declare - macOS

Overview

The declare command, primarily found in Unix-like operating systems with Bash shells, is used to declare shell variables and assign attributes to them. It allows the setting of values to variables and the manipulation of their properties (e.g., defining them as integer, read-only, or exported). This command is effective for managing variable behaviors in scripts, configuring system behaviors, and shell scripting optimization.

Syntax

The basic syntax for the declare command is:

declare [options] [variable_name[=value] ...]
  • variable_name is the name of the variable you want to manipulate.
  • value is the optional value to assign to the variable.

Options/Flags

The declare command supports multiple options that modify its behavior:

  • -a : Declare the variable as an array.
  • -f : Display function definitions. If used alone, it displays all functions.
  • -i : Declare the variable as an integer. Arithmetic operations are allowed on this variable.
  • -r : Declare the variable as read-only. This variable cannot be changed or unset.
  • -x : Mark the variable for export, making it available as an environment variable to child processes.
  • -p : Display the attributes and value of each variable. When no variable name is provided, it displays all declared variables.
  • -g : Declare a global variable. By default, declare inside a function would make a variable local to that function.

Each option tailors the behavior of variables within shell scripts and command-line operations, potentially altering the scope, persistence, and type of data stored.

Examples

  1. Declare an Integer Variable 
    declare -i number
number=5+5
echo $number  # Outputs '10'
  2. Declare a Read-Only Variable 
    declare -r myVar="Final"
echo $myVar  # Outputs 'Final'
# Attempting to modify myVar here would result in an error.
  3. Declare and Display All Functions 
    declare -f
  4. Declaring an Array with Initialized Values 
    declare -a myArray=(one two three)
echo ${myArray[1]}  # Outputs 'two'

Common Issues

  • Permission Errors: When using -r to create read-only variables, attempting to change or unset these variables later can result in permission errors.
  • Scope Confusion: Misunderstanding the scope of variables when using -g inside functions can lead to unexpected results. Variables expected to be local remain global.

Integration

Combine declare with other commands to manage environment variables effectively within scripts:

#!/bin/bash

declare -x PATH="/my/custom/path:$PATH"
some_command_that_uses_modified_path

In this script, declare is used to temporarily extend the PATH environment variable(scope limited to the script), ensuring that any commands executed after its declaration respect the newly modified path.

  • export – Marks each variable to be exported to child processes.
  • unset – Deletes named variables.
  • local – Create a function-local variable within shell functions.

For more detailed examples and technical usage, visit the official Bash documentation or man page (man bash).