Start interactive function definition from shell-script

Question

Using zsh or bash, I want to run a script that may prompt the user for multiple commands and store them as a function, however I'm finding that eval "function $FNCNAME() {" or echo "function $FNCNAME() {" | zsh -is is not valid unless typed directly since the script will not wait for more input before either continuing to the next line or exiting.

How do I block the script process while accepting user input, and continue only when the user has concluded the function definition with a final (unquoted) }?

Requirements are:

• The shells own context aware autocomplete functionality is active.
• The script should not immediately execute the deferred commands but should be able to call or print the definition of the new function

This question is limited to a general piping and interaction problem. I am not asking for:

• How to provide prewritten commands to a script in any way.
• How to provide "the best user experience" or how to improve it.
• Why I should not solve this problem and do something else instead.

Answer 1

In zsh, it could be something like:

make-function() {
  local psvar=$1
  local line= body=
  print -ru2 Please enter the definition of the $psvar function, one line at a time, terminate with an empty line.
  while
    line=
    vared -p '%v> ' line
    [[ -n $line ]]
  do
    body+=$line$'\n'
  done
  functions[$psvar]=$body
}

Then for example:

$ make-function f
Please enter the definition of the f function, one line at a time, terminate with an empty line.
f> echo test
f> return 42
f>
$ f
test
$ echo $?
42
$ functions f
f () {
        echo test
        return 42
}

You could also just do:

vared 'functions[f]'

To edit the body of the f function in the line editor. Note that Enter leaves the editor and you need Alt+Enter or Esc followed by Enter (or Ctrl+v, Ctrl+j like in bash's readline) to enter a literal newline in the body of the function as usual.

vared (variable editor) is a builtin that invokes the zsh line editor (zle) to edit the contents of a variable, $functions is a special associative array that maps function names to a representation of the code in their body.

In bash, you can do something approaching by using IFS= read -rep "$psvar> " line where -e uses readline to edit $line, and you can use eval "$psvar() { $body; }" to create the function from the name and body.

Answer 2

Is this what you're trying to do?

$ cat ./tst.sh
#!/usr/bin/env bash

tmp=$(mktemp) || exit 1
trap 'rm -f "$tmp"; exit' EXIT

fncname='foo'

printf 'List commands, terminated with Control-D\n' >&2

{
    printf '%s () {\n' "$fncname"
    while IFS= read -r line; do
        printf '\t%s\n' "$line"
    done
    printf '}\n'
} > "$tmp" &&
. "$tmp"

declare -f "$fncname"

"$fncname"

$ ./tst.sh
List commands, terminated with Control-D
echo 'hello'
date
echo 'world'
        <- I hit Control-D here
foo ()
{
    echo 'hello';
    date;
    echo 'world'
}
hello
Wed Jan  3 15:48:38 CST 2024
world