shell – Termux: Widget and Emacs: can not start subprocesses

I like performing certain tasks using Emacs on different operating systems.
I use Termux to be able to access these tasks on my Android phone.
To reduce the configuration of the input in the file editing part of this workflow, I installed Termux: Widget and wrote a two-line widget script that

filename = $ (some shell logic)
emacs $ filename

So now my phone has a button on the home screen that I can press and it opens the file I want in the editor. This part works well.

Often, once I have started editing, it is convenient to run a shell script. It's nice to be able to run the shell script in an Emacs buffer so you can use the usual buffer switch commands to interact with it. If I open a new Termux session and I type Emacs at the prompt, so I can do M-x async-shell-command, use the relative file names and tab completion to find the name of the script I want (for example: ../, and run it since Emacs. This part works well.

However, if I started Emacs from Termux: Widget, Emacs can not generate subprocesses. I receive errors like

/data/data/com.termux/files/usr/bin/sh: 1: ../ not found
/data/data/com.termux/files/usr/bin/sh: 1: /data/data/com.termux/files/home/wwhat/ not found

I think the relative and absolute path names are correct because I use tab completion to check them before asking for execution.

This seems to be the kind of problem for which termux-exec was invented. However, it does not work by magic, and the documentation puzzles me.