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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.

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Data Types in Bash

Bash offers two fundamental data types:
  1. String
  2. 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

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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.

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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.

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Other declare Flags

Here’s a quick reference for some common attributes:
FlagDescriptionExample
-iForce integerdeclare -i counter=100
-rRead-only variabledeclare -r PI=3.1415
-uConvert value to uppercasedeclare -u animal="dog" → DOG
-lConvert value to lowercasedeclare -l word="HELLO" → hello

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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.

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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

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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 "${colors[rose]}" # red
Array TypeDeclarationAccess
Indexed Arraydeclare -a arr${arr[index]}
Associative Arraydeclare -A map${map[key]}

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