JEVAL

A tool to help process text, JSON, YAML, TOML or plists on the command line Meant to be used with stdin being piped in from some other command

Install

npm install -g jeval

This puts a jeval command on your PATH.

Usage

Run jeval with no arguments to print the full flag list. The last argument is always a JavaScript expression evaluated with the input available as the variable l:

echo "some line" | jeval "l.replace(/e/, 'a')"

Input is read as one big string by default. Pass --line to evaluate each line on its own, which is also what --match filters on.

For example, turning markdown headers into an indented list:

## Something
some text
### Another thing
more text

## Section
lots of text

### Sub section 1
### Sub section 2

## A header

### A final section

Can be done with npm run test:match

outputs:

-  Something
    -  Another thing
-  Section
    -  Sub section 1
    -  Sub section 2
-  A header
    -  A final section

Or one that I have been using recently on a regular basis:

function alaOpacity() {
    cat ~/.config/alacritty/alacritty.yml | jeval --yaml --yamlout "function r(l, o) { l.background_opacity=Number(o); return l; }  r(l, \"$1\"); " >  ~/.config/alacritty/alacritty.new
    mv ~/.config/alacritty/alacritty.new ~/.config/alacritty/alacritty.yml
}

then:

alaOpacity 0.7

This shows it's ability to parse yaml as well.

It can also parse and print JSON of course and handle plists and TOML.

Input is parsed with --json, --yaml, --toml or --plist, and output can be dumped in any of those formats with --jsonout, --yamlout, --tomlout or --plistout. That makes converting between them a one liner:

cat config.yml | jeval --yaml --jsonout "l"      # yaml  -> json
cat config.json | jeval --json --yamlout "l"     # json  -> yaml
cat config.toml | jeval --toml --jsonout "l"     # toml  -> json
cat config.yml | jeval --yaml --tomlout "l"      # yaml  -> toml

Editing TOML in place

Reading a value, checking it and writing the file back out. Given this alacritty.toml:

[scrolling]
history = 50000
multiplier = 15

[terminal.shell]
program = "/bin/zsh"
args = ["--login"]

[window]
decorations = "buttonless"
dynamic_padding = true

double the scrollback history, but only if it is under 100000:

cat alacritty.toml | jeval --toml --tomlout "l.scrolling.history < 100000 ? (l.scrolling.history *= 2, l) : l"

[scrolling] history becomes 100000 and every other key, type and section is preserved. The guard makes it safe to run repeatedly — a second run leaves the file alone. This is npm run test:toml, which runs against test/alacritty.toml.

Note that comments are not preserved when writing TOML back out; the output is regenerated from the parsed data.

Compiling down into a single file which could be transported to different machines

Use ncc:

npm i -g @vercel/ncc 
ncc build dist/jeval.js -o jeval
chmod +x jeval/index.js
cp jeval/index.js /usr/local/bin/jeval # jeval/index.js is the file you could now use on any machine without deps other than node
rm -rf jeval

Or compiling the NodeJS based command line tool into a binary command line tool...

with a packaged NodeJS interpreter.

alias -g removeFirstLine='tail -n +2'
npm i -g @vercel/ncc pkg
ncc build dist/jeval.js -o jeval
cp jeval/index.js ./jeval-with-hashbang.js 
rm -rf jeval
cat jeval-with-hashbang.js  | removeFirstLine > jeval.js
pkg --target macos-x64 jeval.js
rm jeval-with-hashbang.js jeval.js

Here is an example bash function that will change your Alacritty opacity:

# To use run: alaFontSize 12
function alaFontSize() {
    cat ~/.config/alacritty/alacritty.yml | jeval --yaml --yamlout "(l.font.size=Number(\"$1\")) && l; " > $HOME/.config/alacritty/alacritty.yml.tmp
    mv $HOME/.config/alacritty/alacritty.yml.tmp $HOME/.config/alacritty/alacritty.yml
}
# To use run: alaOpacity 0.8
function alaOpacity() {
    cat ~/.config/alacritty/alacritty.yml | jeval --yaml --yamlout "(l.background_opacity=Number(\"$1\")) && l; " > $HOME/.config/alacritty/alacritty.yml.tmp
    mv $HOME/.config/alacritty/alacritty.yml.tmp $HOME/.config/alacritty/alacritty.yml
}
alaOpacity 0.3

This idea of evalutating JS against text/json/yaml input is actually based off of someone elses idea/tool but I don't know where that repo is and cannot link to it.