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7.17. Modifying a File in Place Without a Temporary File
7.17.1. 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.
7.17.2. 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: $!";
7.17.3. Discussion
As explained in this chapter's 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 Recipe 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 is fine for small files, but 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 truncate $infile: $!";
close(F) or die "can't close $infile: $!";
For other examples of things you can do in-place, look at the recipes
in Chapter 8.This approach is only for the truly determined. It's harder to write,
takes more memory (potentially a lot more),
doesn't keep a backup file, and may confuse other processes trying to
read from the file you're updating. For most purposes, therefore, we
suggest it's probably not worth it.Remember to lock if you're paranoid, careful, or both.
7.17.4. See Also
The seek, truncate,
open, and sysopen functions in
perlfunc(1) and in Chapter 29 of
Programming Perl; Recipe 7.15; Recipe 7.16; Recipe 7.18
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| 7.16. Modifying a File in Place with the -i Switch |  | 7.18. Locking a File |
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