readonly

Mark variables as read-only.

Source: src/execution/builtins.f90

Synopsis

readonly [-aAfp] [name[=value] ...]

Description

The readonly builtin marks variables or functions as read-only. Once marked, their values cannot be changed or unset.

Options

Option	Description
`-a`	Apply to indexed arrays
`-A`	Apply to associative arrays
`-f`	Apply to functions
`-p`	Print all readonly variables

Usage

Make Variable Readonly

readonly PI=3.14159
readonly VERSION="1.0.0"

PI=3   # Error: PI is readonly

Mark Existing Variable

config="/etc/app.conf"
readonly config

List Readonly Variables

readonly -p
# declare -r PI="3.14159"
# declare -r VERSION="1.0.0"

Readonly Arrays

readonly -a COLORS=(red green blue)
COLORS[0]=yellow   # Error

readonly -A CONFIG=([host]=localhost [port]=8080)
CONFIG[host]=other   # Error

Readonly Functions

critical_func() {
    echo "Don't modify me"
}
readonly -f critical_func

Examples

Constants

readonly TRUE=0
readonly FALSE=1
readonly NULL=""

# Use in conditionals
[[ $result -eq $TRUE ]] && echo "Success"

Configuration

# Set once at startup
readonly CONFIG_DIR="/etc/myapp"
readonly LOG_DIR="/var/log/myapp"
readonly DATA_DIR="/var/lib/myapp"

Script Protection

#!/usr/bin/env fortsh
readonly SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
readonly SCRIPT_NAME="$(basename "$0")"

# These can't be accidentally modified

Environment Constants

# Protect critical environment
readonly PATH
readonly HOME
readonly USER

Behavior

Cannot Unset

readonly var="value"
unset var
# Error: var is readonly

Cannot Modify

readonly arr=(1 2 3)
arr+=(4)           # Error
arr[0]=99          # Error
unset 'arr[1]'     # Error

Inheritable

Readonly status is not inherited by child processes.

readonly SECRET="value"
bash -c 'echo $SECRET; SECRET=new'
# Prints "value", then can modify in subshell

Exit Status

Status	Condition
0	Success
1	Invalid option
1	Variable not found (with specific name)

Common Patterns

Module Constants

# mymodule.sh
readonly MYMODULE_VERSION="2.0"
readonly MYMODULE_AUTHOR="John"

mymodule_init() { ... }
mymodule_run() { ... }

Immutable Configuration

load_config() {
    source config.sh
    # Lock down after loading
    readonly DB_HOST DB_PORT DB_USER
}

Prevent Accidents

# Protect against common mistakes
readonly IFS
readonly TERM
readonly SHELL

Notes

Readonly is permanent for shell session
Cannot "un-readonly" a variable
Equivalent to declare -r
Readonly functions prevent redefinition

Comparison

Command	Effect
`readonly var`	Can't modify or unset
`declare -r var`	Same as readonly
`local -r var`	Readonly local in function