NodeJS read and parse each line of stdout

Question

I have a NodeJS script that 'exec's a child process to capture cat dump of a file:

var exec = require('child_process').exec;
var result = '';
var child = exec('./scripts/first.sh',function(err, stdout, stderr) {
    result = stdout.split("=");
});

If in case the file is not there I would take dump of a different file:

var result = '';
var child = exec('./scripts/first.sh',function(err, stdout, stderr) {
    result = stdout.split("=");
    if(stdout.indexOf('No such file or directory') != -1){
        var child = exec('./scripts/second.sh', function(err, stdout, stderr) {
            result = stdout.split("=");
    });
});

Finally I log the value of result variable:

console.log(result);

The files would have data like mentioned below:

line_1 = 7888
line_2 = 8998
line_3 = 9090
line_4 = 9097

I need to parse and extract values of line_1 and line_3?

The result variable does not shows any value. My idea was to get the stdout data in a string variable and use some search mechanism.

Though I am not sure of the approach as I am not much experience on JS / NodeJS.

==EDIT==

Please find a replica of the function I have written.

var exec = require('child_process').exec;

function getdetail() {
        var result = '';
        var child = exec('./scripts/first.sh', function(err, stdout, stderr) {
                if(stdout.indexOf('No such file or directory') != -1){
                        var child = exec('./scripts/second.sh',function(err, stdout, stderr) {
                        result = stdout.toString().split("=");
                        console.log(result);
                        });
                }
                else
                {
                        result = stdout.toString().split("=");
                        console.log(result);
                }
        });
}

The tostring() on stdout stream object did the trick but I get console logs as mentioned below:

[ 'spawn ssh -o UserKnownHostsFile',
  '/dev/null -o StrictHostKeyChecking',
  'no user@www.mybox.com cat ver.txt\r\nWarning: Permanently added \'www.mybox.com,XX.XX.XX.XX\' (RSA) to the list of known hosts.\r\r\nuser@www.mybox.com\'s password: \r\line_1',
  '9400\r\nline_2',
  '3508\r\nline_3',
  '77f3\r\nline_4',
  '/tmp\r\nline_5',
  '/tmp/ramdisk/\r\nline_5',
  '77f3\r\n' ]

How can I extract value of line_1 and line_3?

Yes, I am trying to do some enahancements on an existing product. Note that the file I need to reach is on an another server. The script silently ssh to that box and execute the cat command on the file — Programmer, Oct 18 '12 at 13:29
There is a syntax error in the second code snippet. Is it intentional? Also where do you check the contents of `result`? Inside the exec callback function or after the exec+exec -definition? — jsalonen, Oct 18 '12 at 13:32
Please forgive me for syntax error - it could be while typing. I am trying to store it in a variable and am printing it both inside and outside the exec block — Programmer, Oct 18 '12 at 13:34
I edited and modified the content to include exec definition — Programmer, Oct 18 '12 at 13:36

jsalonen · Accepted Answer · 2013-07-23T09:41:58.337

exec is asynchronous. Thus, if you write something like:

var result = '';
var child = exec('...'), function() { result = 'abc'; } );
console.log(result);

Then result maybe be empty, since console.log(result) can and often will get executed before exec returns to its callback and fill in the new value.

To fix this, you need to process the result asynchronously in the exec's callback function.

Also I'm not sure if the way you check for errors is the best possible. Instead of checking for "no such file or directory", you could simply test if err has non-null value:

if(err) {

Putting this all together we end up with the following code:

var exec = require('child_process').exec;

var result = '';
var processResult = function(stdout) {
    var result = stdout.split("=");
    console.log(result);
};

var child = exec('./scripts/first.sh',function(err, stdout, stderr) {
    if(err) {
        var child = exec('./scripts/second.sh', function(err, stdout, stderr) {                         
            processResult(stdout);
        });
    } else {            
        processResult(stdout);
    }
});

If you need to further process the stdout data, you need to iterate through it to find out all possible occurences of strings containing "key=value". Here is a rough idea:

var processResult = function(stdout) {  
    var lines = stdout.toString().split('\n');
    var results = new Array();
    lines.forEach(function(line) {
        var parts = line.split('=');
        results[parts[0]] = parts[1];
    });

    console.log(results);
};

I hope this gets you started.

Yes, I am doing the same thing but instead of calling processResult I am directly calling console.log(result); at the same place in your code. But still it is coming as empty string — Programmer, Oct 18 '12 at 13:41
Then maybe its because your error checking doesn't work? Check the edit for the code that works for me. — jsalonen, Oct 18 '12 at 13:53
Thanks for your help, I have updated the thread with my results — Programmer, Oct 18 '12 at 13:54
Asked 10 years ago... What to do if calling code needs the result? How do I return a result to the caller of this function? — Holm76, Oct 17 '22 at 10:28
@Holm76 answer to that question can be found for example from https://stackoverflow.com/questions/47629756/how-can-i-get-the-returned-value-from-the-child-process-exec-in-javascript — jsalonen, Nov 02 '22 at 19:28

NodeJS read and parse each line of stdout

1 Answers1