Parsing Node Command Line Arguments

Question

I am learning how to use node.js, but am having trouble parsing command-line arguments. The following line:

node test.js --input=in.txt

Gives a parsing error when the code reaches this command:

var fileName = JSONparse.(process.argv[2]);

Results in the error:

 undefined
--input=in.txt 

Syntax error: Unexpected number

What I am trying to do is create an optional variable for the input file. If it is not specified in the command-line, it should resolve to 'a.txt'.

I have not found an easy way of creating default parameters, or use identifiers such as '--input=' to not have to worry about the order in which arguments are passed (I know it does not matter in this case with one argument).

Incidentally, Syntax error is because the JS syntax is bad. There is no JSONparse, and even if there was, JSONparse.( doesn't make sense; . is to access a member, so you need an identifier after .; ( doesn't work. Perhaps you meant JSON.parse(process.argv[2]), but that doesn't make sense either since the string is not JSON. — Jacob
– Jacob, Commented Feb 28, 2015 at 18:40
You are right @Jacob, I did mean JSON.parse. However:developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — Pablo Prado
– Pablo Prado, Commented Feb 28, 2015 at 19:29

Shekar Mania · Accepted Answer · 2019-03-30 07:19:36Z

2

If you want to parse something, it should be a stringified JSON object or parsing results in error. It is important to know that all the command line arguments in node js are strings

let fileName = 'a.txt'

process.argv.slice(2).forEach((arg) => {
  if(arg.indexOf('--input') !== -1){
    if(arg.indexOf('=') !== -1){
      fileName = arg.split("=")[1].trim()
    } else {
      console.log('Error in providing command line argument')
    }
  }
})

console.log("File Name is ",fileName)

for node index output will be

File Name is a.txt

for node index --input=hello.txt, the output will be

File Name is hello.txt

answered Mar 30, 2019 at 7:19

Shekar Mania

3334 silver badges9 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

dthree · Accepted Answer · 2016-01-14 05:12:26Z

1

Vorpal.js is a framework I made for building interactive CLIs in Node. As part of this, it has a built-in API for easy command parsing.

answered Jan 14, 2016 at 5:12

dthree

20.9k14 gold badges81 silver badges115 bronze badges

1 Comment

starball Jun 30 at 20:04

this answer would be improved by showing how to use it to solve the problem in the question post. if you can edit to make that change, I think future readers would appreciate that.

Jacob · Accepted Answer · 2015-02-28 18:35:46Z

Best option: use an existing command line parser. The yargs module works pretty well.

If you want to roll your own, here's the approach I'd take. First, create an object with all your defaults:

var opts = { 
  inputStream: process.stdin,
  verbose: false
};

...then just loop through the args, replacing the options as you go. If an option wasn't present, then the defaults will be used:

for (var i = 2; i < process.argv.length; i++) {
  var arg = process.argv[i];
  var keyValue = arg.split('=');  
  var key = keyValue[0], value = keyValue[1];
  if (key === '--input' || key === '-i') {
      opts.inputStream = fs.createReadStream(value);
  }
  if (key === '---verbose' || key === '-v')  {
      opts.verbose = true;
  }
  // etc.
}

Collectives™ on Stack Overflow

Parsing Node Command Line Arguments

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related