How do I redefine a bash function in terms of old definition?

Question

Is there any way I can redefine a bash function in terms of its old definition? For example I would like to add the following block of code to the preamble of the function command_not_found_handle (),

# Check if $1 is instead a bash variable and print value if it is
local VAL=$(eval echo \"\$$1\")
if [ -n "$VAL" ] && [ $# -eq 1 ]; then
    echo "$1=$VAL"
    return $?
fi

It is currently defined in /etc/profile.d/PackageKit.sh and sourced by bash start-up scripts.

That way I can query the value of an environment variable at the command prompt by simply typing the variable name (and provided that no such command by that name exists). e.g.

user@hostname ~:$ LANG
LANG=en_AU.utf8

I know I could just copy and paste the current definition and add my own changes in ~/.bashrc, but I am looking for a more elegant way that involves code reuse.

Better ways of achieving my goal or code improvements/extensions are also appreciated.

Answer 1

You can print out the current definition of the function, and then include it in a function definition inside an eval clause.

current_definition=$(declare -f command_not_found_handle)
current_definition=${current_definition#*\{}
current_definition=${current_definition%\}}
prefix_to_add=$(cat <<'EOF'
  # insert code here (no special quoting required)
EOF
)
suffix_to_add=$(cat <<'EOF'
  # insert code here (no special quoting required)
EOF
)
eval "command_not_found_handle () {
  $prefix_to_add
  $current_definition
  $suffix_to_add
}"

Another approach, which I find clearer, is to define the original function under a new name, and call that from your definition. This only works if you don't need to act on the local variables of the original definition.

eval "original_$(declare -f command_not_found_handle)"
command_not_found_handle () {
  …
  original_command_not_found_handle
  …
}

Answer 2

Thanks @mat, @dennis-williamson . After reading your comments, this is what I got

eval 'command_not_found_handle () {

    local VAL=$(echo "${!1}")
    if [ -n "$VAL" ] && [ $# -eq 1 ]; then
            echo "$1=$VAL"
            return $?
    fi

    '$(declare -f command_not_found_handle |
    tail -n +3 | head -n -1)'
}'

Though I think I prefer @Gilles solution.