1) Require the output of which in non-interactive environments to conform to the behavior a reasonable shell script author is likely to have assumed the specification was based on observations. In the case of historic 'which' that would be to print the absolute path to an executable without any other arguments, or a specific error message and exit status.
2) Allow __interactive__ shells to deviate from that behavior. E.G. with a warning, with alias information, etc.
I feel like I'm going to be bit when I try commands interactively and then that act differently in a script. I think the current solution is the pragmatic one, don't standardize things you can't pull off standardizing! #!
The ls command does this: if output is to a tty then it prints in columns. Many commands add color when the output is a tty. I don’t think it would be a big issue.
1) Require the output of which in non-interactive environments to conform to the behavior a reasonable shell script author is likely to have assumed the specification was based on observations. In the case of historic 'which' that would be to print the absolute path to an executable without any other arguments, or a specific error message and exit status.
2) Allow __interactive__ shells to deviate from that behavior. E.G. with a warning, with alias information, etc.