Perl Cookbook

Modifying a File in Place Without a Temporary File

Problem

You need to insert, delete, or change one or more lines in a file, and you don’t want to (or can’t) use a temporary file.

Solution

Open the file in update mode ("+<"), read the whole file into an array of lines, change the array, then rewrite the file and truncate it to its current seek pointer.

open(FH, "+< FILE")                 or die "Opening: $!";
@ARRAY = <FH>;
# change ARRAY here
seek(FH,0,0)                        or die "Seeking: $!";
print FH @ARRAY                     or die "Printing: $!";
truncate(FH,tell(FH))               or die "Truncating: $!";
close(FH)                           or die "Closing: $!";

Discussion

As explained in the Introduction, the operating system treats files as unstructured streams of bytes. This makes it impossible to insert, modify, or change bits of the file in place. (Except for the special case of fixed-record-length files, discussed in Section 8.13.) You can use a temporary file to hold the changed output, or you can read the entire file into memory, change it, and write it back out again.

Reading everything into memory works for small files, but it doesn’t scale well. Trying it on your 800 MB web server log files will either deplete your virtual memory or thrash your machine’s VM system. For small files, though, this works:

open(F, "+< $infile") or die "can't read $infile: $!"; $out = ''; while (<F>) { s/DATE/localtime/eg; $out .= $_; } seek(F, 0, 0) or die "can't seek to start of $infile: $!"; print F $out or die "can't print to $infile: $!"; truncate(F, tell(F)) or die "can't ...