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

Data Types in Bash

Bash offers two fundamental data types:

1. String
2. Integer

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

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:

Read-Only Variables (-r)

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

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

Links and References

• Bash Reference Manual
• KornShell typeset Documentation
• GNU Bash Features