Ubuntu release/renew DHCP IP of a specific interface from CLI

To renew the DHCP lease on the wlp0s20f3 interface in Ubuntu Desktop, you can use the dhclient command. Here’s how:

Open a terminal and type the following command:

sudo dhclient -r wlp0s20f3

This command sends a DHCP release message to the DHCP server, effectively releasing the current DHCP lease on the specified interface.

Then, to obtain a new DHCP lease, type the following command:

sudo dhclient wlp0s20f3

This command requests a new DHCP lease for the specified interface.

After running these commands, your network interface should have a renewed DHCP lease, and it should be able to connect to the network using the new lease.

Scheduling conditional statements with crontab

 

Recently while trying to auto-restart a daemon which is down by identifying with ps aux and if condition, it is not worked as intended with crontab.

I used following command which is perfectly running in from command line but not through crontab.

if [ `ps aux | grep nrpe | grep -v grep | wc -l` -eq 0 ]; then service nagios-nrpe-server restart ;fi

After trying with different commands following thing worked for me:

pgrep nrpe; [ $? != 0 ] && /etc/init.d/nagios-nrpe-server restart

where pgrep returns non zero exit code if the process nrep is not running and with $? is used to get the exit code of previous command (in this case pgrep) and start the process.

Lists/Remove all empty files in a directory

 

To list all empty files in a directory use follwoing command:

for file in `ls`; do if [[ ! -s $file ]]; then echo $file; fi; done

To remove all empty files in a directory use following commad:

for file in `ls`; do if [[ ! -s $file ]]; then echo $file; rm $file; fi; done

Username and password with wget/curl

 

To download a url with basic authentication we need to pass username & password.

To acheive this functionality with wget or curl observer following examples:

With curl:

$ curl -u 'username:password' 'http://www.example.com'

With wget:

$ wget --user='username' --password='password' 'http://www.example.com'

Wget with cookie:

Login to server & save cookie to a file:
$ wget --save-cookies cookiesFile.txt --post-data 'user=username&password=password' http://www.example.com/login.html

Download a page by using saved cookie file:
$ wget --load-cookies cookiesFile.txt -p http://www.example.com/somecontent.html

awk print Nth line after matching a pattern

 

By using awk we can print only Nth line after matching a pattern.

Lets say I have a file test.txt with following content and I want to extract 5 line after matching pattern.

$ cat test.txt

1
2
3
4
5
6
7
8
9

Use following awk code to extract Nth line:

Syntax:

$ awk 'c&&!--c;/pattern/{c=N}' file

where “pattern” is your input pattern and N is a line number to extract after matching pattern.

$ awk 'c&&!--c;/1/{c=5}' test.txt

Output:

6

In above example I am trying to extract 5th line after matching pattern, here my input pattern is 1 and 5th line after matching pattern is 6. So our output is 6.

Linux – check if a remote port is open

 

By using nc(Netcat) command we can check if a port in remote machine is opened or closed.

Here is an example to check if remote port is opened or closed:

STATUS=$(nc -z you_remote_host 80 ; echo $?)

if [ $STATUS -eq 0 ]
then
    echo "Port is opened"
else
    echo "Port is opened"
fi

bash – get previous command status

 

To get previous command status in bash use “$?” variable.

echo $? will return zero if previous command is success & if its non zero it indicates failure.

Example:

$ ls ; echo $?

Output of the above command is list of files, directories and next line it will return number.

We can use condition for check if previous command is success or failure:

any_command
if [ $? -eq 0 ] 
then
    echo SUCCESS
else
    echo FAIL
fi

awk – print last field

 

NF is built in variable in awk which stores Number of fields.

To get last field with awk use following example:

$ echo a b c | awk '{print $NF}'

Output:
c

By default awk will take any white space character(space, tab, …) to split.

To get last but one field with awk:

$ echo a b c | awk '{print $(NF-1)}'

Output:
b

To print only number of fields with awk:

$ echo a b c | awk '{print NF}'

Output:
3

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 192.168.1.1

Use following command to scan specific ports:

$ nmap -sS -O -p80,8080 192.168.1.1

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

$ sudo nmap -sS -O -p80-100 192.168.1.1

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" user@example.com -a attachment.txt < body.txt

or

$ echo "body text here" | mutt -s "PFA" user@example.com -a attachment.txt

Send mail without attachment:

$ mutt -s "PFA" user@example.com < body.txt

or

$ echo "body text here"  | mutt -s "PFA" user@example.com