readonly - Linux

Overview

The readonly command in Linux is used to mark shell variables and functions as unchangeable. Once a variable or function is set as readonly, its value or function definition cannot be changed or unset. This command is particularly useful in scripts where maintaining a consistent environment is crucial, ensuring that key variables are not modified accidentally.

Syntax

The basic syntax for the readonly command is as follows:

readonly [-aAfFp] [name[=value] ...]
  • name is the name of the variable or function to mark as readonly.
  • value is the value to assign to the variable when declaring it as readonly.

Options/Flags

  • -a: Mark each name as an array variable.
  • -A: Mark each name as an associative array variable.
  • -f: Refer to shell functions, rather than variables.
  • -F: Do not display function definitions; only display name when used with -f.
  • -p: Display the list of all readonly names. If no arguments follow this option, the names of all readonly variables and functions are displayed.

Examples

  1. Mark a Variable as Readonly:

    readonly var1="Initial Value"
echo $var1        # Outputs: Initial Value
var1="New Value"  # Error: var1: readonly variable

  2. Define a Readonly Function:

    readonly -f myFunc
myFunc() {
    echo "This function is protected."
}
myFunc              # Outputs: This function is protected.
unset -f myFunc     # Error: myFunc: readonly function

  3. Using -p to Display Readonly Variables:

    readonly var2="Final Value"
readonly -p         # Displays all readonly variables

Common Issues

  • Modification Attempt: Users might attempt to modify or unset a readonly variable or function, leading to errors.
    Solution: Ensure you mark only those variables or functions as readonly which are not intended to be modified.

  • Forgetting to Mark as Readonly: Variables intended to be protected may accidentally be left without the readonly declaration.
    Solution: Conduct thorough code reviews and tests to ensure important variables are properly secured.

Integration

readonly can be effectively combined with other shell scripting features and commands. For example, declaring essential configuration as readonly in a startup script can prevent subsequent scripts from altering them:

readonly CONFIG_FILE="/etc/myapp/config.cfg"
source $CONFIG_FILE
# Now CONFIG_FILE cannot be overwritten later in the script.
  • export: Mark variables for automatic export to the environment of subsequently executed commands.
  • declare: Used to declare variables and give them attributes (like -r for readonly similar to readonly).

For more detailed information, refer to the Bash documentation and specific man pages related to these commands.