Beefy Boxes and Bandwidth Generously Provided by pair Networks
XP is just a number

How can I manipulate fixed-record-length files?

by faq_monk (Initiate)
on Oct 08, 1999 at 00:23 UTC ( #648=perlfaq nodetype: print w/replies, xml ) Need Help??

Current Perl documentation can be found at

Here is our local, out-dated (pre-5.6) version:

The most efficient way is using pack() and unpack(). This is faster than using substr() when take many, many strings. It is slower for just a few.

Here is a sample chunk of code to break up and put back together again some fixed-format input lines, in this case from the output of a normal, Berkeley-style ps:

    # sample input line:
    #   15158 p5  T      0:00 perl /home/tchrist/scripts/now-what
    $PS_T = 'A6 A4 A7 A5 A*';
    open(PS, "ps|");
    print scalar <PS>; 
    while (<PS>) {
        ($pid, $tt, $stat, $time, $command) = unpack($PS_T, $_);
        for $var (qw!pid tt stat time command!) {
            print "$var: <$$var>\n";
        print 'line=', pack($PS_T, $pid, $tt, $stat, $time, $command),

We've used $$var in a way that forbidden by use strict 'refs'. That is, we've promoted a string to a scalar variable reference using symbolic references. This is ok in small programs, but doesn't scale well. It also only works on global variables, not lexicals.

Log In?

What's my password?
Create A New User
[shmem]: well finally.
[stevieb]: nice! I just finished a GPS "take me home" device last week, and did a bunch of software updates to it yesterday. I also created a pseudo chip with an Arduino to simulate an IC, where it responds to register read/writes over the I2C bus...
[stevieb]: ...from an I2C master. It's ugly and there are many changes I'm going to make, but I had not done anything like it before. It's designed for my RPi:: automated test platform; a system that does CI on *all* my RPi modules.

How do I use this? | Other CB clients
Other Users?
Others contemplating the Monastery: (5)
As of 2017-06-25 22:38 GMT
Find Nodes?
    Voting Booth?
    How many monitors do you use while coding?

    Results (572 votes). Check out past polls.