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The final script is more complicated Updating information in either a delimited or fixed-length database requires that you know the location of the data that you want to update With a delimited database, the data can potentially be of any length, and so you need to copy the existing information from the current database into a new file, substituting the updated information in the new file before continuing the copy process It s not possible to insert data into the file you cannot move data within a physical file without moving the information somewhere else first You could do it in memory, but for large databases, this wouldn t be practical Instead, the better solution is to use external files for the process With a fixed-length database, the process is slightly easier The length of each record in the database is the same, so updating a record is as easy as overwriting the updated and packed record in the same physical location within the file To do this you use the seek and tell functions This is a cyclical process in which you read in each record until you find the one you want, remembering the location of the start of the record in each case Then, once the information has been updated, you go back to the start of the record and rewrite the packed record Here s the script that updates our earlier task file entries:
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use Fcntl; my ($taskfile) = "tasksdb"; my ($taskformat) = "A40LL"; my $tasklength = length(pack($taskformat,)); die "Usage: modtaskpl title completed-date\n" if (@ARGV<2); open(D,"+<$taskfile") || die "Couldn't open the task file, $!\n"; while(read(D,$_,$tasklength)) { ($title,$reqdate,$compdate) = unpack($taskformat,$_); last if (($title eq $ARGV[0]) && ($compdate eq 0)); $lastseek=tell(D); } if ($lastseek >= (-s $taskfile)) { die "Couldn't find the task specified\n"; }
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($mday,$mon,$year) = split '/',$ARGV[-1]; $compdate = ($year*10000)+($mon*100)+$mday; seek(D,$lastseek,SEEK_SET); print D pack($taskformat,$title,$reqdate,$compdate); close(D) || die "Couldn't close the database\n"; PROGRAMMING WITH PERL
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To use all three scripts, first call addtaskpl (the first script) to create and add a record to the database, and then update the task you have added by specifying a completion date:
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$ perl -w addtaskpl 'Phone Richard' 25/3/1999 $ perl modtaskpl 'Phone Richard' 26/3/1999 $ perl listtaskpl Title Req Date Phone Richard 25/03/1999
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For simple structures, such as single-record databases, and especially for a sequential series of data, text files are exceedingly practical However, they suffer from performance issues if you want to access the information on a random basis, or where the amount of information you wish to store becomes unmanageable in a text database A better solution is to store the information in a real database where the information can be recorded and extracted using identified and unique keys This enables you to ignore the searching and storage mechanisms that are required for text databases and instead to concentrate on writing the code for using, rather than the code for accessing, the database The standard database under Unix is a system called DBM, which is based on a simple key/value pair, much like Perl s own hash data type Each entry within the database will have a unique key, and attached to this will be the piece of data you want to store To extract information from the database, you simply request the data associated with a particular key Although it sounds complicated, it is really no different from the way hash variables are stored within Perl You access a hashed variable entry within Perl by specifying a textual key With some careful programming and the use of well-worded keys, you can store information in a DBM file in the same way you would store information in any off-the-shelf database
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