Globalways Blog - Scripting (Bash/Perl)

Strace with multiple processes

nospam@example.com (Uli Stärk) — Wed, 25 Aug 2010 09:48:01 +0200

Usually, you trace a single process that does not give a helpful error message. But if you have to trace e.g. a webserver with multiple processes you have to type a lot. Here is a one-liner that makes it a little bit easier:

strace -f -p $(ps -ea f| grep httpsd | cut -f 1 -d " " | xargs | sed -e 's/ / -p /g') 1>/tmp/strace.log 2>&1

Dumping json-data in shell

nospam@example.com (Uli Stärk) — Thu, 17 Jun 2010 15:58:09 +0200

When using web services, its sometimes hard to read the serialized json data. I wrote a small helper script to dump the contents

File: /usr/local/bin/json_decode

#!/usr/bin/php
<?php
error_reporting(E_ALL);
ini_set('display_errors',true);
if(!empty($argv[1])) {
        $json = file_get_contents($argv[1]);
}
else {
        $json = '';
        while(!feof(STDIN)) {
                $json .= fgets(STDIN, 1024);
        }
}

$data = json_decode($json);

if($data === null) {
        fwrite(STDERR, "Invalid json data\n");
        exit(1);
}

print_r($data);

Usage:

wget --quiet -O - "http://domain.tld/file.json" | json_decode

for-loop in bash

nospam@example.com (Uli Stärk) — Wed, 06 Aug 2008 14:45:30 +0200

If you want to update several hosts at once you can use a for-loop in a shell to execute a command on serveral hosts

for i in 1  3 `seq 4 7` 9; do HOST="1.2.3.$i"; ssh root@$HOST ; done;

As you can see it is very easy to create a custom set of items to loop through. On the one hand you can use any value seperated by a space or you can create a sequence of numbers with the seq-command.

Bash: Arithmetic operations using bash

nospam@example.com (Markus Binder) — Wed, 25 Jul 2007 12:06:17 +0200

When using the bash shell sometimes you need arithmetic calculations (like plus, minus, multiply, divide) on variables and numbers. Simple division by 5

i=10
echo $[$i/5];
2

Better use-case: do 10 times the same command

host ~ # i=0; while [ $i -lt 10 ]; do echo $i; i=$[$i+1]; done
0
1
2
3
4
5
6
7
8
9