On 7/23/19 12:12 PM, Sam Liddicott wrote:
> Given what you have explained as intentional, it the difference between 1
> and 2, but it is best understood as a 4-way difference, outlined here:
>
> 1. {var} internal: fd remains open in parent
> 2. {var} external: fd closed in parent
> 3. numeric internal: fd closed in parent
> 4. numeric external: fd closed in parent
>
> 1. {var} internal: fd remains open in parent
> bash -c 'echo 1 {_}>&2 2>&1 1>&${_} {_}<&- ;
> echo done ; lsof -p $$ | grep CHR'
>
> 2. {var} external: fd closed in parent
> bash -c '/bin/echo 1 {_}>&2 2>&1 1>&${_} {_}<&- ;
> echo done ; lsof -p $$ | grep CHR'
>
> 3. numeric internal: fd closed in parent
> bash -c 'echo 1 10>&2 2>&1 1>&10 10<&- ;
> echo done ; lsof -p $$ | grep CHR'
>
> 4. numeric external: fd closed in parent
> bash -c '/bin/echo 1 10>&2 2>&1 1>&10 10<&- ;
> echo done ; lsof -p $$ | grep CHR'
>
> You've indicated that {var} syntax leaves me an fd to do with what I wish.
Yes. The question is whether the {_}<&- should close the file descriptor
`permanently' or whether that close action should be undone because you're
not using `exec' and the file descriptor was created using the variable
assignment syntax. The latter is what bash does now.
> You've also explained what bash is doing that makes this untrue if the
> command was an external command.
Because you never open the file descriptor in the parent shell:
redirections happen in the child. The child process in case 2 (and 4)
can do what it wants with the file descriptor, and its view of the
file descriptor is the same as case 1 (and 3).
> I don't believe that this behaviour is *intended( to depend on the
> non-obvious detail of whether or not the command is external.
Why? There are plenty of things that depend on whether or not a command is
builtin, or whether it's run in the parent shell.
>
> Given the coding pattern of wrapping external commands with functions that
> re-invoke using bash "command"; this can lead to unpredictable behaviours
> when such wrappers are active.
>
> e.g. openssl() { LD_LIBRARY_PATH="$LD_LIBRARY_PATH:/extra" *command*
> openssl "$@"; }
>
> If that function is defined, I get a handle leak*, if it isn't and main
> openssl is called, I don't -- or from the other of view my handle got
> closed without my knowledge, so I can't use it as I wish.
>
> Personally I would wish that "{var} internal" would also close the fd as it
> does for numeric fd and for external {var} fd, because if I really wanted
> to open an fd and have it hang around I would do a naked: exec {xxx}>&2 ;
> type of thing.
Sure. But there's not as good a reason to have that syntax otherwise. It's
just syntactic sugar, a way for historical shells to use file descriptors
greater than 9.
> I also think that based on your description of what bash is doing, it might
> be easier to fix by also closing in the parent, as I describe.
>
> It would bring full consistency and avoid hard to detect and hard to
> code-around bugs.
>
> Ultimately, unless {var} external is intended to behave different to {var}
> internal, then we have a consistency bug.
The internal/external difference doesn't really have anything to do with
the semantics of file descriptors or redirections per se; only that
redirections are performed in the child. There are multiple variations in
behavior depending on whether a command is executed in a subshell, and this
is one of them.
Ultimately, the difference is between cases 1 and 3 and retaining a handle
to the file descriptor instead of closing it when the command finishes.
Everything else is identical between the two commands. That was a design
choice. I don't see changing it now.
Chet
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU [email protected] http://tiswww.cwru.edu/~chet/
