Bash $RANDOM Variable

Understanding and using the $RANDOM internal variable in Bash, including its open-source implementation

February 19, 2026

Bash $RANDOM Variable

In bash, $RANDOM is a built-in variable that returns an integer between 0 and 32767 each time it's called.
When you need a simple random number, it can be used quickly without any external utilities.

Basic Usage

The most basic usage is simply referencing the variable.


Generate random number
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To get a number within a specific range (e.g., 1-100), use the modulo operator (%).


Random number in range 1-100
sh

Seeding RANDOM

Assigning a value to the RANDOM variable sets an integer seed.
Providing the same seed will result in the same random sequence every time.


Setting a seed
sh

Deep Dive: Bash Open Source Code

Let's take a look at how $RANDOM is implemented internally in Bash.
Looking at the Bash source code (variables.c), this variable is treated not as a simple variable, but as a dynamic variable.

1. Dynamic Variable Registration

During the Bash initialization process, the RANDOM variable is registered with special handlers via the initialize_dynamic_variables() function.


variables.c - initialize_dynamic_variables
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2. Generating Random Numbers

When a user calls $RANDOM, it executes in the order get_random() -> get_random_number(), and the actual random number generation calls the brand() function (an internal LCG algorithm).


variables.c - get_random_number
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3. Seeding via Assignment

When a user assigns a value, such as RANDOM=42, the assign_random function is called to initialize the random number generator's seed.


variables.c - assign_random
c

Key Implementation Details

LCG (Linear Congruential Generator): Bash uses the lightweight brand() function for performance.
Its deterministic nature means the same seed will yield the same results.
Subshell Handling: To prevent subshells (like $(echo $RANDOM)) from producing the same random sequence as their parent, the source code checks getpid() and automatically performs seedrand() if the session differs.
att_integer: $RANDOM is assigned an integer attribute at the time of creation, minimizing type conversion overhead during arithmetic operations.
The range of $RANDOM is from 0 to 32767 (2^15 - 1).
It is not a cryptographically secure random number generator (CSPRNG), so it's not suitable for security-related purposes.
If higher precision or security is required, it's recommended to use /dev/urandom.