FAQ 5.8 How can I manipulate fixed-record-length files?


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5.8: How can I manipulate fixed-record-length files?

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

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
my $PS_T = 'A6 A4 A7 A5 A*';
open my $ps, '-|', 'ps';
print scalar <$ps>;
my @fields = qw( pid tt stat time command );
while (<$ps>) {
my %process;
@process{@fields} = unpack($PS_T, $_);
for my $field ( @fields ) {
print "$field: <$process{$field}>\n";
print 'line=', pack($PS_T, @process{@fields} ), "\n";

We've used a hash slice in order to easily handle the fields of each
row. Storing the keys in an array means it's easy to operate on them as
a group or loop over them with for. It also avoids polluting the program
with global variables and using symbolic references.


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