node js post request only works second time

I have one strange issue, when I submit form, it redirects to form action URL then it shows blank page. When I reload it again it will display the data.

index.jade - http://172.18.0.60:3000/

  form#command(action='runcommand', method='post')
    input#cmdls(type='checkbox', name='cmdls', value='ls -la')
    label(for='cmdls') List Files
    br
    input#cmdpwd(type='checkbox', name='cmdpwd', value='pwd')
    label(for='cmdpwd') Print Working Directory
    br
    input#cmddate(type='checkbox', name='cmddate', value='date')
    label(for='cmddate') Date
    br
    input.button(type='submit', value='Run')

app.js

var tmp=""; 
app.post('/runcommand',function(req,res){

    for (var key in req.body) {

            console.log(key);
            function puts(error, stdout, stderr) { sys.puts(stdout) }
            exec(req.body[key], function(error, stdout, stderr) {
              if (!error) {
                    tmp+=stdout;                       
              } else {
                tmp+=stderr;
              }
            });

    }

    res.render("result",{ data: tmp });

});

result.jade - http://172.18.0.60:3000/runcommand

extends layout

block content
    h1= "Result"   
    pre=  data

When I submit form it will redirect to http://172.18.0.60:3000/runcommand only displaying h1, when I reload it again it display data.

why it is behave like this?

via Bilal Usean