passing delimiters on command line

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hello Monks,

I am trying to pass a delimiter string via a command line argument like this :

./script.pl "\t" ARG1 ARG2
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so inside the script I would like to set the $" to do some interpolation on a list like this :

my $delim = shift @ARGV;
$" = $delim;
...
print "@list";
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but with any of the backslashed characters what prints is something like this :

list_item1\tlist_item2\tlist_item3
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and not what I am going for which is to have the line print tab-delimited, in this case :

list_item1   list_item2    list_item3
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I don't understand why this happens because if I set the delimiter in the script like this :

$" = "\t"
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everything is fine. I have tried every kind of escaping or quoting I can think of and nothing seems to work.

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Re: passing delimiters on command line by Joost (Canon) on Sep 13, 2004 at 09:54 UTC
The problem here is that "\t" is not interpreted by the shell to be a tab character, but just a regular backslash followed by a "t": `> echo "\t" \t` [download] Perl also doesn't interpret this input as a tab, because that would make it impossible to have strings with a literal "\t" sequence. \escape sequences are only interpreted in double quoted string literals (and regular expressions). The way to pass special characters in bash is to use `$'\code'`, ie $'\t' for tab: `> perl -e '$"=shift;print "@ARGV"' $'\t' a b c d e a b c d e` [download] If you use another shell, look in its documentation. Ofcourse you could also do `$" = eval '"'.shift(@ARGV).'"';` but that would complicate matters if anyone needs to pass something that can't be put in double quotes. update: fixed typo "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply] [d/l] [select]
Re: passing delimiters on command line by ikegami (Patriarch) on Sep 13, 2004 at 09:52 UTC
If you trust the user, `$" = eval '"' . $delim . '"';` works. Use eval with extreme caution, for security reasons. You could parse it yourself: `%lookup_slash = ( t => "\t", n => "\n", ... );` `$delim =~ s/\\(.)/$lookup_slash{$1}/ge;` But do you really expect slashed delimiters other than "\t"? If not, the previous snippet can be simplified to: `$delim =~ s/\\t/\t/g;` Or you could go fancier args...	[reply] [d/l] [select]
Re^2: passing delimiters on command line by Anonymous Monk on Sep 13, 2004 at 16:01 UTC
The basic problem with `$" = eval '"' . $delim . '"'` is that someone can pass something like `rm -rf /` as the delimiter argument to your script. A QND workaround would be to just reject anything over two characters in length for the delimiter argument... like so: `die "some trouble is brewing\n" if length($ARGV[0]) > 2; # Freely borrowed from ikegami $delim = eval '"' . $ARGV[0] . '"'; print "Field1" . $delim . "Field2" . $delim . "Field3" . $/;` [download] Good luck	[reply] [d/l] [select]
Re: passing delimiters on command line by thor (Priest) on Sep 13, 2004 at 10:55 UTC
On Unix, you can bypass the shell's tab-completion (if it has it) by hitting Ctrl-v before hitting the tab. This will give you a literal tab character that should be recognized by perl. The same thing will work for literal newlines and escapes, too. thor `Feel the white light, the light within Be your own disciple, fan the sparks of will For all of us waiting, your kingdom will come`	[reply]
Re: passing delimiters on command line by Crian (Curate) on Sep 13, 2004 at 09:32 UTC
I use to pass a tab (or other) delimiter to a Perl script too. There I give the delimiter as the last parameter and don't escape it at all (it works at least under Win2k). The output is delimited by tabs as I wanted it. Perhaps the difference is, that I don't use `$" = "\t"`, I print out the delimiter I got from the user between each data "per hand" in the print statement, as `print $data1, $delimiter, $data2, $delimiter, ..., $datan, "\n";`. If you want to put the delimiter into $", you could use `sprintf` to print it into that variable. That may be safer than eval.	[reply]
Re: passing delimiters on command line by BrowserUk (Patriarch) on Sep 13, 2004 at 09:57 UTC
As a one-liner: `>perl -wle"${\"} = eval qq[\"$ARGV[0]\"]; my @a = 1 .. 10; print \"@a\ +"" \t 1 2 3 4 5 6 7 8 9 + 10` [download] Or more simply in a program: `#! perl -slw use strict; $" = eval qq["$ARGV[ 0 ]"]; my @a = 1 .. 10; print "@a"; __END__ P:\test>test.pl \t 1 2 3 4 5 6 7 8 9 + 10` [download] Caveat: If you type in a dangerous command in place of the delimiter, it will do dangerous things! Update: As a less dangeruos alternative you could do one of: `>perl -wle"${\"} = chr $ARGV[0]; my @a = 1 .. 10; print qq[@a]" 9 1 2 3 4 5 6 7 8 9 + 10 >perl -wle"${\"} = chr hex $ARGV[0]; my @a = 1 .. 10; print qq[@a]" 0x +9 1 2 3 4 5 6 7 8 9 + 10 >perl -wle"${\"} = chr oct $ARGV[0]; my @a = 1 .. 10; print qq[@a]" 11 1 2 3 4 5 6 7 8 9 + 10` [download] Examine what is said, not who speaks. "Efficiency is intelligent laziness." -David Dunham "Think for yourself!" - Abigail "Memory, processor, disk in that order on the hardware side. Algorithm, algorithm, algorithm on the code side." - tachyon	[reply] [d/l] [select]
Re: passing delimiters on command line by Anonymous Monk on Sep 13, 2004 at 13:34 UTC
`./script.pl \^V^I ARG1 ARG2` [download] If you have a shell where in interactive mode tab means something special (like in bash), you need to escape the tab twice. First to take the tab as-is - this is done by typing a CNTL-V character before the tab. Then you need to tell the parser that it should take the tab as a symbol, and not as whitespace separating the symbols. This can be done by putting a backslash in front of it - or by putting it inside quotes.	[reply] [d/l]


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