2014-10-09 23:17 GMT+02:00 Cheer Xiao <xiaqq...@gmail.com>:
> 2014-10-09 19:10 GMT+02:00 Greg Reagle <greg.rea...@umbc.edu>:
> > I was reading an article about fish at
> > http://arstechnica.com/information-technology/2005/12/linux-20051218/2/
> > which has:
> >
> > Most other Unix shells include built-in implementations of common
> > commands like echo, kill, printf, pwd, time, and test. These internal
> > implementations are usually slightly incompatible with the standard
> > Unix tools they replace, making portability an even bigger hurdle.
> > Making more and more commands a part of the shell also has stability
> > implications, since a bug in the implementation of a command risks to
> > crash the entire shell instead of only crashing a separate process.
> >
> > Also http://fishshell.com/docs/current/index.html has:
> >
>
> The argument is flawed.
>
> There is no such thing like "standard Unix tool", since all those
> traditional Unix tools have subtly different behaviors on different
> platforms.
>
I disagree. There is a POSIX specification which describes the behaviour of
"standard tools".
>
> Consider the most basic command echo(1), and you want to echo exactly
> two backslashs (which is not an unrealistic task in the context of
> shell scripting). With GNU echo you write either ` echo '\\\\' ` or `
> echo -E '\\' `, since GNU echo interprets escape sequences by default
> and need the -E switch to turn it off. However, with busybox echo,
> both prints out four backslashes, since busybox echo does not
> interpret escape sequences by default and need a -e switch to turn it
> on, and -E is a no-op for it. Things get more interesting when you run
> it on BSD: The former prints "\\\\" and the latter "-E \\\\". And it
> turns out that BSD echo never interprets escape sequences and respect
> no flag other than -n.
>
GNU is Not Unix!
http://pubs.opengroup.org/onlinepubs/9699919799/utilities/echo.html
Only the "-n" option seems to be specified, and the only mandatory option
is the string argument.
>
> If you refer to the POSIX manpage, it contains the following desperate
> paragraph:
>
> It is not possible to use echo portably across all POSIX
> systems unless
> both −n (as the first argument) and escape sequences are omitted.
>
If you are using a XSI compliant echo, you can use -n with escaped
characters in a specified way.
>
> To echo something in a portable way, you should use `printf '%s'`
> instead. Which is not the most obvious thing.
>
> Echo is an extreme case. But basically all "standard" Unix tools
> suffer from similar problems. Anyone who has ever attempted to write
> shell scripts that work consistently on GNU, busybox, BSD and Mac OS X
> userlands will remember the experience of comparing manpages side by
> side and manually picking out the least common denominator.
>
Just stick with POSIX specification, and let users port the stuff where
they do not use compliant tools.
>
> Since "standard" Unix tools are actually so fragmented these days,
> implementing (a subset of) them as a builtins will improve portability
> insteads of hurdling it. If echo is a builtin in the shell, we can
> always rely on it working consistently across different platforms (or
> more exactly, different sets of userlands). It will not work the same
> way as in other shells, but fish is not meant to compatible with other
> shells anyway.
>
> > To avoid code duplication, and to avoid the confusion of subtly
> > differing versions of the same command, fish generally only implements
> > builtins for actions which cannot be performed by a regular command.
> >
> > However:
> > $ type echo printf pwd test
> > echo is a builtin
> > printf is a builtin
> > pwd is a builtin
> > test is a builtin
> >
> > I wonder why these commands were made built-in.
> >
>
>
> --
> Best regards,
> 肖骐 Qi Xiao
>
>
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