How to clear swap space – Ubuntu/Linux


To clear swap space in Ubuntu/Linux we need to use swapoff command as shown below:

$ swapoff -a

Above command will turn of swap completely.

Before running swapoff command you need to have enough free RAM available, since swapoff command will move all swap data to RAM.

If you wont have enough free RAM this command may make your system unstable.

Again to turn on swap you need to use swapon command ash shown below:

$ swapon -a

You need to have root/sudo permissions to run above commands.

Number of open files or file descriptors associated with a process

Since in *nix every thing is a file, When you try to find open files or file descriptor associated with a process you will see Actual file log, Unix Sockets, Library files, Network Sockets,  /dev files and others.

lsof is a command to list all open files.

To list all open files or file descriptors by a process:

$ lsof -p PID


$ lsof -a -p PID


$ ls /proc/PID/fd

where PID is a process ID of any process that is running.

To count all open files by a process:

$ lsof -p PID | wc -l


$ ls /proc/PID/fd | wc -l

To list all open files:

$ lsof 

To count all open files:

$ lsof | wc -l

Nmap – Scan for open ports


nmap is a network exploration tool and security/port scanner.

We can list all open ports using nmap for a given IP/Host.

Use following command to list all open ports:

$ sudo nmap -v -sS

Use following command to scan specific ports:

$ nmap -sS -O -p80,8080

To can specific range of ports(scan from port 80 to 100):

$ sudo nmap -sS -O -p80-100

mutt – send mails with attachments


By using mutt we can send mails with attachments from command line.

Install mutt:

$ apt-get install mutt

Send mail with attachment:

$ mutt -s "PFA" -a attachment.txt < body.txt


$ echo "body text here" | mutt -s "PFA" -a attachment.txt

Send mail without attachment:

$ mutt -s "PFA" < body.txt


$ echo "body text here"  | mutt -s "PFA"

CentOS – Hostname Change


To change host name in CentOS use following steps:

Open /etc/sysconfig/network file

$ vim /etc/sysconfig/network

Now modifiy HOSTNAME= value is desired host for my machine, similarly we can use other valid domain names.

Next edit /etc/hosts file, here we need to change the host that is associated to your main IPaddress for your server. my

After completing above two changes restart network by using following command:

$ service network restart 

Now run hostname command:

$ hostname


Above procedure will work both in CentOS 5 and 6 versions.

HAProxy – Starting proxy webfarm: cannot bind socket

Today morning when I am trying to start haproxy in one of server I got following error:

[ALERT] 096/213653 (26549) : Starting proxy webfarm: cannot bind socket

I have spent around 2 hours to debug this issue.

Here is the solution:

HAProxy will try to bind to Virtual IP which will only available in active node. Since our virtual IP is already bind to active node, this server unable to bind.

To resolve this issue we need to append net.ipv4.ip_nonlocal_bind=1 in /etc/sysctl.conf file.

Open /etc/sysctl.conf and append following line:


After adding above line run following command to check if its updated or not:

$ sysctl -p


net.ipv4.ip_nonlocal_bind = 1

In output of sysctl -p we can see net.ipv4.ip_nonlocal_bind = 1.

Now start HAProxy:

$ service haproxy restart

* Restarting haproxy haproxy

After starting check HAProxy status:

$ service haproxy status

haproxy is running.

To find where is your virtual IP

To find to which host your virtual IP is bind use following command:

$ ip addr

In output check if you can find your virtual IP.

Start HAProxy with init script

To start HAProxy with init script set the ENABLED option to in /etc/default/haproxy file

Open /etc/default/haproxy file

$ vim /etc/default/haproxy

Append following line:


After adding above line restart HAProxy.

$ service haproxy restart 

* Restarting haproxy haproxy

vim convert to lowercase or uppercase


By using simple substitution we can convert  text in vim either to lowercase or to uppercase.

I have a following text in in one of file:


To convert complete text to uppercase use following substitution code:




Similarly to convert complete text to lowercase use substitution code:




Python convert string to list/tuple/dictionary

By using ast modules literal_eval function we can convert any python string to valid literal.

ast modules literal_eval function will only evaluates string that are valid python literals.

Lets try with an example:

I have file with some valid python literals and I want to evaluate each of them.

Content of my input file is following:

[1, 2, 3]
(1, 2, 3)
{‘a’:1, ‘b’:2, ‘c’:3}

Here I want to detect type of literal in each line.

First I will try with a simple example:

import os, sys
f = open(sys.argv[1], 'r')
lines = f.readlines()
print lines
for i in lines:
    print i.strip(), type(i)


[1, 2, 3] <type ‘str’>
(1, 2, 3) <type ‘str’>
{‘a’:1, ‘b’:2, ‘c’:3} <type ‘str’>
‘abc’ <type ‘str’>
123 <type ‘str’>

Just observer output, by default all lines are detected as string.

Now try with ast modules literal_eval function.

import os, sys, ast
f = open(sys.argv[1], 'r')
lines = f.readlines()
#print lines
for i in lines:
    print i, type(ast.literal_eval(i))


[1, 2, 3] <type ‘list’>
(1, 2, 3) <type ‘tuple’>
{‘a’:1, ‘b’:2, ‘c’:3} <type ‘dict’>
‘abc’ <type ‘str’>
123 <type ‘int’>

Now observer above output, each line is detected and converted as its own literal type.

I tested this example in Python 2.7.3

awk/sed print from line number to end of file

I have file with 10 lines where content of first line in 1, second line 2, third line is 4 …… and tenth line is 10.

Here I will show some operations related to read read in lines based their line numbers.

Print from line number 5 to end of file:

$ awk 'NR>=5' inputFile
$ sed '1,4d' inputFile

Print from line number 4 to 9 or print lines between linen number 3 and 10:

$ awk 'NR>=4 && NR<=9'  inputFile
$ sed -n '4,9p' inputFile

Print all even lines numbers:

$ awk '{if(NR%2 == 0) {print $1}}' inputFile

Print all odd lines numbers:

$ awk '{if(NR%2 != 0) {print $1}}' inputFile

Vim case insensitive search


By default vim editor is enabled to search case sensitive search.



Above example will looks for linux only not Linux or LINUX.

To enable case insensitive search use \c at start/end of your search key word.





Any of above example will ignore case and will search for linux.