Helpers for Bash like shell scripting in JavaScript
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Helpers for Bash like shell scripting in JavaScript

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Write your shell scripts using jBash and you get the best of both worlds: JavaScript and Bash. jBash is a small JavaScript library that provides helper aliases and functions that are similar to Bash syntax, allowing you to write shell scripts in JavaScript that are simple and familiar.

Quick Start

Create a file called script.js:

#!/usr/bin/env npx jbash

echo("Hello jBash")

Make the file executable, run it, and you should see "Hello jBash" printed:

chmod +x script.js

> Hello jBash


Bash jBash notes
echo "Hello" echo("Hello") print text to console (stdout)
$0 $0 file path of current script
$1, $2, ... $1; $2; ... $1, $2, etc. variables contain args passed in
args[0], args[1] arguments passed in are also in args array
$HOME $HOME environment variables
env.HOME all env variables are also mapped on env var
set -x set("-x") echos all commands
options.xtrace=true alternative to set("-x")
set -e set("-e") throw when a command exits with non-zero status
options.errexit=true alternative to set("-x")
cd "/usr/bin" cd("/usr/bin") change current working directory
exit 1 exit(1) exit with code
result=$( result=$("") $(...) buffers output as return value
eval ping eval("ping") eval() streams output to console (stdout)
exec("ping") exec() is an alias for eval()
config=$(cat cnf.txt) config=cat("cnf.txt") read text from file
echo "World" > cnf.txt echo("World", "cnf.txt") save text to file

Command Execution

To run commands in jBash you can use either the $() or eval() helper, depending upon whether you need to capture the output of the commands. Both of these commands are synchronous and will block execution until completion.


When you want to run a command and buffer the output (stdout) of that command as a return value, you'll want to use $(). As the command is running, stdout will not be printed to the console but will instead be captured and returned as the result. This helper is intended for short running commands that do not produce a large amount of output. Example: To grab the output of git status --porcelain and store in variable named result:

// Will wait for `git status` to complete and assign
// output to result variable.  Nothing will be
// printed to the console.

let result=$(`git status --porcelain`);


eval() should be used for running commands where the output does not need to be captured, but only printed to the console (stdout). This helper is intended for long running commands or those where output does not need to be captured. Example: To run npm install:

// Will print `npm install` output immediately as it happens
// eval() will return null

eval(`npm install`)

Reading and Writing Files

In Bash, reading files is usually done with the cat command (i.e. config=$(cat cnf.txt)) and writing to files is usally done with cat or echo and piping (i.e. echo $cnf > cnf.txt). The same approaches can be used in jBash, using $() (reading: config=$(`cat cnf.txt`); writing: $(`echo ${cnf} > cnf.txt`)). But jBash also provides the cat and echo helpers for convenience.


The cat helper works just like the cat command where you pass in a file path argument and it returns the contents. Unlike Bash, the output from the cat helper will be returned but not sent to the console (stdout). If you need to also print it to stdout you can simply call echo with the result of cat. By default, "utf-8" encoding will be used when reading the file but you can pass an alternative encoding to be used as the second parameter.

// Read contents of cnf.txt and store in config variable

// Print to console (stdout)


The echo helper will print text to console (stdout) when passed a single argument (echo("Hello")) but when specifying a file path as a second argument, the first argument (string) will be used to replace the contents of that file. This is equivalent to echo $config > cnf.txt in Bash. By default, "utf-8" encoding will be used when writing to the file but you can pass an alternative encoding to be used as the second parameter.

// Save config variable value to cnf.txt
echo(config, "cnf.txt")

Error Handling

If a command exits with a non-zero status, the stderr will be echoed on console (stdout). If the command was run with $(), the stderr will also be returned. To throw an error and therefore halt the script unless the error is handled with try / catch, you can enable errexit option by calling set("-e") which behaves just like set -e in Bash. The error will have properties { message, status, stderr } that contain detail of the error.


// This command will error but will not throw because errexit is not enabled
// The string "cat: invalid.txt: No such file or directory" will be returned and
// assigned to `content`.
let content=$(`cat invalid.txt`)

// Turn on errexit

try {
  // This command will throw because errexit is enabled
  eval(`cat invalid.txt`)
} catch (err) {
  console.log(err.status) // 1
  console.log(err.stderr) // "cat: invalid.txt: No such file or directory"



By far the easiest way to use jBash is with a npx shebang which will allow your script to be executed directly without an npm install needed.


#!/usr/bin/env npx jbash

echo("Hello jBash")


Since Linux does not support multiple arguments in the shebang, you need to call npx at its absolute path. Usually npx is installed in /usr/local/bin/ but you can run which npx to find out.

#!/usr/local/bin/npx jbash

echo("Hello jBash")

npm global install

If you don't want to use npx, you can install jBash globally with npm:

npm install -g jbash

Once it is installed globally, you can write your script with a jBash shebang which will allow your script to be executed directly, with the globally installed jBash loaded at runtime.


echo(`Hello jBash`)


Rather than installing jBash globally, you can simply download it to a local folder and reference it directly from your script using a require statement. This is a good option for scripts running on a remote system where you may not have the ability to use npx or be able to install npm packages globally. Node.js will still need to be available, though.

First, download jBash:

wget -O jbash.js

Then, in your script:

#!/usr/bin/env node

echo(`Hello jBash`)

It's Still JavaScript

When you write your shell scripts in jBash, you get to use a simple Bash like syntax but remember, it's still JavaScript! This means you can install npm packages and use them to your ❤️'s content.


npm install uuid

echo(uuidv4()) // -> '110ec58a-a0f2-4ac4-8393-c866d813b8d1'

TypeScript Support

TypeScript declarations for jBash are available and specifed with "types": "index.d.ts" in the package.json file. A clean way to use TypeScript with jBash is by using ts-node.

First, install ts-node, typescript, and jBash globally:

npm install -g ts-node typescript jbash

Then, create your jBash script file using a .ts file extension.


#!/usr/bin/env ts-node

const contents: string = "Hello jBash from TypeScript";

Run it:

chmod +x ./myscript.ts

And you should see the following printed to the console:

"Hello jBash from TypeScript


Looking at real-world scripts is a good way to get a feel for the library.