I recently discovered that it is possible to set attributes on readonly variables
$ declare -p PPID declare -ir PPID="759437" $ declare -l PPID $ declare -p PPID declare -irl PPID="759437" I noticed SHELLOPTS and BASHOPTS among the default readonly variables set by bash. They are modified by bash whenever you set a set option or shopt. I tried setting the -i attribute on them, and got this result: $ declare -i SHELLOPTS $ set -f bash: set: braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor:noglob: syntax error in expression (error token is ":emacs:hashall:histexpand:history:interactive-comments:monitor:noglob") $ declare -p SHELLOPTS declare -i SHELLOPTS="braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor" $ set -C bash: set: braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor:noclobber:noglob: syntax error in expression (error token is ":emacs:hashall:histexpand:history:interactive-comments:monitor:noclobber:noglob") $ declare -p SHELLOPTS declare -i SHELLOPTS="braceexpand:emacs:hashall:histexpand:history:interactive-comments:monitor" $ SHELLOPTS=123; declare -p SHELLOPTS declare -i SHELLOPTS="123" $ set +B emacs:hashall:histexpand:history:interactive-comments:monitor:noclobber:noglob: syntax error in expression (error token is ":hashall:histexpand:history:interactive-comments:monitor:noclobber:noglob") $ declare -p SHELLOPTS declare -i SHELLOPTS="123" $ declare +i SHELLOPTS=hello; declare -p SHELLOPTS declare -- SHELLOPTS="hello" $ set -o posix $ declare -p SHELLOPTS declare -r SHELLOPTS="emacs:hashall:histexpand:history:interactive-comments:monitor:noclobber:noglob:posix" The arithmetic syntax error caused by trying to assign the new list of options to SHELLOPTS prevents bash from changing its value, and re-adding the readonly attribute to SHELLOPTS. Then SHELLOPTS can be modified, and unset freely. :-) The same is possible with BASHOPTS. o/ emanuele6
