On Sun, Dec 17, 2000 at 01:31:19PM -0600, David Talkington <[EMAIL PROTECTED]>
wrote:
| Cameron Simpson wrote:
| >One thing I have done in my own shell setup is alias (well, shell
| >function, but the idea's the same) rm, cp, chmod, chown, chgrp and a
| >couple of others to an equivalent of
| > ( set -x; rm ${1+"$@"}; )
| >i.e. I get to see the results of my shell expansion [...]
|
| Ok, y'got me ... what is the thingy in the brackets doing? set -x
| shows me the expansion, but I don't understand what the rest of it is
| doing. In a test script, that structure returned the same output as
| $@ by itself (just the positional parameters). Enlighten me?
That is the One True Gimme-The-Args Incantation.
Some background:
As you know, $* is the command line args stuck together with spaces (hmm,
with the first char of $IFS in some shells, which is normally a space).
It acts exactly like every other shell var in that "$*" is a single
string.
When you're passing your arguments to another command (as we are above)
you naturally don't want them stuck together as a single argument, but $*
on its own is subject to space interpretation, so calling our wrapper with:
"this and that" "foo"
and saying
$*
would make four strings, not two.
So...
Enter $@. Outside quotes, $@ acts like $* and any other var: space
interpretation happens. BUT... The special incantation
"$@"
keeps the original quoting intact. Now, for reasons I have a few theories
about, there's a catch with this. If there are _no_ arguments then "$@"
doesn't result in no arguments, it results in exactly one empty argument.
So...
What you do in a robust portable script is to only use "$@" when you know
there's at least one argument. Now, the slickest way to do that is with the
handy ${+} incantation. From the manual (under "Parameter Substitution")
you'll know that the incantation:
${var+foo}
says ``if var is set, substitute foo, otherwise nothing''. So we say
``if $1 is set (which means $# > 0: there's at least one argument)
substitute "$@", otherwise nothing'', which neatly steps around this
corner case for "$@".
So you will find
${1+"$@"}
in portable wrapper scripts.
Aside: ${1:+"$@"} is wrong wrong wrong. As an exercise, tell me what it would
break.
Thus endeth the lesson for today. Make sense? Cheers,
--
Cameron Simpson, DoD#743 [EMAIL PROTECTED] http://www.zip.com.au/~cs/
It's a vague science. - Rory Tate, circle researcher.
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