Centering fixed-width plaintext files in widescreen terminals

How to pad out 80-ish column plain text documents such that they're placed in the center of a much wider terminal (or other application window), for better readability.

Plain text documents in your web browser

Simplest CSS that could possibly work, assuming an 80-column plain text document. Requires Stylus or a similar browser extension. I use "URLs matching the regexp" .*\.(|TXT|txt|rst|md|ME|me)$.

body {
  width: 80ch;
  margin: 0 auto;
}

Center a document based on the actual longest line, rather than just guessing 80ch. Only lightly tested. Requires a user script extension like Tampermonkey.

// ==UserScript==
// @name         Center plaintext docs
// @namespace    https://github.com/ernstki
// @version      0.1
// @description  Pad fixed-width text documents so they're centered within the window
// @author       Kevin Ernst
// @include      /.*\.(|TXT|txt|rst|md|ME|me)$/
// @icon         none
// @grant        none
// ==/UserScript==

(function() {
    'use strict';
    var max = 0;
    var style = document.createElement('style');

    document.body.textContent.split(/\n/).forEach((l) => {
        if (l.length > max) max = l.length;
    })

    // not sure why '+2' should be necessary, but it is in Firefox at least
    style.textContent = 'body { width: ' + (max+2) + 'ch; margin: 0 auto; }'
    document.head.appendChild(style);
})();

man7.org

Manual pages on man7.org are displayed in a fixed-width font, but left aligned. You can fix this with Stylus, or another user style manager:

body {
  width: 88ch;
  margin: 0 auto;
}

Anything `less` can display

source

cless() {
    local contentwidth=${WIDTH:-100}
    local termwidth=$(tput cols)

    sed "s/^/$(printf "%$(((termwidth-contentwidth)/2))s")/" ${1:+"$@"} \
      | less
}

alias lessc=cless  # for convenience, in case you forget

Markdown via `~/.lessfilter` and rich-cli

Requires rich-cli. Put this in your ~/.lessfilter and mark it executable with chmod u+x ~/.lessfilter.

Note that ~/.lessfilter is not a feature of less. It's a convention that probably originated with Debian, and seems to be supported in CentOS/RHEL, but not the less from Macports, for example. In environments where ~/.lessfilter doesn't seem to do anything, try putting export LESSOPEN="|$HOME/.lessfilter '%s'" in your login scripts.

See the "INPUT PREPROCESSOR" section of less(1) for details.

#!/bin/bash
contentwidth=${WIDTH:-100}
termwidth=$(tput cols)

case "$1" in
    *.md|*.markdown)
        if type rich &>/dev/null; then
            command rich --force-terminal -d 0,$(((termwidth-contentwidth)/2)) "$@"
            exit 0
        else
            exit 1
        fi
        ;;
esac
exit 1

Anything `rich` can display

Requires rich-cli.

# source: https://github.com/Textualize/rich-cli/issues/19#issuecomment-1052873593
rich() {
    local contentwidth=${WIDTH:-100}
    local termwidth=$(tput cols)
    local stdin=-
    
    [[ -t 0 ]] && stdin=
    command rich -d 0,$(((termwidth-contentwidth)/2)) --force-terminal $stdin "$@" | less -XR
}

Man pages

source

cman() { 
    local DEFAULT_WIDTH=100
    local manpage margin pad
    local width=$DEFAULT_WIDTH
    local termwidth=$(tput cols)

    while (( $# )); do
        case $1 in
            -w*|--width*)
                if [[ $1 =~ --?w(idth=)?([1-9][0-9]*)$ ]];
                then
                    width=${BASH_REMATCH[2]}
                else
                    shift
                    if [[ -z ${1:-} || ! $1 =~ ^[1-9][0-9]*$ ]]; then
                        echo "ERROR: The -w / --width option expects an" \
                             "integer argument." >&2
                        return 1
                    fi
                    width=$1
                fi
                ;;
            -*)
                echo "ERROR: Unrecognized option '$1'." >&2
                return 1
                ;;
            *)
                if [[ ${manpage:-} ]]; then
                    echo "ERROR: One manual page at a time, please!" >&2
                    return 1
                fi
                manpage=$1
                ;;
        esac
        shift
    done

    if (( termwidth < width )); then
        echo "WARNING: '-w $width' is wider than the terminal." >&2
        width=$termwidth
    else
        margin=$(( (termwidth - width) / 2 ))
        pad=$(printf "%${margin}s")
    fi

    if [[ -z $manpage ]]; then
        echo "What manual page do you want?"
        return  # on macOS/BSD anyway, this *doesn't* return failure
    fi

    (
        (( ${DEBUG:-} )) && set -x
        set -eo pipefail

        manpath=$(man -w $manpage)
    
        { [[ ${manpath##*.} == gz ]] && gzip -dc $manpath || cat $manpath; } \
          | groff -Tascii -rLL=${width}n -man \
          | sed "s/^/$pad/" \
          | ${PAGER:-less -FRX}
    )
}

alias manc=cman  # for convenience, in case you forget

Vim buffers

Courtesy of @vcavallo, via this gist.

" source: https://gist.github.com/vcavallo/c3147fca261c78bd9c4a3648089b22e8
" centers the current pane as the middle 2 of 4 imaginary columns
" should be called in a window with a single pane

 function CenterPane()
   lefta vnew
   wincmd w
   exec 'vertical resize '. string(&columns * 0.75)
 endfunction

" optionally map it to a key:
"nnoremap <leader>e :call CenterPane()<cr>

License

Unless otherwise noted, I am the original author of the code snippets in this gist, and reuse is permitted under the terms of the MIT license.

ernstki/centering-plaintext.md