in reply to Reading and writing to files

Inserting lines into text files can only be done by rewriting the text files. You will have to create a new file, write the three lines into it and then append monthly.txt to that file, and then rename that file to monthly.txt.

Update: See the below node by Anonymous Monk for stuff I left out and additional hints.

Some untested code : 

#/usr/bin/perl -w
use strict;

my $daily = "daily.txt";
my $monthly = "monthly.txt";

# These will hold all lines from the files
my @daily_lines;
my @monthly_lines;

# First extract the last three lines from $daily
local *DAILY;
open DAILY, "< $daily" or die "Can't open $daily: $!\n";
@daily_lines = <DAILY>;
close DAILY;

@daily_lines = @daily_lines[-3..-1];
# Now, @daily_lines contains the three last lines from the
# file named $daily

# Now we will recreate the monthly file :
local *MONTHLY;
open MONTHLY, "< $monthly" or die "Can't open $monthly : $!\n";
@monthly_lines = <MONTHLY>;
close MONTHLY;

# And write the new file :
open MONTHLY, "> $monthly.tmp";
print MONTHLY join "", @daily_lines;
print MONTHLY join "", @monthly_lines;
close MONTHLY;

# Now we delete $monthly and move our temp file over it :
unlink $monthly or die "Couldn't remove old file $monthly : $!\n";
rename( "$monthly.tmp", "$monthly" ) or die "Couldn't rename $monthly.
+tmp to $monthly : $!\n";

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RE: Re: Reading and writing to files
by Anonymous Monk on Aug 09, 2000 at 14:40 UTC
    That's correct: you cannot add something to the beginning of a file without rewriting the whole file.

    As it seems, the code does not implement one of the required tasks: 'then remove those 3 lines from daily.txt'.

    Of course, this can be fixed: 

    #!/usr/bin/perl -w

my $daily="daily.txt";
my $weekly="weekly.txt";
my $lines=3;
# alternatively, read the names from the command line:
# my($daily,$weekly,$lines)=@ARGV;

# read the daily file:
open DAILY, $daily or die "open $daily: $!";
my @daily=<DAILY>;
close DAILY;

# read the weekly file:
open WEEKLY, $weekly or die "open $weekly: $!";
my @weekly=<WEEKLY>;
close WEEKLY;

# if the last line of @daily has no \n, this line will
# be merged with the first line of @weekly.  We are
# paranoid and make sure it is there!
$daily[-1]=~/\n$/ or $daily[-1].="\n";
# Well. That's good for Unix, not for Windows. You would
# have to change "\n" to whatever is appropriate.

# move the last $lines lines from the end of @daily to
# the beginning of @weekly. Note:
# splice(@daily, -$lines) removes and returns the last 
# $lines lines from @daily;
# unshift @weekly, @some_lines adds some lines at the
# beginning of @weekly.  Together:
unshift @weekly, splice(@daily, -$lines);

# Write back the weekly file:
open WEEKLY, ">$weekly" or die "open >$weekly: $!";
print WEEKLY @weekly;
close WEEKLY;

# Write back the daily file:
open DAILY, ">$daily" or die "open >$daily: $!";
print DAILY @daily;
close DAILY;

    [download]

    You might be interested in the splice and unshift manpages for documentation of the main step of this program.

    By the way: It is possible to append to the end of a file by opening it with open FILE, ">>$name".

    With the truncate function, you can truncate a file (chop off the end leaving the beginning in place). However, I wouldn't recommend that: this function may not be implemented on some systems. Also, it's much simpler to overwrite the file.

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