readonly - set parameter read-only attribute
readonly [-p] [parameter[=value]] ...
This command is a Korn-shell built-in command.
The readonly(1) command sets the read-only attribute of the named parameters. If values are given, parameters are set to them before setting the attribute. Once a parameter is made read-only, it cannot be unset, and its value cannot be changed.
If no parameters are specified, the names of all parameters with the read-only attribute are printed one per line, unless the -p option is used, in which case readonly commands defining all read-only parameters, including their values, are printed.