Is Perl faster than bash?


Question

I have a bash script that cuts out a section of a logfile between 2 timestamps, but because of the size of the files, it takes quite a while to run.

If I were to rewrite the script in Perl, could I achieve a significant speed increase - or would I have to move to something like C to accomplish this?

#!/bin/bash

if [ $# -ne 3 ]; then
  echo "USAGE $0 <logfile(s)> <from date (epoch)> <to date (epoch)>"
  exit 1
fi

LOGFILES=$1
FROM=$2
TO=$3
rm -f /tmp/getlogs??????
TEMP=`mktemp /tmp/getlogsXXXXXX`

## LOGS NEED TO BE LISTED CHRONOLOGICALLY
ls -lnt $LOGFILES|awk '{print $8}' > $TEMP
LOGFILES=`tac $TEMP`
cp /dev/null $TEMP

findEntry() {
  RETURN=0
  dt=$1
  fil=$2
  ln1=$3
  ln2=$4
  t1=`tail -n+$ln1 $fil|head -n1|cut -c1-15`
  dt1=`date -d "$t1" +%s`
  t2=`tail -n+$ln2 $fil|head -n1|cut -c1-15`
  dt2=`date -d "$t2" +%s`
  if [ $dt -ge $dt2 ]; then
    mid=$dt2
  else
    mid=$(( (($ln2-$ln1)*($dt-$dt1)/($dt2-$dt1))+$ln1 ))
  fi
  t3=`tail -n+$mid $fil|head -n1|cut -c1-15`
  dt3=`date -d "$t3" +%s`
  # finished
  if [ $dt -eq $dt3 ]; then
    # FOUND IT (scroll back to the first match)
    while [ $dt -eq $dt3 ]; do
      mid=$(( $mid-1 ))
      t3=`tail -n+$mid $fil|head -n1|cut -c1-15`
      dt3=`date -d "$t3" +%s`
    done
    RETURN=$(( $mid+1 ))
    return
  fi
  if [ $(( $mid-1 )) -eq $ln1 ] || [ $(( $ln2-1)) -eq $mid ]; then
    # FOUND NEAR IT
    RETURN=$mid
    return
  fi
  # not finished yet
  if [ $dt -lt $dt3 ]; then
    # too high
    findEntry $dt $fil $ln1 $mid
  else
    if [ $dt -ge $dt3 ]; then
      # too low
      findEntry $dt $fil $mid $ln2
    fi
  fi
}

# Check timestamps on logfiles
LOGS=""
for LOG in $LOGFILES; do
  filetime=`ls -ln $LOG|awk '{print $6,$7}'`
  timestamp=`date -d "$filetime" +%s`
  if [ $timestamp -ge $FROM ]; then
    LOGS="$LOGS $LOG"
  fi
done

# Check first and last dates in LOGS to refine further
for LOG in $LOGS; do
    if [ ${LOG%.gz} != $LOG ]; then
      gunzip -c $LOG > $TEMP
    else
      cp $LOG $TEMP
    fi
    t=`head -n1 $TEMP|cut -c1-15`
    FIRST=`date -d "$t" +%s`
    t=`tail -n1 $TEMP|cut -c1-15`
    LAST=`date -d "$t" +%s`
    if [ $TO -lt $FIRST ] || [ $FROM -gt $LAST ]; then
      # This file is entirely out of range
      cp /dev/null $TEMP
    else
      if [ $FROM -le $FIRST ]; then
        if [ $TO -ge $LAST ]; then
          # Entire file is within range
          cat $TEMP
        else
          # Last part of file is out of range
          STARTLINENUMBER=1
          ENDLINENUMBER=`wc -l<$TEMP`
          findEntry $TO $TEMP $STARTLINENUMBER $ENDLINENUMBER
          head -n$RETURN $TEMP
        fi
      else
        if [ $TO -ge $LAST ]; then
          # First part of file is out of range
          STARTLINENUMBER=1
          ENDLINENUMBER=`wc -l<$TEMP`
          findEntry $FROM $TEMP $STARTLINENUMBER $ENDLINENUMBER
          tail -n+$RETURN $TEMP
        else
          # range is entirely within this logfile
          STARTLINENUMBER=1
          ENDLINENUMBER=`wc -l<$TEMP`
          findEntry $FROM $TEMP $STARTLINENUMBER $ENDLINENUMBER
          n1=$RETURN
          findEntry $TO $TEMP $STARTLINENUMBER $ENDLINENUMBER
          n2=$RETURN
          tail -n+$n1 $TEMP|head -n$(( $n2-$n1 ))
        fi
      fi
    fi
done
rm -f /tmp/getlogs??????

