Why am I getting "Command 'deb' not found"?

deb is not a command. It is used in /etc/apt/sources.list file to indicate a Debian software repository.

From Ubuntu Manpage - sources.list:

The source list is designed to support any number of active sources and a variety of source media. The file lists one source per line, with the most preferred source listed first. The format of each line is: type uri args. The first item, type determines the format for args. uri is a Universal Resource Identifier (URI), which is a superset of the more specific and well-known Universal Resource Locator, or URL.

The deb type describes a typical two-level Debian archive, distribution/component. The format for a sources.list entry using the deb and deb-src types is:

deb [ options ] uri distribution [component1] [component2] [...]

The URI for the deb type must specify the base of the Debian distribution, from which APT will find the information it needs. distribution can specify an exact path, in which case the components must be omitted and distribution must end with a slash (/). This is useful for when the case only a particular sub-section of the archive denoted by the URI is of interest. If distribution does not specify an exact path, at least one component must be present.

So, if I have deb http://archive.ubuntu.com/ubuntu/ quantal main restricted in the sources.list file - it says: I have a Debian archive which is based on http://archive.ubuntu.com/ubuntu/, the distribution is quantal and the components are main and restricted.

Like the answer by @Eric Carvalho deb is not command line If you have deb then url like this:

deb http://download.virtualbox.org/virtualbox/debian trusty contrib 

Edit

Like commit of @muru, you need to create new file with the extension .list into /etc/apt/source.list.d/ folder:

Example: I want to download Oracle virtualbox, create new file :

sudo gedit /etc/apt/sources.list.d/oracle-virtualbox-trusty.list 

Then copy and paste the line of deb into this file

deb is not a unix command. If you have a line like the following (source for docker):

deb https://apt.dockerproject.org/repo ubuntu-xenial main

it is a line that must be available in your ubuntu sources.list so that apt-get can find future packages from this new source.

However, it's not a good practice to edit the /etc/apt/sources.list file directly. Instead add the deb line as an entry to a new .list file inside the /etc/apt/sources.list.d/ directory. We will create a docker.list file like this:

echo "deb https://apt.dockerproject.org/repo ubuntu-xenial main" | sudo tee /etc/apt/sources.list.d/docker.list

Once done, remember to perform a sudo apt-get update and you should now be able to find new packages from this source easily.

On Ubuntu, you don't have to manually edit the source.list to add the repository. Instead, you can use add-apt-repository, as you would do for a PPA.

For example, to add the LLVM repository, you can call:

sudo add-apt-repository 'deb http://apt.llvm.org/trusty/ llvm-toolchain-trusty main'

As you can see, we must pass the deb line as a single argument to the command.

Deb isn't actually a command (I thought so too, at first) -- Assuming you are trying to download/install a deb, do this (for example):

wget http://whatever.com/whatever.deb

then dpkg -i whatever.deb

then run the commands:

sudo apt-get update

This is not the CLI command.

This is information about some Debian package repository (used also by Ubuntu).

If you want to use this repository:

1. Open the file /etc/apt/sources.list with administrator privileges.

2. Add to this file a line with information about the repository starting with deb ...

3. Save the file.

4. Run this command:

   apt-get update

Now you will be able to use this repository.