od command

 

Command: od (Octal Dump)

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🔍 Purpose

The od command is used to display file contents in various numeric formats, such as:

• Octal (default)

• Hexadecimal

• Decimal

• Character

• ASCII

It allows you to view raw binary data — the exact bytes stored in memory or a file — which is extremely useful in:

• Debugging programs

• Examining binary or data files

• Understanding file formats

• Learning how text and numbers are represented internally

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Basic Syntax

od [options] filename

Example:

od myfile.txt

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🧠 Default Behavior

By default, od displays the file’s contents in octal format.

Example:

Create a simple file:

echo "OS" > test.txt

Run:

od test.txt

Output:

0000000 117 123 012
0000003

Explanation:

• The first column (0000000, 0000003) → byte offset in the file

• The numbers (117 123 012) → ASCII values of characters in octal

  • 117 = O

  • 123 = S

  • 012 = newline (\n)

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🧩 Viewing in Different Formats

You can change how the bytes are displayed using options.

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🔹 1. Hexadecimal Output

od -x test.txt

Output:

0000000 534f 000a
0000003

🧩 Explanation:

• 534f = Hex representation of “OS” (O=4F, S=53)

• 000a = newline (\n)

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🔹 2. Character Output

od -c test.txt

Output:

0000000   O   S  \n
0000003

🧩 Explanation:
Displays the character equivalents of bytes — much easier to read for text files.

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🔹 3. ASCII + Octal Together

od -a test.txt

Output:

0000000   O   S  nl
0000003

🧩 Explanation:

• Shows ASCII names instead of characters (e.g., nl for newline).

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🔹 4. Decimal Output

od -i test.txt

Output:

0000000 12335 00010
0000003

🧩 Displays integer (decimal) representation of the bytes.

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🔹 5. Binary Output

od -b test.txt

Output:

0000000 117 123 012
0000003

🧩 Same as default — octal representation of bytes.

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🔹 6. Hex Dump (Most Common Use)

od -t x1 test.txt

Output:

0000000 4f 53 0a
0000003

🧩 Explanation:

• 4f → O

• 53 → S

• 0a → newline
  Shows each byte in hexadecimal (x1 = one-byte units).

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🔹 7. Combine Character and Hex

od -tx1 -c test.txt

Output:

0000000  4f  53  0a
          O   S  \n
0000003

🧩 Displays both — useful for teaching how ASCII characters map to byte values.

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Viewing Specific Number of Bytes

Use the -N option:

od -c -N 5 myfile.txt

🧩 Displays only the first 5 bytes.

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Skipping Bytes

Use the -j option to skip bytes from the beginning:

od -c -j 3 myfile.txt

🧩 Skips the first 3 bytes, then prints the rest.

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Starting Offset

Use the -A option to control the address format in the first column:

od -A x -t x1 test.txt

Output:

000000  4f 53 0a
000003

🧩 -A x displays offsets in hexadecimal.

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Display Memory or Binary Files

You can also examine binary files like compiled executables:

od -t x1 a.out | head

🧩 Displays the raw binary structure of the compiled program — great for understanding what’s actually stored.

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Simple Lab Exercise

Aim:

To study the use of the od command for displaying file contents in octal, hexadecimal, and character formats.

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Procedure:

1. Create a text file:

   echo "Operating Systems" > sample.txt
   

2. Display in different formats:

   od sample.txt
   od -c sample.txt
   od -x sample.txt
   od -t x1 sample.txt
   od -a sample.txt
   

3. Observe and record differences in the output.

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Sample Observation Table

Command	Description	Output (example)
od sample.txt	Octal representation	117 160 145 ...
od -c sample.txt	Character view	O p e r ...
od -x sample.txt	Hexadecimal view	704f 6572 ...
od -t x1 sample.txt	One byte per hex value	4f 70 65 ...
od -a sample.txt	ASCII names	O p e r ...

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🧠 Learning Outcomes

Students will:

• Understand how characters and data are represented in memory.

• Learn to interpret ASCII, octal, and hexadecimal representations.

• Get a hands-on feel for binary-level file inspection.

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Quick Reference Summary

Option	Meaning
-b	Display octal bytes
-c	Display ASCII characters
-a	Display named ASCII characters
-x	Display hexadecimal words
-t x1	Display hexadecimal bytes
-i	Display decimal integers
-N n	Display only n bytes
-j n	Skip n bytes from start
-A x	Display addresses in hex