1
11
8/27/2009 8:19:37 PM

Accepted Answer

Updated script based on Brent's comment: This one is untested.

#!/usr/bin/perl

use strict;
use warnings;

my %months = (
    jan => 1, feb => 2,  mar => 3,  apr => 4,
    may => 5, jun => 6,  jul => 7,  aug => 8,
    sep => 9, oct => 10, nov => 11, dec => 12,
);

while ( my $line = <> ) {
    my $ts = substr $line, 0, 15;
    next if parse_date($ts) lt '0201100543';
    last if parse_date($ts) gt '0715123456';
    print $line;
}

sub parse_date {
    my ($month, $day, $time) = split ' ', $_[0];
    my ($hour, $min, $sec) = split /:/, $time;
    return sprintf(
        '%2.2d%2.2d%2.2d%2.2d%2.2d',
        $months{lc $month}, $day,
        $hour, $min, $sec,
    );
}


__END__

Previous answer for reference: What is the format of the file? Here is a short script which assumes the first column is a timestamp and prints only lines that have timestamps in a certain range. It also assumes that the timestamps are sorted. On my system, it took about a second to filter 900,000 lines out of a million:

#!/usr/bin/perl

use strict;
use warnings;

while ( <> ) {
    my ($ts) = split;
    next if $ts < 1247672719;
    last if $ts > 1252172093;
    print $ts, "\n";
}

__END__
12
7/15/2009 11:09:56 PM

Perl is absurdly faster than Bash. And, for text manipulation, you can actually achieve better performances with Perl than with C, unless you take time to write complex algorithms. Of course, for simple stuff C can be unbeatable.

That said, if your "bash" script is not looping, just calling other programs, then there isn't any gain to be had. For example, if your script looks like "cat X | grep Y | tr -f 3-5 | sort | uniq", then most of the time is spent on cat, grep, tr, sort and uniq, NOT on Bash.

You'll gain performance if there is any loop in the script, or if you save multiple reads of the same file.

You say you cut stuff between two timestamps on a file. Let's say your Bash script looks like this:

LINE1=`grep -n TIMESTAMP1 filename | head -1 | cut -d ':' -f 1`
LINE2=`grep -n TIMESTAMP2 filename | head -1 | cut -d ':' -f 1`
tail +$LINE1 filename | head -$(($LINE2-$LINE1))

Then you'll gain performance, because you are reading the whole file three times: once for each command where "filename" appears. In Perl, you would do something like this:

my $state = 0;
while(<>) {
  exit if /TIMESTAMP2/;
  print $_ if $state == 1;
  $state = 1 if /TIMESTAMP1/;
}

This will read the file only once and will also stop once you read TIMESTAMP2. Since you are processing multiple files, you'd use "last" or "break" instead of "exit", so that the script can continue to process the files.

Anyway, seeing your script I'm positive you'll gain a lot by rewriting it in Perl. Notwithstanding the loops dealing with file names (whose speed WILL be improved, but is probably insignificant), for each file which is not fully inside or outside scope you do:

  1. Read the WHOLE file to count lines!
  2. Do multiple tails on the file
  3. Finish by "head" or "tail" the file once again

Furthermore, head your tails. Each time you do that, some piece of code is reading that data. Some of those lines are being read up to 10 times or more!


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