On 10/10/10 7:09 PM, Devin Teske wrote:
On Oct 9, 2010, at 10:25 PM, Julian Elischer wrote:
For what it matters, I'v enever found the [ "x$foo" = "x" ] construct to be
useful.
the quoting seems to work for everything I've ever worked on.
There have been times where I had scripts which could get errors unless
"x$foo" was used, but it's been more than 10 years since I last hit that
situation. Of course, ever since I did hit it, I tend to write my 'test'
parameters in ways which avoid the problem. It might have only been when
checking if the variable was set to anything. Eg, using:
if [ "$foo" ] ; then ....
instead of:
if [ -n "$foo" ] ; then ...
Or it might have been when combining multiple checks in a single 'test'
(using -a's, etc). However, I'm not going to try to dream up a
pathological example of that right now.
I agree... I think that the "x" syntax came around for when people were using
non-quoted syntax... for example...
[ x$foo = x ]
is still very useful in that it prevents the error when $foo expands to "-e".
The non-quoted example is dangerous in the case where $foo has multiple
words in it. The "x" does not save you from that problem. I have a
'list_args' script which just lists out the parameters it is called with:
# Test="this is a multi-word test"
# list_args x$Test
list_args at 19:22:27 Oct 10: $# = 5
ARG[000] l=005: 'xthis'
ARG[001] l=002: 'is'
ARG[002] l=001: 'a'
ARG[003] l=010: 'multi-word'
ARG[004] l=004: 'test'
However, enclosing the argument (as the 'x$foo' portion is really just the
first argument to the '[' built-in) in quotes:
[ "$foo" = x ]
makes it so that the expansion is taken as:
[ "-n" = x ]
rather than:
[ -n = x ]
The former not causing an error, while the latter does.
The latter does not cause an error. Try it:
# [ "-n" = x ] ; echo $?
1
# [ -e = "no" ] ; echo $?
1
# [ -e = -n ] ; echo $?
1
Quite functionally, at a C-level, if you have your array, ...
argv[0] = "[\0";
argv[1] = "\"-n\"\0"; /* quoted syntax */
argv[2] = "=\0";
argv[3] = "x\0";
and
argv[0] = "[\0";
argv[1] = "-n\0"; /* non-quoted syntax */
argv[2] = "=\0";
argv[3] = "x\0";
You won't see the double-quotes in the C program. The shell processes
single and double quotes, and passes the result to the C program which
is running. It might be different for built-in functions, but /bin/test
would never see the double-quotes if they were used. And since the
built-in function has to work the same as standalone function, I doubt
the built-in would be any different.
# list_args "-n"
list_args at 19:36:15 Oct 10: $# = 1
ARG[000] l=002: '-n'
# list_args -n
list_args at 19:36:22 Oct 10: $# = 1
ARG[000] l=002: '-n'
(note that the single-quotes you see there are printed by the list_args
script itself for display purposes).
/disclaimer: I think this is the first post that I've made with the new
"open-source edition" of Eudora, and I have no idea if this will be
formatted the way I'm expecting it be!/
--
Garance Alistair Drosehn = dro...@rpi.edu
Senior Systems Programmer or g...@freebsd.org
Rensselaer Polytechnic Institute; Troy, NY; USA
_______________________________________________
freebsd-hackers@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-hackers
To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"
