...or enabling Neovim undercurl on Red Hat Enterprise Linux 7
Firstly what is 
and why would I want it? I believe undercurl is a powerful option for your editor to give you feedback on what you have been writing, and Mozilla, Microsoft, Google, etc. have all implemented it many products.
As I switch away from Visual Studio Code to Neovim, basing my config on the excellent video from ThePrimeagen, I have faced several issues getting a modern copy of Neovim (>0.9) running. The source of the issues is both the age of the server operating system - Red Hat Enterprise Linux 7 (RHEL 7 from now on), and that I cannot install any software as root. That limits me to local installs in my home directory.
To get undercurl working we need a terminal which supports it, and software which will use it.
Enabling undercurl
Background
To get started we need to talk about Terminals, and a bit of terminal history. Way back terminals were monochrome (no color yet!) freestanding, simple computers running a fixed ROM program made by various manufacturers to various standards. They connected to a central server using a dedicated serial line each, and supported displaying and typing characters. Pretty quickly various manufacturers implemented features like colors, bold, italic, more characters, etc. by means of special codes that do not printed to the screen but turn on and off the features. Even graphic support arrived, e.g. the Tektronix 4010.
With all that variety came a bunch of different features, novel bugs, and programmers soon grew very, very, tired with supporting all these different terminals. Each manufacturer had their individual quirks as well as standards they supported. To address this, software libraries were created where the programmer writes software to talk to the library, and the library queries the terminal for its feature set, and converts the program instructions into a stream of character codes for that specific terminal.
The most common software library used these days is ncurses, and its corresponding database of terminal capabilities is terminfo.
Testing your terminal for the undercurl feature
The code \e[4:3m starts the curly underline, and the code \e[0m resets everything to normal, stopping it. If you have undercurl support you will see this:
If you do not see this then your client terminal program (e.g. Cygwin mintty, Alacritty, etc.) does not support undercurl and you need to find either another client terminal program or fix your current one.
Neovim
I
have had experience with this issue before when supporting either very
old or newer proprietary CAD tools on Linux (and Solaris before that),
and believe me it is a tedious process.
Step one, prepare your environment:
- Create ~/.local/bin and ~/.local/lib directories
- Set environment variables in .bashrc (or equivalent) so any programs you run will look in these directories first:
- export PATH=${HOME}/.local/bin:${PATH}
- export LD_LIBRARY_PATH=${HOME}/.local/lib
- export GIT_EXEC_PATH=${HOME}/local/git-2.27/opt/rh/rh-git227/root/usr/libexec/git-core/ [I had to add this later to get git-2.27 running, but I have put this here to save you time]
Step two, iterate running nvim and downloading missing libraries:
- Execute nvim
- Note error message about a missing library or binary
- Search for missing library from RHEL 7 compatible CentOS SCLo repository or the project homepage
- Unpack the .rpm into a folder in my home directory
- e.g. rpm2cpio ../sclo-git25-git-core-2.5.5-1.2.el7.x86_64.rpm | cpio -idmv
- Symlink libraries and binaries into .local/bin or .local/lib
- e.g. cp --symbolic-link ${HOME}/local/git-2.27/opt/rh/rh-git227/root/usr/bin/* ${HOME/.local/bin/
- ... repeat until it works
Eventually I ended up installing the following (your list may well be different!):
- nvim-linux64.tar.gz - Neovim stable
- rh-git227-git-core-2.27.0-3.el7.x86_64.rpm - for lazy.vim plugin manager
- httpd24-libcurl-7.61.1-2.el7.x86_64.rpm - for git-2.27
- httpd24-libnghttp2-1.7.1-7.el7.x86_64.rpm - for git-2.27
- ripgrep-13.0.0-x86_64-unknown-linux-musl.tar.gz - for telescope.nvim plugin
- fd-v8.7.0-x86_64-unknown-linux-musl.tar.gz - for telescope.nvim plugin
Neovim undercurl support
So now load up Neovim and test undercurl support with the following two commands:
:hi Foo guifg=Red guibg=Blue gui=undercurl guisp=Green
:hi Foo
and check the output:
But ... that is not a curly underline! It is a straight underline! What is happening??
Well, the printf test statement forces the server to send the characters exactly as typed in the statement, which the terminal software recognizes and draws the curly underline below "Hello World" correctly. However Neovim is built on the ncurses library which looks up the terminal features by searching the terminfo data for the terminal named in the environment variable TERM. On RHEL 7 the database of terminals, terminfo, does not have ANY terminals listed that support undercurl. Why would it, as it was written before the undercurl feature existed. The upshot of this is that when you load Neovim ncurses finds no undercurl support and falls back to the available feature, a straight underline, instead.
The excellent Neovim documentation on this topic specifies a way around this problem by installing a local modern terminfo database into your home directory:
curl -LO https://invisible-island.net/datafiles/current/terminfo.src.gz
gunzip terminfo.src.gz
tic terminfo.srcHowever if you try this tic, the terminfo compiler, will generate maybe a hundred errors of the form:
"terminfo.src", line 12153, col 29, terminal 'wy60': unknown capability 'kF3'
... and Neovim will still not generate undercurl in the test.
The problem is that the old tic program does not know about many new Terminal capabilities and as a result it can't build a terminfo database to include them ... or can it? Luckily such a problem was forseen and accomodated by this flag:
-x Treat unknown capabilities as user-defined.
So running:
tic -x terminfo.srcand retesting Neovim finally shows undercurl working properly:
