1-3" %%A IN (transactions.csv) DO (
       ECHO %%A, %%B, %%C >> khalti_log.txt
     )
     

3. Pathao Driver Dispatch:

   • Tool: SORT and FIND commands.
   • How: Pathao’s legacy systems (for older drivers) use DOS commands to sort ride requests by proximity.
   • Example:

     TYPE ride_requests.txt | FIND "status=pending" | SORT > pending_requests.txt
     

4. NEPSE Stock Data Processing:

   • Tool: TYPE + FINDSTR.
   • How: Analysts extract specific stock prices from daily logs.
   • Example:

     TYPE stock_data.txt | FINDSTR "NEPSE:5000" > top_gainers.txt
     

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Worked Example: Daraz Order Queue Simulation

Scenario: Daraz uses a first-in-first-out (FIFO) queue for order processing. Simulate this with DOS commands.

Steps:

1. Create an order log:

   ECHO Order1,User123,Pending > orders.txt
   ECHO Order2,User456,Pending >> orders.txt
   ECHO Order3,User789,Pending >> orders.txt
   

2. Process orders in order (FIFO):

   @ECHO OFF
   SET /P order=<orders.txt
   ECHO Processing: %order%
   SHIFT
   FOR %%G IN (%*) DO (
     ECHO Next order: %%G
     TIMEOUT /T 2 > NUL
   )
   

Output:

Processing: Order1,User123,Pending
Next order: Order2,User456,Pending
Next order: Order3,User789,Pending

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6. DOS vs. Modern CLI Tools

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7. Common Pitfalls and Troubleshooting

Common Errors and Fixes

Example Trace: Problem: COPY file.txt C:\Backup\ fails with "Access denied." Trace:

1. Check permissions: ATTRIB file.txt → Shows "R" (read-only).
2. Fix: ATTRIB -R file.txt.
3. Retry: COPY file.txt C:\Backup\ → Success.

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Hidden DOS Commands

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Exam Tip

How to Score Full Marks

1. Command Syntax:

   • Always include flags (e.g., DIR /P not just DIR).
   • Use full paths in examples (e.g., C:\Users\file.txt).

2. Batch Scripts:

   • Structure answers with:
     • @ECHO OFF (to hide commands).
     • Clear comments (REM This copies files).
     • Error handling (IF ERRORLEVEL).

3. Real-World Applications:

   • Link commands to Nepali contexts (e.g., NTC, Khalti, Daraz).
   • Example: For PING, mention "diagnosing slow internet in Pokhara."

4. Diagrams:

   • Draw directory trees for file structure questions.
   • Use sequence diagrams for protocol-like operations (e.g., batch script flow).

5. Common Exam Questions:

   • Create a directory structure: Use MD + CD commands.
   • Batch script for calculations: Use SET /A for math (e.g., SET /A sum=5+3).
   • Networking commands: Always include PING and IPCONFIG.

Example Answer Structure: Question: Write DOS commands to create the following structure:

C:\Data
├── Reports\
│   ├── Q1.txt
│   └── Q2.txt
└── Backup\
    └── old_files\

Answer:

MD C:\Data
CD C:\Data
MD Reports
MD Backup\old_files
CD Reports
COPY CON Q1.txt
[Press F6, Enter]
COPY CON Q2.txt
[Press F6, Enter]

Visual:

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Past Exam Patterns

• Short Answer: Define COPY command + example (2 marks).
• Long Answer: Write a batch script to:
  • Backup files (3 marks).
  • Calculate statistics (e.g., sum of numbers in a file) (5 marks).
• Scenario-Based: "A company uses DOS to log sales. Write commands to filter sales > NPR 10,000" (4 marks).

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Pro Tips

• Memorize flags: /P, /S, /A (for DIR, COPY, ATTRIB).
• Practice typing: DOS is case-insensitive but flags are exact (e.g., /PAGE ≠ /page).
• Use HELP: In DOS, type HELP or command /? to see examples.

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