When trying to read random bytes from /dev/urandom, many beginners attempt commands like:

read /dev/urandom 3

This fails because read is a shell built-in designed for user input, not file operations. The proper approach requires using file handling commands.

The most reliable method is using dd with controlled parameters:

dd if=/dev/urandom bs=1 count=3 2>/dev/null

Key parameters:

• if=/dev/urandom: Input file
• bs=1: Block size of 1 byte
• count=3: Number of blocks to read
• 2>/dev/null: Suppresses statistics output

For different use cases:

1. Head Command (Simple Cases)

head -c 3 /dev/urandom

2. Hexdump Representation

dd if=/dev/urandom bs=1 count=3 2>/dev/null | hexdump -v -e '/1 "%02x"'

3. Storing in Variable

random_bytes=$(dd if=/dev/urandom bs=1 count=3 2>/dev/null | base64)

For reading larger chunks (e.g., 1MB):

dd if=/dev/urandom bs=4096 count=256 2>/dev/null

This reduces system calls by increasing block size while maintaining the same total byte count.

Remember:

• /dev/urandom is cryptographically secure for most use cases
• For extremely security-sensitive operations, consider /dev/random (though it may block)
• Always verify your system's entropy pool if generating large quantities of random data

# Generate 8-character alphanumeric string
random_string=$(dd if=/dev/urandom bs=8 count=1 2>/dev/null | base64 | tr -d '=' | tr '+/' '-_')
echo $random_string

Things to avoid:

• Using cat without limiting byte count (will run indefinitely)
• Forgetting to redirect stderr when using dd
• Assuming ASCII representation of raw bytes (random bytes may include non-printable characters)

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The Linux special file /dev/urandom provides an interface to the kernel's cryptographically secure pseudorandom number generator. Unlike /dev/random which may block when the entropy pool is exhausted, /dev/urandom will never block, making it preferable for most use cases.

The naive approach of trying to use read directly won't work because:

# This doesn't work as expected
read /dev/urandom 3

The read builtin is for reading shell input, not files. We need proper file operations.

Here are several reliable ways to read N random bytes:

Method 1: Using dd

dd if=/dev/urandom bs=1 count=3 2>/dev/null

This reads exactly 3 bytes. The 2>/dev/null suppresses status output.

Method 2: Using head

head -c 3 /dev/urandom

Simpler syntax that reads the specified number of bytes.

Method 3: Storing in a Variable

random_bytes=$(head -c 3 /dev/urandom | od -An -t x1)
echo "Random bytes: $random_bytes"

This shows the bytes in hexadecimal format.

Here's how to use this in real scripts:

Generating Random Strings

# Generate 16 random bytes as hex string
random_string=$(head -c 16 /dev/urandom | od -An -t x1 | tr -d ' ')
echo "Random string: $random_string"

Creating Temporary Passwords

# Generate 8-character alphanumeric password
password=$(head -c 8 /dev/urandom | base64 | tr -d '=' | tr '+/' '-_')
echo "Temporary password: $password"

For reading large amounts of random data, consider these optimizations:

# Read 1MB efficiently
dd if=/dev/urandom of=random_data.bin bs=1M count=1

Adjust block size (bs) based on your needs for optimal performance.