How to generate random number in Bash?

Generating Random Numbers in Bash: A Comprehensive Guide

Learn various methods to generate random numbers in Bash, from basic integers to cryptographically secure values, and understand their use cases.

Generating random numbers is a common requirement in shell scripting for tasks such as creating temporary filenames, simulating events, generating passwords, or implementing simple games. Bash provides several built-in mechanisms and external commands to achieve this. This article will explore the most common and effective ways to generate random numbers, discussing their strengths and appropriate use cases.

Using $RANDOM for Pseudo-Random Integers

The simplest way to generate a pseudo-random integer in Bash is by using the built-in variable $RANDOM. This variable returns a different pseudo-random integer each time it is referenced. The range of numbers generated by $RANDOM is typically between 0 and 32767 (inclusive), as it's a 16-bit unsigned integer. It's important to note that $RANDOM is not cryptographically secure and should not be used for security-sensitive applications.

echo $RANDOM

# Generate a random number between 1 and 100
RANDOM_1_100=$(( $RANDOM % 100 + 1 ))
echo "Random number (1-100): $RANDOM_1_100"

# Generate a random number between 50 and 150
MIN=50
MAX=150
RANGE=$(( MAX - MIN + 1 ))
RANDOM_MIN_MAX=$(( $RANDOM % RANGE + MIN ))
echo "Random number (50-150): $RANDOM_MIN_MAX"

Examples of using $RANDOM and limiting its range

flowchart TD
    A[Start] --> B{Access $RANDOM}
    B --> C[Get 0-32767]
    C --> D{Need specific range?}
    D -- Yes --> E[Apply Modulo and Offset]
    E --> F[Result: In-range Integer]
    D -- No --> F
    F --> G[End]

Flowchart for generating random numbers using $RANDOM

Generating Cryptographically Secure Random Numbers with /dev/urandom

For applications requiring higher security, such as generating passwords, encryption keys, or session tokens, /dev/urandom is the preferred source of randomness. This special file provides an interface to the kernel's cryptographically secure pseudo-random number generator (CSPRNG). It will never block, making it suitable for scripts that need to generate random data quickly without waiting for sufficient entropy.

# Generate 16 random bytes (128 bits) in hexadecimal format
head /dev/urandom | tr -dc A-Za-z0-9_ | head -c 16 ; echo

# Generate a random integer between 1 and 100 using /dev/urandom
# This method is more complex but cryptographically stronger
RAND_BYTE=$(head /dev/urandom | od -An -N1 -i)
RAND_1_100=$(( RAND_BYTE % 100 + 1 ))
echo "Secure random number (1-100): $RAND_1_100"

# Generate a random password of 12 characters
PASSWORD=$(head /dev/urandom | tr -dc A-Za-z0-9_@#$%^&*-+= | head -c 12 ; echo)
echo "Generated password: $PASSWORD"

Examples using /dev/urandom for secure random data

Using shuf for Random Permutations and Selections

The shuf command (from GNU coreutils) is excellent for shuffling lines from a file or generating random permutations of a sequence of numbers. It's particularly useful when you need to select a random item from a list or generate a unique sequence of numbers within a range.

# Select a random line from a file
cat /etc/passwd | shuf -n 1

# Generate 5 unique random numbers between 1 and 100
seq 1 100 | shuf -n 5

# Pick a random fruit from a list
FRUITS=("Apple" "Banana" "Cherry" "Date" "Elderberry")
RANDOM_FRUIT=$(printf "%s\n" "${FRUITS[@]}" | shuf -n 1)
echo "Random fruit: $RANDOM_FRUIT"

Examples of using shuf for random selections and permutations

Choosing the right method for generating random numbers in Bash depends entirely on your specific needs. For simple, non-critical tasks, $RANDOM is quick and easy. For security-sensitive operations, /dev/urandom is the go-to. And for shuffling or selecting unique items, shuf offers a convenient solution. Understanding these options allows you to write robust and appropriate Bash scripts.