Niklas Smedemark-Margulies <niklas...@gmail.com> added the comment:
Good point - the phrasing I suggested there is not accurate, and there is more complicated behavior available than simply specifying a single executable. Here's the bash manual's info about "-c" flag: """ If the -c option is present, then commands are read from the first non-option argument command_string. If there are arguments after the command_string, the first argument is assigned to $0 and any remaining arguments are assigned to the positional parameters. The assignment to $0 sets the name of the shell, which is used in warning and error messages. """ So the command_string provided (the first word or the first quoted expression) is interpreted as a shell program, and this program is invoked with the remaining words as its arguments. As you point out, this command_string can be a terminal expression like `true`, a function definition like you provided, an executable, or other possibilities, but in any case it will be executed with the remaining args. (This also matches how the library code assigns `executable`: https://github.com/python/cpython/blob/master/Lib/subprocess.py#L1707) As you say, simply slapping quotes around all the args produces a subtle difference: the arg in the position of `$0` is used as an actual positional parameter in one case, and as the shell name in the other case: $ bash -c 'f() { printf "%s\n" "$@"; }; f "$@"' - foo bar baz foo bar baz $ bash -c 'f() { printf "%s\n" "$@"; }; f "$@" - foo bar baz' - foo bar baz (Unless I am misunderstanding the behavior here). It's a bit frustrating that this approach would not work to simplify the usage, but (assuming my explanation is correct) I concede that code might certainly be depending on this behavior and setting the shell name with args[1] (and they would not want this to become a positional parameter instead). Improving on my first attempt, here's another possible phrasing for the docs: """ Using `shell=True` invokes the sequence of args via `<SHELL> -c` where <SHELL> is the chosen system shell (described elsewhere on this page). In this case, the item at args[0] is a shell program, that will be invoked on the subsequent args. The item at args[1] will be stored in the shell variable `$0`, and used as the name of the shell. The subsequent items at args[2:] will be stored as shell parameters (`$1`, `$2`, etc) and available as positional parameters (e.g. using `echo $@`). """ I would certainly be happy to defer on giving a precise and thorough statement for the docs, but clarifying/highlighting this behavior definitely seems useful. Thanks again ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue39692> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
