07-27-2011
Methods For Debugging Perl Problems
Note: Not a programmer by profession but occasionally have to program.
I am looking for general methods and freely/readily available tools employed to debug problems during development of perl scripts. Anything that has really helped you out with problems you just couldn't find.
A couple of problems I'm seeing:
- apparent corruption of $_[0], $_[1], etc
- regexp failures that should be matching
I have a lot of subroutines that I unit tested and found all to be working as expected. When I put everything together into a single script, I noticed unexpected results.
For the $_[0] case, I put in debug statements to see how the subroutine was executing and noticed that the value was correct on entry into the routine but then, when processing, it seemed as if it had been truncated (based on how the routine behaved). After I couldn't really figure out what was happening to it, I copied the parameters to temporary variables on entry to the subroutine and then used those variables throughout. And that solved that problem. But again, if I isolate the subroutine it works fine without the temporary variables.
I have a long list of regexps I'm searching for and I know that the expressions, themselves are good. Everyone of them was tested to verify that it was matching what it was supposed to match. But with everything together, the regexps are failing. And I'm talking even simple regexps... like just matching an 11-digit number. More complicated ones are matching and then, on the same line, simple ones are failing to match. During my testing, I tested with multiple expressions on the same line and there was no problem so I don't think it has anything to do with how many
are on the line. But I don't know. I can't understand why the expressions are failing to match. Would it have anything to do with pos? One thing I did change (some time) before I ever noticed any problem was that, once I found a match, I perform a global substitution. I don't think that should be the source of this problem (but I will take out the global substitution in just a minute, just to see) as I am not doing global matches to try to identify the expressions and each attempt at a match uses a new block.
Any debugging suggestions would be greatly appreciated.
------------------------------------------------------------
Edited to add:
I removed the global substitutions and it made no difference in the regexp matching. Some lines aren't even matching any of the regexps that are there so the global substitution never would have even come into play in those cases.
Another thing I just tried... inside my loop where I read the data file, I manually set the line variable to one of the lines in the file (thus ignoring the actual read-in lines) and the matches that are failing when the data is read from the file match this way. The fact that I can take the same file data and do a manual assignment and it will succeed, but it fails when actually reading the file, makes no sense because I just copied/pasted the data so I'm fairly sure there's nothing wrong with the data file.
----------------------------------------------
Edited to add:
It just keeps getting better. Now I deleted the direct assignment and tried the file again. No joy. But then I put the direct assignment back into the loop and it no longer matches there either. Guess it's time to pull it all apart once again and have another go at it.
Last edited by Vi-Curious; 07-27-2011 at 05:20 PM..
Reason: Follow-up re: direct assignment of line data
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GREP(1) General Commands Manual GREP(1)
NAME
grep, egrep, fgrep - search a file for a pattern
SYNOPSIS
grep [ option ] ... expression [ file ] ...
egrep [ option ] ... [ expression ] [ file ] ...
fgrep [ option ] ... [ strings ] [ file ]
DESCRIPTION
Commands of the grep family search the input files (standard input default) for lines matching a pattern. Normally, each line found is
copied to the standard output. Grep patterns are limited regular expressions in the style of ex(1); it uses a compact nondeterministic
algorithm. Egrep patterns are full regular expressions; it uses a fast deterministic algorithm that sometimes needs exponential space.
Fgrep patterns are fixed strings; it is fast and compact. The following options are recognized.
-v All lines but those matching are printed.
-x (Exact) only lines matched in their entirety are printed (fgrep only).
-c Only a count of matching lines is printed.
-l The names of files with matching lines are listed (once) separated by newlines.
-n Each line is preceded by its relative line number in the file.
-b Each line is preceded by the block number on which it was found. This is sometimes useful in locating disk block numbers by con-
text.
-i The case of letters is ignored in making comparisons -- that is, upper and lower case are considered identical. This applies to
grep and fgrep only.
-s Silent mode. Nothing is printed (except error messages). This is useful for checking the error status.
-w The expression is searched for as a word (as if surrounded by `<' and `>', see ex(1).) (grep only)
-e expression
Same as a simple expression argument, but useful when the expression begins with a -.
-f file
The regular expression (egrep) or string list (fgrep) is taken from the file.
In all cases the file name is shown if there is more than one input file. Care should be taken when using the characters $ * [ ^ | ( ) and
in the expression as they are also meaningful to the Shell. It is safest to enclose the entire expression argument in single quotes ' '.
Fgrep searches for lines that contain one of the (newline-separated) strings.
Egrep accepts extended regular expressions. In the following description `character' excludes newline:
A followed by a single character other than newline matches that character.
The character ^ matches the beginning of a line.
The character $ matches the end of a line.
A . (period) matches any character.
A single character not otherwise endowed with special meaning matches that character.
A string enclosed in brackets [] matches any single character from the string. Ranges of ASCII character codes may be abbreviated
as in `a-z0-9'. A ] may occur only as the first character of the string. A literal - must be placed where it can't be mistaken as
a range indicator.
A regular expression followed by an * (asterisk) matches a sequence of 0 or more matches of the regular expression. A regular
expression followed by a + (plus) matches a sequence of 1 or more matches of the regular expression. A regular expression followed
by a ? (question mark) matches a sequence of 0 or 1 matches of the regular expression.
Two regular expressions concatenated match a match of the first followed by a match of the second.
Two regular expressions separated by | or newline match either a match for the first or a match for the second.
A regular expression enclosed in parentheses matches a match for the regular expression.
The order of precedence of operators at the same parenthesis level is [] then *+? then concatenation then | and newline.
Ideally there should be only one grep, but we don't know a single algorithm that spans a wide enough range of space-time tradeoffs.
SEE ALSO
ex(1), sed(1), sh(1)
DIAGNOSTICS
Exit status is 0 if any matches are found, 1 if none, 2 for syntax errors or inaccessible files.
BUGS
Lines are limited to 256 characters; longer lines are truncated.
4th Berkeley Distribution April 29, 1985 GREP(1)