Run A Mysql Query From Command Line

To run a mysql query directly from command line, without entering the interactive mode, use -e flag, like below

$ mysql -u user -p -e 'show tables;' mydbname

In the above example, the output would be, a list of tables inside mydbname, displayed on the command line, after you have put in the mysql user password.

Downloading torrent using command line

To download torrents using a command line, the easiest tool to use is transmission-cli. This tool is available in debian based distro and redhat based distro alike.

To use this tool, you have to install it first.

In debian based distro:

$ sudo apt install transmission-cli -y

In redhat based distro:

$ sudo yum install transmission-cli -y 

To use it with torrent file, first download the torrent file, and then run transmission-cli against the file

$ wget https://releases.ubuntu.com/20.04/ubuntu-20.04.2-live-server-amd64.iso.torrent

$ transmission-cli ubuntu-20.04.2-live-server-amd64.iso.torrent

To use it with magnet link, just run transmission-cli against the magnet link

$ transmission-cli magnet:?xt=urn:btih:eb6354d8d9b9427458af8bee90457101a4c1e8e3&dn=archlinux-2021.05.01-x86_64.iso

Checking your public IP from terminal

Sometimes, when you are setting up a linux machine, there is a need to know what IP our machine is being connected to the public internet. There are many websites that provide this kind of service, but below are some of my favorites that work well with commands. 

To be able to use this, we need a text based web browser. We will use curl in this case. Install curl if your machine does not have one.

# yum install curl -y

To get our ip address, run curl like below. Choose whichever you like :).

$ curl ipecho.net/plain

210.210.210.210

$ curl icanhazip.com

210.210.210.210

The best, in my opinion is ipinfo.io, because it will give you a nicely formatted information regarding your public IP address, and if you want a specific info, just put it as a slash being ipinfo.io. For example, if you want to know what is your city, just curl ipinfo.io/city

$ curl ipinfo.io

{

  "ip": "210.210.210.210",

  "city": "Kuala Lumpur",

  "region": "Kuala Lumpur",

  "country": "MY",

  "loc": "3.1390,101.6869",

  "org": "AS4818 Digi Telecommunications Sdn. Bhd.",

  "postal": "50505",

  "timezone": "Asia/Kuala_Lumpur",

  "readme": "https://ipinfo.io/missingauth"

}

$ curl ipinfo.io/ip

210.210.210.210

$ curl ipinfo.io/city

Kuala_Lumpur

Creating image from a running instance

Sometimes we want to spawn off a few new instances, with the same spec and operating systems, but we do not want to go through the hassle of setting up each OS manually, and then update it one by one. In order to do that efficiently, openstack provides a very good way, which is to create an image from a running instance, and this image can be used to spawn off new instances afterwards.

Before we turn any instance to an image, we need to know its instance ID

$ openstack server list

We can then create an image from the above instance ID

$ openstack server image create --name centos7-updated-20180525 21e78f23-8b67-423a-9622-d46c8487f829

To make sure our image is created correctly, check using:

$ openstack image list

To create a new instance from the image, please refer here

Creating a new instance on openstack

In order to create new instance (it is called server in openstack command), you need to know beforehand a few information to feed to the create instance command. Refer below for those information:

check available flavor

$ openstack flavor list

check available images

$ openstack image list

check available network

$ openstack network list

check available security group

$ openstack security group list

check available keypair

$ openstack keypair list

Once you get all the above information, to create the new instance, just use below command, providing the above information as option to openstack server create command

$ openstack server create \

--image centos-7-20180520 \

--key-name my-keypair \

--flavor m1.medium \

--security-group defaults \

--network private-140 \

thenewinstancename

To check whether your new instance has been created and active:

$ openstack server list

The definition of ls column headers in long listing mode

Thanks to http://www.unix.com/unix-dummies-questions-answers/41832-ls-l-column-headings.html, I have managed to find out what is the meaning of all the column header in ls -l command. Below are the details:

# ls -l
total 52
drwxrwxrwx   2 root root    41 Jan 14  2013 archive
drwxr-xr-x   2 root root  3072 Aug  3  2013 bin
-rw-r--r--   1 root root   578 Jan 14  2013 README-archive.txt 

lrwxrwxrwx   1 root root     3 Feb 17  2013 run -> tmp

1st column is file type(d for DIR, - for Files, l for links) and access details for UGO (User Group Others aka file permission).

2nd column is Number of links (2,1), the number of names there are for the file. Generally an ordinary file will only have one link, but a directory will have more, because you can refer to it as ``dirname'', ``dirname/.'' where the dot means ``current directory'', and if it has a subdirectory named ``subdir'', ``dirname/subdir/..'' (the ``..'' means ``parent directory'').

3rd Column is File/directory owner (root)

4th Column is File/directory group (root)

5th Column is Size of the file and Dir (41, 3072)

6th Column is Date the file / DIR created. ( Jan 14  2013)

7th Column is File Directory name (archive, bin)

Using curl to check on site availability

The easy way to check if a site is available, is by using curl. For example, if you want to check whether www.google.com is available or not, jut run:

$ curl -I www.google.com

HTTP/1.1 200 OK

Date: Thu, 25 Apr 2013 06:03:06 GMT

Expires: -1

Cache-Control: private, max-age=0

Content-Type: text/html; charset=ISO-8859-1

Set-Cookie: PREF=ID=f4eacf44ddfe9913:FF=0:TM=1366869786:LM=1366869786:S=pD4jQT9xbgTOjuKG; expires=Sat, 25-Apr-2015 06:03:06 GMT; path=/; domain=.google.com

Set-Cookie: NID=67=KULBN37y3Mw7TIYNurxqV3L9OAm0gaj4VhRxz0_OsayoTS8C7nPN9QLCMovAzkVxhKfoop1EcHjWiBWjv7Vxl2C5iQ-Z8J0zcVtv4YfrJXs2ypRegbp2Y8MPcJjTyX1p; expires=Fri, 25-Oct-2013 06:03:06 GMT; path=/; domain=.google.com; HttpOnly

P3P: CP="This is not a P3P policy! See http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=151657 for more info."

Server: gws

X-XSS-Protection: 1; mode=block

X-Frame-Options: SAMEORIGIN

Transfer-Encoding: chunked 

with -I for fetching the HTTP-header only. In this case the return code is 200, and it means the site is available.

sudo: unable to execute /bin/rm: Success

I found a weird error message when I tried to delete some files in my linux box today. The command I used was

sudo rm -rf *.js

to delete some .js files, and I got an error message:

sudo: unable to execute /bin/rm: Success
After googling around, I found this wonderful site, and the solution is to use find coupled with -exec flag rather than rm:
sudo find . -iname '*.js' -exec rm {} \;
The problem is, according to the site, was caused by buffer overflow when expanding the *. 
The single quote that we used in the find command will prevent it from expanding and causing overflow.

So, that's all folks.