Use child_process.exec's return value in if statements?

Question

I have this function:

function checkfType(a,b){
  exec("file '"+a+"'",function(err,stdout,stderr){
    if(stdout.containsString(b)===true){return true}else{return false}
  })
}

However, if I use it in an if statement like this: if(checkfType(".","directory"){}, it just goes "false". I tested the exec function as a non-function and using it instead of the if statement:

exec("file '.'",function(err,stdout,stderr){
  if(stdout.containsString("directory")===true){
    console.log("It works!);
  }else{
    console.log("It doesn't work.";}
});

Which works just fine.

I am led to believe tha the exec function is async (or similar), which is where my problem lies.

Is there any way to use exec's output in an if statement?

Assuming `containsString` returns `true` or `false`, `if(stdout.containsString(b)===true){return true}else{return false}` is a *really* long-winded way to write `return stdout.containsString(b);` If it may return something truthy that you don't want to consider `true` (as your statement filters out other truthy values), it's a long way to write `return stdout.containsString(b)===true;` — T.J. Crowder, Jul 11 '14 at 12:33

score 1 · Accepted Answer · answered Jul 11 '14 at 12:29

1

Is there any way to use exec's output in an if statement?

Yes, but not as a return value for the function calling it.

I am led to believe that the exec function is async (or similar), which is where my problem lies.

Right. Your function will need to accept a callback that it will pass the flag to when the exec completes:

function checkfType(a,b,callback){
  exec("file '"+a+"'",function(err,stdout,stderr){
    callback(stdout.containsString("directory"));
  })
}

Usage:

checkfType("whatever", "directory", function(flag) {
  if (flag) {
    // It's a directory
  } else {
    // It isn't
  }
});

answered Jul 11 '14 at 12:29

T.J. Crowder

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Thanks for the quick answer! Is my best option then to wrap the `exec` function in another function (to not make my code a mess), and use it instead of an if statement? – Jacob Pedersen Jul 11 '14 at 12:32
@JacobPedersen: That doesn't change anything, then you'd just have to have *that* function accept a callback. You *cannot* convert an asynchronous result into a function's return value. – T.J. Crowder Jul 11 '14 at 12:34
1

I see your edit, I think I understand async callbacks now, thanks. By "wrapping", I meant something similar to what you did in your edit. – Jacob Pedersen Jul 11 '14 at 12:35

Use child_process.exec's return value in if statements?

1 Answers1