Novice surprise
Robert Steven Glickstein
Today I ran my very first Perl program; it was generated by a2p. The
awk program was:
BEGIN {x = 0}
$0 == "" {x = 1}
x == 1
The perl program produced was:
eval '$'.$1.'$2;' while $ARGV[0] =~ /^([A-Za-z_]+=)(.*)/ && shift;
# process any FOO=bar switches
$x = 0;
while (<>) {
chop; # strip record separator
if ($_ eq '') {
$x = 1;
}
print $_ if $x == 1;
}
The idea: skip the first lines of the input until a blank line is
found, then start echoing the lines. The awk program worked fine, the
perl program did not. I kept getting "cannot open file" errors.
It was two hours of staring at this stupid thing before I realized
that the problem was this: The filenames that I was passing as
arguments happened to begin with a "+". Since I didn't have
write-access, I got a "cannot open file" error. I couldn't figure out
a way to circumvent this in Perl, so instead of the command-line
perlscript +a +b +c
I used
perlscript ./+a ./+b ./+c
The big question: What is the *right* way around this problem?
______________ _____________________________
Bob Glickstein | Internet: bo...@andrew.cmu.edu
Information Technology Center | Bitnet: bobg%and...@cmuccvma.bitnet
Carnegie Mellon University | UUCP: ...!harvard!andrew.cmu.edu!bobg
Pittsburgh, PA 15213-3890 |
(412) 268-6743 | Sinners can repent, but stupid is forever
Larry Wall
: It was two hours of staring at this stupid thing before I realized
: that the problem was this: The filenames that I was passing as
: arguments happened to begin with a "+". Since I didn't have
: write-access, I got a "cannot open file" error. I couldn't figure out
: a way to circumvent this in Perl, so instead of the command-line
:
: perlscript +a +b +c
:
: I used
:
: perlscript ./+a ./+b ./+c
:
: The big question: What is the *right* way around this problem?
Er, don't use a toy language for a real problem... :-)
I think that most people avoid the problem by not using filenames beginning
with + or >. If you really want to write a bulletproof script, you might
put the following before the initial loop:
for (@ARGV) { s#^([^/])#./$1#; s#$#\0#; }
while (<>) {...
In general, filenames starting with + aren't quite as dangerous as those
starting with -, but there are some programs that use + to introduce
switches, so it's probably a bad idea. You can't, for example, say
sort +a +b +c
Just don't create a file called -rf. :-)
Larry
Harald Fuchs
>I think that most people avoid the problem by not using filenames beginning
>with + or >. If you really want to write a bulletproof script, you might
>put the following before the initial loop:
>for (@ARGV) { s#^([^/])#./$1#; s#$#\0#; }
>while (<>) {...
Shouldn't that be
for (@ARGV) { s#^[^./]#./$&#; s#$#\0#; }
Note the period: ^
--
Harald Fuchs <fu...@tmipi4.telematik.informatik.uni-karlsruhe.de>
<fu...@telematik.informatik.uni-karlsruhe.dbp.de> *gulp*
Larry Wall
: lw...@jpl-devvax.JPL.NASA.GOV (Larry Wall) writes:
:
: >I think that most people avoid the problem by not using filenames beginning
: >with + or >. If you really want to write a bulletproof script, you might
: >put the following before the initial loop:
:
: >for (@ARGV) { s#^([^/])#./$1#; s#$#\0#; }
: >while (<>) {...
:
: Shouldn't that be
: for (@ARGV) { s#^[^./]#./$&#; s#$#\0#; }
: Note the period: ^
You can if it makes you feel better, but it doesn't hurt (much) to add an
extra ./ on the front of any relative pathname (even those beginning with .
or ..). The current directory will even be in the cache already, so there's
no extra disk access.
Larry
Tom Neff
>I think that most people avoid the problem by not using filenames beginning
>with + or >.
It's not always up to us, is it! Sometimes we need our Perl tools
just to DEAL with the weird stuff other peoples' programs create.
> If you really want to write a bulletproof script, you might
>put the following before the initial loop:
>
>for (@ARGV) { s#^([^/])#./$1#; s#$#\0#; }
>while (<>) {...
Nah, too complicated. Just
grep(ARGV,s//</);
Tom Christiansen
:>[Larry said]
:> If you really want to write a bulletproof script, you might
:>put the following before the initial loop:
:>
:>for (@ARGV) { s#^([^/])#./$1#; s#$#\0#; }
:>while (<>) {...
:
:Nah, too complicated. Just
:
: grep(ARGV,s//</);
Hello? Don't you mean
grep(s//</, @ARGV);
It's kind of a pity perl doesn't flag this as an error.
And I would use s/^/</ myself -- I always think // will
be a remembered /pattern_match/, although in s/// it doesn't
seem to me. Maybe it only counts for m// things. Larry?
Also, Larry's works on things that Tom Neff's doesn't: you
can't open a file named "-" nor any with trailing white space
unless you use Larry's quotation mechanism, or it's functional
equivalent, like
grep(s#^([^/])(.*)#./$1$2\0#, @ARGV);
--tom
--
"All things are possible, but not all expedient." (in life, UNIX, and perl)
Larry Wall
: Nah, too complicated. Just
:
: grep(ARGV,s//</);
Not complicated enough. (Not to mention backwards.) What if the filename
begins with &? What if // invokes a previous pattern? Try
grep(s/.*/< $&\0/, @ARGV);
And that still doesn't handle leading whitespace.
By the way, /foo/ && s//bar/ is busted in 44, seemingly. I'm looking at it.
/f../ && s//bar/ works right though. Looks like the constant string optimizer
got me.
Larry
Larry Wall
: Also, Larry's works on things that Tom Neff's doesn't: you
: can't open a file named "-" nor any with trailing white space
: unless you use Larry's quotation mechanism, or it's functional
: equivalent, like
:
: grep(s#^([^/])(.*)#./$1$2\0#, @ARGV);
Unfortunately, that's not a functional equivalent, since it won't put the
trailing null on an absolute pathname.
Larry
Tom Neff
>
>Hello? Don't you mean
>
> grep(s//</, @ARGV);
>
>It's kind of a pity perl doesn't flag this as an error.
Groooooaaannn! :-( --> :-)
I'll tell you what's really sick -- mine worked too! Go figure.