SNMP Poller tool to monitor any thing on network

Few years ago, I had created one SNMP Poller tool using perl and snmp utilites that can poll OID informations from any network devices, which is kind of passive monitoring mechanism.

Thought to make it Opensource under GNU GPL.

The details of the utility with its usage can be found here, if anyone is interested to use it.

Nagios Plugin Developed@NagiosExchange

Long ago, while working in one of the previous organization, there were lots of components like services and servers running in production environment. I had deployed all products one by one from scratch and the count kept on increasing. There were components like PLM Servers, DB Server, License Mgmt, internal portal, Cotainer based virtualization system and a lots.

But there was no proper tools to monitor all the components at a time. As the count kept increasing , it becomes difficult to keep an eye on UP/DOWN time of all.

So I decided to deploy Nagios Monitoring system in the Data Center and developed many plugins to use.

I have opensourced few of the plugins, which I thought can help other people in world, those may facing these kind of challenges.

Also I posted them on Nagios Exchange on 4 years ago and now they are huge success. They each are downloaded 50k+ times  and I received many thanks from many people from around the world and feel happy.

They can be found from here:

Fix SSH Issue : Offending key for IP in ~/.ssh/known_hosts:n

When ever we connect to another host with ssh client, it asks the user to add the public key of the remote host in the ~/.ssh/known_hosts file of that user. This is by default enabled by “StrictHostKeyChecking yes” which can be altered to no in /etc/ssh/ssh_config file.

But sometimes what happened, the host name remain same, but the key value might have been changed, due to some reason like : may be a new system with same host name OR the system may be compromised OR any other reason, the error will come as the fingerprint will mismatch:

Offending key for IP in ~/.ssh/known_hosts:n

Where ‘n’ is the line number in that file.

So in this case manually open the   ~/.ssh/known_hosts and remove the line number “n”.

Or a simply perl pie will search and replace the line with an empty.

  perl -pi -e ‘s/\Q$_// if ($. == n);’ ~/.ssh/known_hosts

One liner tricks using perl/bash

Sometime small small hacks saves a lot of our time. The regexes used here can be changed as per need. In some cases try alternatively with single/double quote if dont work.

Count the number of times a specific character appears in each line

  • This counts the number of quotation marks in each line and prints it
    perl -ne ‘$cnt = tr/”//;print “$cnt\n“‘ inputFileName.txt

Add string to beginning of each line

  • Adds string(slc) to each line,
    perl -pe “s/(.*)/slcn\$1/” in.txt > out.txt

 Add string to end of each line

  • Append a string( to each line
     perl -pe “s/(.*)/\$” in.txt > out.txt

Print only alternate values in a list

  • Sometime froma list we have to print only alternate values, We can use the special $| of perl which stores onlt 1 or 0, Below will print the alternate from a list (a..z ) starting with a.
    perl -E  ‘say grep –$|, a..z’

Print only some columns of a file

  • Columns separated by a space
    cut fileWithLotsOfColumns.txt -d” “ -f 1,2,3,4 > fileWithOnlyFirst4Cols.txt

Print all columns except the first

  • cut -d” “ -f 1 –complement filename > filename.

 Replace a pattern with another one inside the file with backup

  • Replace all occurrences of pattern1 (e.g. [0-9]) with pattern2
    perl -p -i.bak -w -e ‘s/pattern1/pattern2/g’ inputFile

 Print only non-uppercase letters

  • Go through file and only print words that do not have any uppercase letters.
    perl –ne ‘print unless m/[A-Z]/’ allWords.txt > allWordsOnlyLowercase.txt

 Print one word per line

  • Go through file, split line at each space and print words one per line.
    perl –ne ‘print join(“\n”, split(/ /,$_));print(“\n”)’ someText.txt > wordsPerLine.txt

 Kill all screen sessions (no remorse)

  • Since there’s no screen command that would kill all screen sessions regardless of what they’re doing, here’s a perl one-liner that really kills ALL screen sessions without remorse.
    screen -ls | perl –ne ‘/(\d+)\./;print $1’ | xargs -l kill –9
  • The killall command may also do the job…

Return all unique words in a text document (divided by spaces), sorted by their counts (how often they appear)

  • assuming no punctuation marks:
    perl -ne ‘print join(“\n“, split(/\s+/,$_));print(“\n“)’ documents.txt > wordsOnePerLine.txt
    cat wordsOnePerLine.txt | sort | uniq -c  | sort -n > wordCountsSorted.txt

 Delete all special characters

  • delete every character that is not a letter, white space or line end (replace with nothing)
    perl -pne ‘s/[^a-zA-Z\s]*//g’ text_withSpecial.txt > text_lettersOnly.txt

 Lower case everything

  • perl -pne ‘tr/[A-Z]/[a-z]/’ textWithUpperCase.txt > textwithoutuppercase.txt;

Combine lower-casing with word counting and sorting

  • perl -pne ‘tr/[A-Z]/[a-z]/’ sentences.txt | perl –ne ‘print join(“\n”, split(/ /,$_));print(“\n”)’ | sort | uniq -c | sort -n

 Print only one column

  • Print only the second column of the data when using tabular as a separator
    perl –ne ‘@F = split(“\t”, $_); print “$F[1]”;’ columnFileWithTabs.txt > justSecondColumn.txt

 Print only text between tags

  • perl –ne ‘if (m/\<a\>(.*?)\<\/a\>/g){print “$1\n”}’ textFile
  • Extracting multiple multiline patterns between a start and an end tag
    • Here, we want to extract everything between <parse> and </parse>.
    • #!/usr/bin/perl -w
      local $/;open(DAT, “yourFile.xml”) || die(“Could not open file!”);
      my $content = <DAT>;while ($content =~ m/<parse>(.*?)<\/parse>/sg){
      print “$1\n

Sort lines by their length

  • perl -e ‘print sort {length $a <=> length $b} <>’ textFile

Print second column, unless it contains a number

  • perl -lane ‘print $F[1] unless $F[1] =~ m/[0-9]/’ wordCounts.txt

Trim/ Collapse white spaces and replace new lines by something else

  • echo “The cat sat    on    the  mat
    asd  sad  das   “
    | perl  –ne ‘s/\n/ /; print $_; print(“;”)’ | perl –ne ‘s/\s+/ /g; print $_’

Get the average of one column from certain lines

  • grep “another criterion” thisDataFile.txt |  perl –ne ‘@F = split(“,”, $_); print “$F[29]\n”;’ | awk ‘{sum+=$1} END { print “Average = “,sum/NR}’

How to sort a file by a column

  • Columns are separated by a space, we sort numerically (-n) and we sort by the 10’th column (-k10)
  • sort -t’ ‘ -n -k10 eSet1_both.txt

 Replace specific space but also copy a group of matches

  • matches a group of numbers in the beginning of a line
  • perl -p -i.bak -w -e ‘s/^([0-9]+) “/$1\t”/g’ someFile.txt