felix-u / shgen

Evaluate shell expansions in any file

shgen

shgen is a POSIX shell script to evaluate shell expansions within any file. It is fast and lightweight, requiring only coreutils.

Usage

Make an alias or copy shgen to somewhere in your $PATH. Then run shgen, providing as arguments an existing input file and an output file location. If the output file already exists, it will be overwritten.

shgen /path/to/inputfile /path/to/outputfile

Examples

1. One very handy use of shgen is to "hack in" support for your Xresources file in any configuration file, including on Wayland. Put this script somewhere in your $PATH. In this scenario the script is named query, and $XRESOURCES is the path to your Xresources file.

grep "$1:" "$XRESOURCES" | awk '{print $2}'

Alternatively, if you use X11, you might like to take advantage of xrdb, though in my experience it comes with no performance benefits and is generally more hassle than the option above.

xrdb -query | grep "$1" | awk '{print $NF; exit}'

Now run shgen on any type of file. Here's a snippet of my zathura config:

set notification-error-fg       "$(query color1)" #red
set notification-warning-fg     "$(query color3)" #yellow
set notification-fg             "$(query color2)" #green
set inputbar-fg                 "$(query color12)" #bright-blue
set highlight-color             "$(query color3)" #yellow
set highlight-active-color      "$(query color4)" #blue

and the result:

set notification-error-fg       "#dc322f" #red
set notification-warning-fg     "#b58900" #yellow
set notification-fg             "#859900" #green
set inputbar-fg                 "#268bd2" #bright-blue
set highlight-color             "#b58900" #yellow
set highlight-active-color      "#268bd2" #blue

2. For the poor X11 users who don't benefit from Wayland's flawless fractional scaling, managing configurations for multiple devices can be frustrating. In this situation, shgen can be used to multiply any pixel values by the device's scale factor. Have the following function saved as dpi somewhere in your $PATH:

#!/usr/bin/env bash
xquery () {
    xrdb -query | grep "$1" | awk '{print $NF; exit}'
}

XDPI=$(bc <<< "scale=2; $(xquery dpi)/96")

dpi () {
    VAL=$(bc <<< "scale=2; $1 * $XDPI"); # multiply input by DPI
    # return value rounded to nearest integer
    printf "%.0f\n" "$(bc <<< "scale=2; $VAL + 0.01")"
            # add 0.01 so that 2.5 rounds to 3, not 2
}

Then:

[bar/laptop]
height = $(dpi 50)

The height will evaluate to 100 on my laptop at 2x scaling, and 63 on my desktop at 1.25x scale.

3. Specifying your network interface in your status bar's wifi module is sometimes needed. With shgen, you can write

interface: $(ip -o route show to default | awk '{print $5}')

which in my case will evaluate to interface: wlan0.

Limitations

You will have to escape shell-sensitive characters such as $ and ^ in input files, though this is easy to fix and is a problem far less often than you might think.