Declare

The declare (also known as typeset) built-in in Bash lets you assign attributes and data types to your variables. While Bash is dynamically typed, using declare can help enforce types (like integers), create read-only variables, and define arrays. In this article, we’ll cover:

Data types in Bash
Dynamically typed variables
Enforcing integer types with declare -i
Other useful declare flags
Working with arrays using declare -a

The image features a list of programming-related topics with checkmarks, including explaining data types, dynamically typed syntax, the declare command in Bash, and using declare for arrays. It also includes simple line drawings of a person and a lightbulb.

Data Types in Bash

Bash offers two fundamental data types:

String
Integer

The image shows a split screen with "String" labeled as 1 on the left and "Integer" labeled as 2 on the right, under the title "arrays-declare."

You can assign values using the typical syntax:

# String assignment
name="John Doe"
echo "$name"   # John Doe

# Integer assignment
count=10
echo "$count"  # 10

Dynamically Typed Variables

Bash determines the type at runtime, based on context:

#!/usr/bin/env bash
var="10"
echo $(( var + 1 ))   # 11

You can even switch types on the fly:

#!/usr/bin/env bash
i="42"
echo $(( i + 8 ))   # 50

i="hello"
echo "$i"           # hello

Bash’s dynamic typing means it tries to interpret values at runtime—no compile-time errors for type mismatches.

Enforcing Integer Types with `declare -i`

To enforce integer semantics (mimicking static typing), use:

#!/usr/bin/env bash
declare -i num=5
echo $(( num + 2 ))   # 7

num="not a number"
echo "$num"           # 0

Non-numeric assignments reset the variable to 0.

Other `declare` Flags

Here’s a quick reference for some common attributes:

Flag	Description	Example
-i	Force integer	`declare -i counter=100`
-r	Read-only variable	`declare -r PI=3.1415`
-u	Convert value to uppercase	`declare -u animal="dog"` → `DOG`
-l	Convert value to lowercase	`declare -l word="HELLO"` → `hello`

Read-Only Variables (`-r`)

#!/usr/bin/env bash
declare -r VERSION="1.0.0"
echo "$VERSION"
VERSION="2.0.0"  # Error: readonly variable

Attempting to modify a readonly variable will terminate your script with an error.

Case Conversion (`-u`, `-l`)

#!/usr/bin/env bash
declare -u upper
declare -l lower

upper="Bash Rocks"
lower="Bash Rocks"

echo "$upper"  # BASH ROCKS
echo "$lower"  # bash rocks

Arrays with `declare -a`

Bash supports both indexed and associative arrays:

#!/usr/bin/env bash

# Indexed array
declare -a fruits=("apple" "banana" "cherry")
echo "${fruits[1]}"    # banana

# Associative array
declare -A colors
colors[sky]=blue
colors[rose]=red
echo "${colors[rose]}" # red

Array Type	Declaration	Access
Indexed Array	`declare -a arr`	`${arr[index]}`
Associative Array	`declare -A map`	`${map[key]}`

Introduction

Conventions

Refresher

Streams

Expansions - Part One

Globs

Expansions - Part Two

Special Shell Variables

Arrays

Good Practices applied

awk

sed

Data Types in Bash

Dynamically Typed Variables

Enforcing Integer Types with `declare -i`

Other `declare` Flags

Read-Only Variables (`-r`)

Case Conversion (`-u`, `-l`)

Arrays with `declare -a`

Links and References

Watch Video

​Data Types in Bash

​Dynamically Typed Variables

​Enforcing Integer Types with declare -i

​Other declare Flags

​Read-Only Variables (-r)

​Case Conversion (-u, -l)

​Arrays with declare -a

​Links and References

Watch Video

Data Types in Bash

Dynamically Typed Variables

Enforcing Integer Types with `declare -i`

Other `declare` Flags

Read-Only Variables (`-r`)

Case Conversion (`-u`, `-l`)

Arrays with `declare -a`

Links and References