You want to read in an old record from a binary file, change its values, and write back the record.
reading the old record,
up the updated values,
seek to the previous
address, and write it back.
use Fcntl; # for SEEK_SET and SEEK_CUR $ADDRESS = $RECSIZE * $RECNO; seek(FH, $ADDRESS, SEEK_SET) or die "Seeking: $!"; read(FH, $BUFFER, $RECSIZE) == $RECSIZE or die "Reading: $!"; @FIELDS = unpack($FORMAT, $BUFFER); # update fields, then $BUFFER = pack($FORMAT, @FIELDS); seek(FH, -$RECSIZE, SEEK_CUR) or die "Seeking: $!"; print FH $BUFFER; close FH or die "Closing: $!";
You don’t have to use anything fancier than
read is not
sysread actually is
join are opposites, but there’s no
speak to match
kill, and no
The example program shown in Example 8.4, weekearly , takes one argument: the user whose record you want to backdate by a week. (Of course, in practice, you wouldn’t really want to (nor be able to!) mess with the system accounting files.) This program requires write access to the file to be updated, since it opens the file in update mode. After fetching and altering the record, it packs it up again, skips backwards in the file one record, and writes it out.
Example 8-4. weekearly
#!/usr/bin/perl # weekearly -- set someone's login date back ...