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 ...