deb: command not found and add-apt-repository: command not found: How to Add an APT Repository

Applies to: Debian 13 and 12, Ubuntu 26.04 and 24.04; tested with apt 3.0.3 on Debian 13.7, software-properties-common 0.120.1 on Ubuntu 26.04.1 and 0.99.49.4 on Ubuntu 24.04.5

deb: command not found means a repository line was typed into the shell. A line such as deb https://nginx.org/packages/debian trixie nginx is not a command: it belongs in a file in /etc/apt/sources.list.d/. The similar error add-apt-repository: command not found means the helper tool isn’t installed, and on Debian 13 it doesn’t exist at all.

What the errors look like:

bash: deb: command not found
sudo: deb: command not found
bash: add-apt-repository: command not found

Add the repository by hand (works everywhere)

This works on every current Debian and Ubuntu release and needs no extra tools. The example adds the official nginx.org repository on Debian 13; take the address, suite, component and key from the instructions of the software you are installing.

1. Download the repository’s signing key into /etc/apt/keyrings, the folder the apt manual recommends for keys you manage yourself:

   sudo install -d -m 0755 /etc/apt/keyrings
   sudo curl -fsSL https://nginx.org/keys/nginx_signing.key -o /etc/apt/keyrings/nginx.asc

   If curl is missing, install it first with sudo apt install curl. apt reads the text key (-----BEGIN PGP PUBLIC KEY BLOCK-----) directly, so you don’t need gpg --dearmor.
2. Create /etc/apt/sources.list.d/nginx.sources with this content:

   Types: deb
   URIs: https://nginx.org/packages/debian/
   Suites: trixie
   Components: nginx
   Signed-By: /etc/apt/keyrings/nginx.asc

3. Update the package lists and check that the new repository is used:

   $ sudo apt update
   $ apt-cache policy nginx
   nginx:
     Installed: (none)
     Candidate: 1.30.5-1~trixie

If the instructions give you a one-line deb ... entry, you can also save it as a .list file, with the key in square brackets:

echo "deb [signed-by=/etc/apt/keyrings/nginx.asc] https://nginx.org/packages/debian trixie nginx" | sudo tee /etc/apt/sources.list.d/nginx.list

The apt manual calls the one-line format deprecated, but it still works. The file name must end in .list for one-line entries and .sources for the block format. On Debian 13, sudo apt modernize-sources converts old .list files and keeps a .list.bak copy.

Install add-apt-repository on Ubuntu

On Ubuntu, the tool is in the software-properties-common package:

sudo apt update
sudo apt install software-properties-common
sudo add-apt-repository ppa:git-core/ppa

On Ubuntu 26.04 and 24.04 it writes a .sources file (for example git-core-ubuntu-ppa-resolute.sources) with the signing key built in, and refreshes the package lists. To take a repository away again: sudo add-apt-repository --remove ppa:git-core/ppa.

add-apt-repository on Debian

• Debian 13: software-properties-common isn’t in the archive (packages.debian.org lists it for Debian 11, 12 and unstable only), so apt install can’t find it. Add repositories by hand as shown above, or use Debian’s extrepo tool for well-known third-party repositories: sudo apt install extrepo, then sudo extrepo enable nginx. In our test it created /etc/apt/sources.list.d/extrepo_nginx.sources with the key already in place.
• Debian 12: the package can still be installed (version 0.99.30), but the manual method is simpler.
• Don’t add Ubuntu PPAs to Debian. PPAs are built for Ubuntu releases, and mixing them into Debian can break updates.

Errors after adding a repository

• Missing key 8540A6F1..., which is needed to verify signature followed by The repository ... is not signed: the key file is missing or the Signed-By path is wrong. Debian 13 checks signatures with sqv, which is why the message names /usr/bin/sqv.
• The repository '... trixi Release' does not have a Release file: the suite name is wrong (here a typo for trixie), or the repository doesn’t support your release. Check your codename with cat /etc/os-release.

If you have a .deb file instead

A downloaded package file is not a program either. Running ./hello_2.10-5_arm64.deb gave syntax error near unexpected token `newline'. Install it with apt, which also pulls in its dependencies:

sudo apt install ./hello_2.10-5_arm64.deb

And if the error is apt: command not found, the server isn’t Debian or Ubuntu: see apt-get: command not found.

Related: add, enable and remove repositories on Ubuntu and convert between RPM and DEB packages.

Official documentation: sources.list(5) on Debian 13 and Debian wiki: using third-party repositories.