The readonly built-in makes variables and functions read-only.
Syntax
-
readonly [-pxX] [name[=value]…] -
readonly -f[p] [name…]
Description
The readonly built-in is equivalent to the typeset
built-in with the -gr option.
Notes
The readonly built-in is a special built-in.
The POSIX standard defines the -p option only;
other options cannot be used in the POSIXly-correct mode.
The POSIX does not allow using the option together with operands.