On Tue, Apr 03, 2018 at 01:43:10PM +0200, Johannes Schindelin wrote: > > I don't have time or interest to work on this now, but thought it was > > interesting to share. This assumes that something in shellscript like: > > > > while echo foo; do echo bar; done > > > > Is no slower on Windows than *nix, since it's purely using built-ins, as > > opposed to something that would shell out. > > It is still interpreting stuff. And it still goes through the POSIX > emulation layer. > > I did see reports on the Git for Windows bug tracker that gave me the > impression that such loops in Unix shell scripts may not, in fact, be as > performant in MSYS2's Bash as you would like to believe: > > https://github.com/git-for-windows/git/issues/1533#issuecomment-372025449 The main problem with `read` loops in shell is that the shell makes one read() syscall per character. It has to, because doing otherwise is user-visible in cases where the descriptor may get passed to a different process. There's unfortunately no portable way to say "please just read this quickly, I promise nobody else is going to read the descriptor". And nor do I know of any shell which is smart enough to know that it's going to consume to EOF anyway (as you would for something like "cmd | while read"). If you know you have bash, you can use "-N" to get a more efficient read: $ echo foo | strace -e read bash -c 'read foo' [...] read(0, "f", 1) = 1 read(0, "o", 1) = 1 read(0, "o", 1) = 1 read(0, "\n", 1) = 1 $ echo foo | strace -e read bash -c 'read -N 10 foo' [...] read(0, "foo\n", 10) = 4 read(0, "", 6) = 0 but then you have another problem: how to split the resulting buffer into lines in shell. ;) But if we're at the point of creating custom C builtins for busybox/dash/etc, you should be able to create a primitive for "read this using buffered stdio, other processes be damned, and return one line at a time". -Peff
