Fixing My Shell


For an embarassingly long time, my shell has unnecessarily tried to initialize a console font in every kind of interactive terminal.

This leaves the following error message in my terminal:

Couldn't get a file descriptor referring to the console.

It even shows up twice when running tmux!

Clearly I’ve done something horrible to my configuration, and now I’ve got to clean it up.

How does Shell Initialization Work?

The precise files a shell reads at start-up is somewhat complex, and defined by this excellent chart 1:

Shell Startup Flowchart

For the purposes of what I’m trying to fix, there are two paths that matter.

  • Interactive login shell startup
  • Interactive non-login shell startup

As you can see from the above, trying to distinguish these two paths in bash is an absolute mess. zsh, in contrast, is much cleaner and allows for a clear distinction between these two cases, with login shell configuration files as a superset of configuration files used by non-login shells.

How did we get here?

I keep my configuration files in a config repository.

Some time ago I got quite frustrated at this whole shell initialization thing, and just linked everything together in one profile file:

.mkshrc -> config/profile
.profile -> config/profile

This appears to have created this mess.

Move to ZSH

I’ve wanted to move to zsh for a while, and took this opportunity to do so. So my new configuration files are .zprofile and .zshrc instead of .mkshrc and .profile (though I’m going to retain those symlinks to allow my old configurations to continue working).

mksh is a nice simple shell, but using zsh here allows for more consistency between my home and $WORK environments, and will allow a lot more powerful extensions.

Updating my Prompt

ZSH prompts use a totally different configuration via variable expansion. However, it also uses the PROMPT variable, so I set that to the needed values for zsh.

There’s an excellent ZSH prompt generator at that I used to get these variables, though I’m sure they’re in the zsh documentation somewhere as well.

I wanted a simple prompt with user (%n), host (%m), and path (%d). I also wanted a % at the end to distinguish this from other shells.

PROMPT="%n@%m%d%% "

Fixing mksh prompts

This worked but surprisingly mksh also looks at PROMPT, leaving my mksh prompt as the literal prompt string without expansion.

Fixing this requires setting up a proper shrc and linking it to .mkshrc and .zshrc.

I chose to move my existing aliases script to this file, as it also broke in non-login shells when moved to profile.

Within this new shrc file we can check what shell we’re running via $0:

if [ "$0" = "/bin/zsh" ] || [ "$0" = "zsh" ] || [ "$0" = "-zsh" ]

I chose to add plain zsh here in case I run it manually for whatever reason. I also added -zsh to support tmux as that’s what it presents as $0. This also means you’ll need to be careful to quote $0 or you’ll get fun shell errors.

There’s probably a better way to do this, but I couldn’t find something that was compatible with POSIX shell, which is what most of this has to be written in to be compatible with mksh and zsh2.

We can then setup different prompts for each:

if [ "$0" = "/bin/zsh" ] || [ "$0" = "zsh" ] || [ "$0" = "-zsh" ]
	PROMPT="%n@%m%d%% "
	# Borrowed from
	PS1='$(id -un)@$(hostname -s)$PWD$ '

Setting Console Font in a Better Way

I’ve been setting console font via setfont in my .profile for a while. I’m not sure where I picked this up, but it’s not the right way. I even tried to only run this in a console with -t but that only checks that output is a terminal, not specifically a console.

if [ -t 1 ]
	setfont /usr/share/consolefonts/Lat15-Terminus20x10.psf.gz

This also only runs once the console is logged into, instead of initializing it on boot. The correct way to set this up, on Debian-based systems, is reconfiguring console-setup like so:

dpkg-reconfigure console-setup

From there you get a menu of encoding, character set, font, and then font size to configure for your consoles.

VIM mode

To enable VIM mode for ZSH, you simply need to set:

bindkeys -v

This allows you to edit your shell commands with basic VIM keybinds.

Getting back Ctrl + Left Arrow and Ctrl + Right Arrow

Moving around one word at a time with Ctrl and the arrow keys is broken by vim mode unfortunately, so we’ll need to re-enable it:

bindkey "^[[1;5C" forward-word
bindkey "^[[1;5D" backward-word

But of course we’re now using zsh so we can do better than just the same configuration as we had before.

There’s an excellent substring history search plugin that we can just source without a plugin manager3

source $HOME/config/zsh-history-substring-search.zsh
# Keys are weird, should be ^[[ but it's not
bindkey '^[[A' history-substring-search-up
bindkey '^[OA' history-substring-search-up
bindkey '^[[B' history-substring-search-down
bindkey '^[OB' history-substring-search-down

For some reason my system uses ^[OA and ^[OB as up and down keys. It seems ^[[A and ^[[B are the defaults, so I’ve left them in, but I’m confused as to what differences would lead to this. If you know, please let me know and I’ll add a footnote to this article explaining it.

Back to history search. For this to work, we also need to setup history logging:

export SAVEHIST=1000000
export HISTFILE=$HOME/.zsh_history
export HISTFILESIZE=1000000
export HISTSIZE=1000000

Why did it show up twice for tmux?

Because tmux creates a login shell. Adding:


to profile and:

echo SHRC

to shrc confirms this with:


For now, profile sources shrc so that running twice is expected.

But after this exploration and diagram, it’s clear we don’t need that for zsh. Removing this will break remote bash shells (see above diagram), but I can live without those on my development laptop.

Removing that line results in the expected output for a new terminal:


And the full output for a new tmux session or console:


So finally we’re back to a normal state!

This post is a bit unfocused but I hope it helps someone else repair or enhance their shell environment.

If you liked this4, or know of any other ways to manage this I could use, let me know at

  1. This chart comes from the excellent Shell Startup Scripts article by Peter Ward. I’ve generated the SVG from the graphviz source linked in the article. 

  2. Technically it has be compatible with Korn shell, but a quick google seems to suggest that that’s actually a subset of POSIX shell. 

  3. I use oh-my-zsh at $WORK but for now I’m going to simplify my personal configuration. If I end up using a lot of plugins I’ll reconsider this. 

  4. Or if you’ve found any typos or other issues that I should fix.