On 01/04/2023 19:46, Denys Vlasenko wrote:
ash allows PATH to contain "%builtin" and "DIR%func"
pseudo-directories.
%builtin shows in what order builtins to be found
relative to searching directories for external commands.
Indeed. This is a desirable feature, regardless of whether the current
implementation is the right way of doing it. POSIX intends for it to be
possible to have a way to specify that a directory in PATH takes
precedence over built-in commands. The way it tried to specify this was
broken, and did not have the desired effect. The way it is implemented
in dash using %builtin is one way of doing something that works.
I am not a fan of the precise way %builtin is implemented, and took that
out in my fork, but do intend to make the feature available in *some*
way again at some point and would suggest not just dropping it without a
replacement in dash.
DIR%func allows to have a directory of "auto-loadable" functions.
(dash git tree seems to have an example of it as src/funcs/*).
When I used dash, I did use this feature. It is roughly equivalent to
ksh's FPATH feature. In my own fork of dash I modified it to support
that instead, and dropped %func since it no longer served a purpose, but
if I were still using dash, I would still be using %func.
I tried a few searches on the internet and this feature
seems to be almost entirely not documented. I found exactly one
manpage documenting it:
https://www.unix.com/man-page/minix/1/ash/
Path Search
When locating a command, the shell first looks to see if it has
a shell function by that name. Then, if PATH does not contain an entry for
"%builtin", it looks for a builtin command by that name.
Finally, it searches each entry in PATH in turn for the command.
The value of the PATH variable should be a series of entries
separated by colons. Each entry consists of a directory name, or a
directory
name followed by a flag beginning with a percent sign. The
current directory should be indicated by an empty directory name.
If no percent sign is present, then the entry causes
the shell to search for the command in the specified directory. If the
flag is
``%builtin'' then the list of shell builtin commands is
searched. If the flag is ``%func'' then the directory is searched for a
file which
is read as input to the shell. This file should define a
function whose name is the name of the command being searched for.
Here is an example of a user having a problem because he has a PATH with
directory containing "GNU%2fLinux" string in its name:
https://unix.stackexchange.com/questions/126955/percent-in-path-environment-variable
This is unfortunate. It is well-known that arbitrary directory names
cannot be used in PATH -- directory names containing : cannot be -- but
it is less well-known that this extends to other directory names.
So... maybe we can drop it?
I assume it's rarely (never?) used in the wild.
It interferes with valid directories with percents in names.
It's non-standard, and not a typical feature of other Bourne-like shells
(maybe we (ash family) are the only ones?)
Code complication to support it is a chore.
It is not clear to me whether you mean "non-standard" as "conflicts with
the standard", or "not required by the standard". It is an extension,
but it is now a permitted extension: POSIX is being changed to permit
this behaviour. In the latest POSIX draft, PATH lookup specifies:
If PATH is unset or is set to null, or if a path prefix in PATH
contains a <percent-sign> character ('%'), the path search is
implementation-defined.
Cheers,
Harald van Dijk
