How can I get the returned value from the child_process.exec in javascript?

Question

I tried the following code, it shows the res is undefined. How can I return the stdout?

function run_shell_command(command)
{
    var res
    exec(command, function(err,stdout,stderr){
     if(err) {
        console.log('shell error:'+stderr);
    } else {
        console.log('shell successful');
    }
    res = stdout
    // console.log(stdout)
    });
    return res
} 

Answer 1

you can't get the return value except you use synchronous version of the exec function. if you are still hellbent on doing that, you should use a callback

function run_shell_command(command,cb) {   
    exec(command, function(err,stdout,stderr){
        if(err) {
          cb(stderr);
        } else {
          cb(stdout);
       }
   });
}

run_shell_command("ls", function (result) { 
   // handle errors here
} );

or you can wrap the exec call in a promise and use async await

const util = require("util");
const { exec } = require("child_process");
const execProm = util.promisify(exec);

async function run_shell_command(command) {
   let result;
   try {
     result = await execProm(command);
   } catch(ex) {
      result = ex;
   }
   if ( Error[Symbol.hasInstance](result) )
       return ;

   return result;
}


run_shell_command("ls").then( res => console.log(res) );