Learning Perl on Win32 Systems

Variable-Length ( Text) Databases

Some system databases (and quite a few user-created databases) are a series of human-readable text lines, with one record per line. For example, the TCP/IP hosts file contains one line per hostname.

Most often, these databases are updated with simple text editors. Updating such a database consists of reading it all into a temporary area (either memory or another disk file), making the necessary changes, and then either writing the result back to the original file or creating a new file with the same name (after deleting or renaming the old version). You can think of this process as a copy pass: the data is copied from the original database to a new version of the database, and changes are made during the copy.

Perl supports a copy-pass-style edit on line-oriented databases using inplace editing. Inplace editing is a modification of the way the diamond operator ( <>) reads data from the list of files specified on the command line. Most often, this editing mode is accessed by setting the -i command-line argument, but we can also trigger inplace editing mode from within a program, as shown in the examples that follow.

To trigger the inplace editing mode, set a value into the $^I scalar variable. The value of this variable is important and will be discussed in a moment.

When the <> construct is used and $^I has a value other than undef, the steps marked ##INPLACE## in the following code are added to the list of implicit actions the diamond operator ...