Globalways Blog - Linux

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

Find symbolic links

nospam@example.com (Uli Stärk) — Thu, 12 Nov 2009 16:59:22 +0100

find -lname "*" | xargs ls -l

Flushing swap space

nospam@example.com (Uli Stärk) — Thu, 16 Jul 2009 13:31:12 +0200

From time to time machines use some swap space due to heavy load. But Linux doesn't free the swap space itself if enough memory is available.

An easy method to flush all disk swap to memory is to disable and re-enable the swap. Eg:

# free -tm
             total       used       free     shared    buffers     cached
Mem:          4096       3269        826          0         31         76
-/+ buffers/cache:       3161        934
Swap:          509        114        395
Total:        4605       3384       1221

# swapoff /dev/xvdb1
# swapon /dev/xvdb1

# free -tm
             total       used       free     shared    buffers     cached
Mem:          4096       3375        720          0         31         78
-/+ buffers/cache:       3265        830
Swap:          509          0        509
Total:        4605       3375       1230

Uptime-Devil

nospam@example.com (Uli Stärk) — Wed, 17 Jun 2009 18:24:24 +0200

18:23:51 up 666 days,  2:55,  1 user,  load average: 0.01, 0.02, 0.00

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.

Locales support

nospam@example.com (Uli Stärk) — Fri, 23 Nov 2007 12:52:28 +0100

I tried setting up a new blog that should display german date format, but none of the (usually valid) locales worked. The php function setlocale() always returned false. So I searched and found a debian package that fixed my problem, by installing all locales:

apt-get install locales-all

Setting up a MoinMoinWiki

nospam@example.com (globalways Admin) — Sat, 08 Sep 2007 17:45:03 +0200

As the MediaWiki is not my favorite choice, when installing an wiki - MoinMoinWiki is currently my first choice, because it offers some kind of Page-Hierarchy to create a simple structure in your wiki articles. Installing and configuring the wiki is quite easy on a debian system.
"Setting up a MoinMoinWiki" vollständig lesen

Removing files and free disk space

nospam@example.com (globalways Admin) — Fri, 07 Sep 2007 15:53:52 +0200

Today I noticed, that the free disk space on one of our servers went low. A quick look pointed to a very large mysqld.err logfile.

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              45G   41G  4.4G  91% /
# cd /var/log/mysql
# ls -al
total 27062512
drwxr-x--- 2 mysql mysql          82 Jul 17 00:05 .
drwxr-xr-x 9 root  root         4096 Aug 29 19:32 ..
-rw-r--r-- 1 mysql mysql           0 Jul 17 00:05 .keep_dev-db_mysql-0
-rw-rw---- 1 mysql mysql           0 Jul 17 00:06 mysql.err
-rw-rw---- 1 mysql mysql           0 Jul 17 00:06 mysql.log
-rw-rw---- 1 mysql mysql 27712011858 Sep  7 15:32 mysqld.err

So after resolving the spamming issue I removed the error logfile.

# rm mysqld.err
# /etc/init.d/syslog-ng restart
 \* Stopping syslog-ng ... [ ok ]
 \* Starting syslog-ng ...  [ ok ]
# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              45G   41G  4.4G  91% /
udev                  512M   24K  512M   1% /dev

Huh? No effect on the free disk space? The problem is, that mysqld had still an open filehandle on the error logfile. So the filesystem didn't free the disk space. Restarting mysqld resolved this issue.

# /etc/init.d/mysql restart
 \* Stopping mysql ... [ ok ]
 \* Stopping mysqld (0)  [ ok ]
 \* Starting mysql ...
 \* Starting mysql (/etc/mysql/my.cnf)  [ ok ]

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/sda1              45G   15G   31G  33% /
udev                  512M   24K  512M   1% /dev

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