Override a shell function, keep a reference to the original one

Question

Is it possible to override a shell function and keep a reference to the original one?

f()  { echo original; }
f()  { echo wrapper; ...; }
f

The output of this should be:

wrapper
original

Is this possible in a semi-portable way?

Rationale: I'm trying to test my program by replacing parts of it with shell functions which record their calls into a log file. This works fine as long as I only wrap commands and builtins, and as long as I don't mind indiscriminate logging. Now I'd like to make the test suite more maintainable by only recording the interesting piece in each test.

So let's say my program consists of

f
g
h

where f, g, h are all shell functions, and I'd like to trace the execution of just g.

Answer 1

The answer by Jens is correct. Just adding below code for completeness.

You can simply use it as below:

eval "`declare -f f | sed '1s/.*/_&/'`" #backup old f to _f

f(){
    echo wrapper
    _f # pass "$@" to it if required.
}

I had used same logic here: https://stackoverflow.com/a/15758880/793796

Answer 2

Many shells (zsh, ksh, bash at least) support typeset -f f to dump the contents of f(). Use this to save the current definition to a file; then, define f() as you want. Restore f() by sourcing the file created with typeset.

If you slightly modify the dumped function (renaming f() to _f() on the first line; a bit trickier when f() is recursive or calls other functions you frobbed in the same way), you can likely get this to produce the output you desired.