Useful linux commands

OS Details

$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 10.04.4 LTS
Release: 10.04
Codename: lucid

System Information

$ uname -a
Linux p3307963.pubip.serverbeach.com 2.6.32-46-generic #108-Ubuntu SMP Thu Apr 11 15:56:25 UTC 2013 x86_64 GNU/Linux

$ uname -v

x86_64 (means system is 64 bit machine, otherwise its 32 bit machine)

User specific resource limit get/set

$ulimit -a
core file size          (blocks, -c) 0
data seg size           (kbytes, -d) unlimited
scheduling priority             (-e) 20
file size               (blocks, -f) unlimited
pending signals                 (-i) 16382
max locked memory       (kbytes, -l) 64
max memory size         (kbytes, -m) unlimited
open files                      (-n) 1024
pipe size            (512 bytes, -p) 8
POSIX message queues     (bytes, -q) 819200
real-time priority              (-r) 0
stack size              (kbytes, -s) 8192
cpu time               (seconds, -t) unlimited
max user processes              (-u) unlimited
virtual memory          (kbytes, -v) unlimited
file locks                      (-x) unlimited

Above command is very very important. Many a times TCP/IP socket connection grows a lot, if you sense that kind of problem, too many connections at a time, try to change "open files" limit to higher value, 20000 is  good enough. How to change the limit, you should read this article How to set ulimit in ubuntu/debian linux systems

Which user has opened how many files in sort order

$ lsof | awk '{if(NR>1) print $3}' | sort | uniq -c | sort -nr
   1256 root
    655 nishal
     16 www-data
      4 syslog
      4 ntp
      4 daemon

Check listening ports
$ netstat -nlp

change the open file limit in debian/ubuntu/linux system

For every user there is a resource limit configuration in linux. If it is not specified, it picks the default values, you can check them by command 
$ ulimit -a

Change Open file limit, ulimit on Debian/Ubuntu/Linux systems

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Now to change it first of all pam limits by default is not loaded in ubuntu

$ vi /etc/pam.d/su
Un-comment the following line
#session    required   pam_limits.so
to
session    required   pam_limits.so

Now
$ vi /etc/security/limits.conf

and add the following lines to the end of the file (before the line # End of file)
*       soft  nofile   16000                                                      
*       hard nofile  64000

Save the file and quit vi.

Now To bring this in affect, you must restart the machine.

Below two lines will change the limit only for mysql user
mysql               soft    nofile          10240
mysql               hard   nofile          10240

Usually we need to change the "open file" limit from 1024(which is default) to higher value. But before that you should check which user is opening how many files.
$ lsof | awk '{if(NR>1) print $3}' | sort | uniq -c | sort -nr
   1256 root
    655 nishal
     16 www-data
      4 syslog
      4 ntp
      4 memcache
      4 daemon

The above result clearly says that root user has opened 1256 files, in such cases default limit of 1024 will start creating IO wait issue, connection timeout issue. To fix them, above article is beautiful.

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How to change Open file limit, ulimit on CentOS/Fedora/Red Hat

Command to check
ulimit -n
ulimit -a


1. vi sysctl.conf  and add this line  fs.file-max = 65536

2. vi /etc/security/limits.conf
*          soft     nproc          16384
*          hard     nproc          65535
*          soft     nofile         16384
*          hard     nofile         65535

3. Restart the server

4. ulmiit -n
16384

Updating and installing package on debian machine

Its very simple :)
APT (Advance Package Tool) is a free user interface which is used in debian machine to install/remove/update any software. For the same in Red Hat machine is "yum".

How to search a package and Install?

Points :

1. $ apt-cache search "Is the command to search a package"
2. $ apt-get install "Is the command to install a package"
3. apt-cache - query the APT cache
4. apt-cache search/madison are two important commands you should know
5. $ apt-cache search - "Performs a full text search on all available package lists for the POSIX regex pattern given"
6. $ apt-cache madison - "Command attempts to mimic the output format and a subset of the functionality of the Debian archive management tool, madison. It displays available versions of a package in a tabular format"
7. sudo apt-get install =version - "Is the command to install a package with a certain version"
8. dpkg -s - "Is the command to about the package"

Example :

$ apt-cache search mysql-server
cacti - Frontend to rrdtool for monitoring systems and services
phpbb2-conf-mysql - Automatic configurator for phpbb2 on MySQL database
torrentflux - web based, feature-rich BitTorrent download manager
mysql-server - MySQL database server (meta package depending on the latest version)
mysql-server-5.0 - MySQL database server binaries

$ apt-cache madison mysql-server
mysql-server | 5.0.96-0ubuntu3 | http://us.archive.ubuntu.com hardy-security/main Packages
mysql-server | 5.0.96-0ubuntu3 | http://us.archive.ubuntu.com hardy-updates/main Packages
mysql-server | 5.0.51a-3ubuntu5 | http://us.archive.ubuntu.com hardy/main Packages

Now if you want to install a certain version package, you should use
$ apt-get install mysql-server=5.0.96-0ubuntu3

Command to know about the package
$ dpkg -s mysql-server

References

1. http://manpages.ubuntu.com/manpages/natty/man8/apt-cache.8.html

2. http://manpages.ubuntu.com/manpages/hardy/man8/apt-get.8.html