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.
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.
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.
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.
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
------------------------------------------------------------------------------
Meet PCI DSS 3.0 Compliance Requirements with EventLog Analyzer
Achieve PCI DSS 3.0 Compliant Status with Out-of-the-box PCI DSS Reports
Are you Audit-Ready for PCI DSS 3.0 Compliance? Download White paper
Comply to PCI DSS 3.0 Requirement 10 and 11.5 with EventLog Analyzer
http://pubads.g.doubleclick.net/gampad/clk?id=154622311&iu=/4140/ostg.clktrk
_______________________________________________
Fish-users mailing list
Fish-users@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/fish-users
