How to overload / customize bash (or any other shell) commands handler?

Question

When in bash some non existing command is run, corresponding error message appears:

$ non-existent-command
non-existent-command: command not found

Is it possible to customize this behavior?

I would like to achieve the following. When the entered command matches, for example, the pattern s\d\d\d, it should be handled specially:

• s should be extracted and used as a command,
• \d\d\d should be extracted and used as an argument for the s command.
• The command created by parsing the above should be run.

This can normally be achieved by means of a function named s that takes the needed arguments. A normal call is used to run such a function. For example:

s 123

But I want to customize the behavior of bash to be able to run such a function with, for example, the following calls:

• s123
• s500

I tried to find related information in man bash by the following keywords: overload, custom, override and did not succeed.

As I understand it, I need to somehow customize the bash handler that is used for processing input. I did not investigate bash source code to understand if this is feasible.

It will be best to come up with solution that is usable for any shell.

Probably creating a custom bash wrapper/adapter with the needed logic and using it instead of standard bash binary will solve this.

Answer 1

In bash:

$ s() { printf 'Called s %s\n' "$*" >&2; }

$ cat $HOME/bin/command_not_found_handle
#!/usr/bin/env bash

command_not_found_handle() {
    if [[ "$*" =~ ^(s)([0-9]{3})$ ]]; then
        "${BASH_REMATCH[1]}" "${BASH_REMATCH[2]}"
    else
        printf 'Command not found: %s\n' "$*" >&2
        return 127
    fi
}

$ . $HOME/bin/command_not_found_handle

$ s123
Called s 123

$ s500
Called s 500

$ echo $?
0

$ s5001
Command not found: s5001

$ echo $?
127

FYI 127 is the standard "command not found" exit status. See https://www.gnu.org/software/bash/manual/bash.html#Command-Search-and-Execution for more information.

I'm only using BASH_REMATCH[] as above to call s because I know it's safe to do so given that $* contains the command as a single letter and its args just 3 digits, for anything more complicated see https://mywiki.wooledge.org/BashFAQ/050.

There is no solution that will work in every shell, see How to define a function that handles `command not found`? to find information on approaches to the above for some other shells.