readonly

Additional Notes

readonly is a shell built-in command that marks variables or functions as read-only in the current shell session. Once marked, the variable or function cannot be changed or unset for the duration of the shell session.

It is used in shell scripts to define constants and protect important variables from accidental modification. Common uses include defining configuration values, paths, and flags that should remain unchanged throughout script execution.

Syntax

readonly [options] [name[=value]...]
readonly -f [name...]

Parameters

• name: Variable or function name to mark as read-only.
• =value: Optional assignment of value at the time of marking.

Common Options

• -p: Print a list of all read-only variables and their values.
• -f: Operate on shell functions instead of variables.
• -a: Operate on indexed arrays.
• -A: Operate on associative arrays.

Examples

readonly MAX_RETRIES=5

Declare a constant variable.

readonly CONFIG_FILE="/etc/myapp/config.ini"

Make a configuration path constant.

readonly -p

List all read-only variables and their values.

readonly -f myfunc

Mark a shell function as read-only.

declare -r VERSION="1.0.0"

Declare a read-only variable using the declare built-in (alternative syntax).

readonly PATH_SEPARATOR=":" IFS=" "

Mark multiple variables as read-only in one command.

readonly -a DAYS=(Mon Tue Wed Thu Fri Sat Sun)

Make an indexed array read-only.

Practical Notes

• Once a variable is marked readonly, it cannot be modified with any assignment or unset with unset.
• readonly only affects the current shell session. Subshells and child processes inherit the value but not the read-only restriction.
• The -p option lists all read-only variables, useful for debugging.
• In bash scripts, use readonly or declare -r for constants at the top of the script.
• Attempting to modify a read-only variable results in an error: bash: VAR: readonly variable.
• The environment variables PATH, HOME, UID, EUID, BASH_VERSINFO, and others are automatically read-only.