How do I debug "Error: spawn ENOENT" on node.js?
NOTE: This error is almost always caused because the command does not exist, because the working directory does not exist, or from a windows-only bug.
I found a particular easy way to get the idea of the root cause of:
Error: spawn ENOENT
The problem of this error is, there is really little information in the error message to tell you where the call site is, i.e. which executable/command is not found, especially when you have a large code base where there are a lot of spawn calls. On the other hand, if we know the exact command that cause the error then we can follow @laconbass' answer to fix the problem.
I found a very easy way to spot which command cause the problem rather than adding event listeners everywhere in your code as suggested in @laconbass' answer. The key idea is to wrap the original spawn call with a wrapper which prints the arguments send to the spawn call.
Here is the wrapper function, put it at the top of the index.js
or whatever your server's starting script.
(function() {
var childProcess = require("child_process");
var oldSpawn = childProcess.spawn;
function mySpawn() {
console.log('spawn called');
console.log(arguments);
var result = oldSpawn.apply(this, arguments);
return result;
}
childProcess.spawn = mySpawn;
})();
Then the next time you run your application, before the uncaught exception's message you will see something like that:
spawn called
{ '0': 'hg',
'1': [],
'2':
{ cwd: '/* omitted */',
env: { IP: '0.0.0.0' },
args: [] } }
In this way you can easily know which command actually is executed and then you can find out why nodejs cannot find the executable to fix the problem.
Step 1: Ensure spawn
is called the right way
First, review the docs for child_process.spawn( command, args, options ):
Launches a new process with the given
command
, with command line arguments inargs
. If omitted,args
defaults to an empty Array.The third argument is used to specify additional options, which defaults to:
{ cwd: undefined, env: process.env }
Use
env
to specify environment variables that will be visible to the new process, the default isprocess.env
.
Ensure you are not putting any command line arguments in command
and the whole spawn
call is valid. Proceed to next step.
Step 2: Identify the Event Emitter that emits the error event
Search on your source code for each call to spawn
, or child_process.spawn
, i.e.
spawn('some-command', [ '--help' ]);
and attach there an event listener for the 'error' event, so you get noticed the exact Event Emitter that is throwing it as 'Unhandled'. After debugging, that handler can be removed.
spawn('some-command', [ '--help' ])
.on('error', function( err ){ throw err })
;
Execute and you should get the file path and line number where your 'error' listener was registered. Something like:
/file/that/registers/the/error/listener.js:29
throw err;
^
Error: spawn ENOENT
at errnoException (child_process.js:1000:11)
at Process.ChildProcess._handle.onexit (child_process.js:791:34)
If the first two lines are still
events.js:72
throw er; // Unhandled 'error' event
do this step again until they are not. You must identify the listener that emits the error before going on next step.
Step 3: Ensure the environment variable $PATH
is set
There are two possible scenarios:
- You rely on the default
spawn
behaviour, so child process environment will be the same asprocess.env
. - You are explicity passing an
env
object tospawn
on theoptions
argument.
In both scenarios, you must inspect the PATH
key on the environment object that the spawned child process will use.
Example for scenario 1
// inspect the PATH key on process.env
console.log( process.env.PATH );
spawn('some-command', ['--help']);
Example for scenario 2
var env = getEnvKeyValuePairsSomeHow();
// inspect the PATH key on the env object
console.log( env.PATH );
spawn('some-command', ['--help'], { env: env });
The absence of PATH
(i.e., it's undefined
) will cause spawn
to emit the ENOENT
error, as it will not be possible to locate any command
unless it's an absolute path to the executable file.
When PATH
is correctly set, proceed to next step. It should be a directory, or a list of directories. Last case is the usual.
Step 4: Ensure command
exists on a directory of those defined in PATH
Spawn may emit the ENOENT
error if the filename command
(i.e, 'some-command') does not exist in at least one of the directories defined on PATH
.
Locate the exact place of command
. On most linux distributions, this can be done from a terminal with the which
command. It will tell you the absolute path to the executable file (like above), or tell if it's not found.
Example usage of which and its output when a command is found
> which some-command
some-command is /usr/bin/some-command
Example usage of which and its output when a command is not found
> which some-command
bash: type: some-command: not found
miss-installed programs are the most common cause for a not found command. Refer to each command documentation if needed and install it.
When command is a simple script file ensure it's accessible from a directory on the PATH
. If it's not, either move it to one or make a link to it.
Once you determine PATH
is correctly set and command
is accessible from it, you should be able to spawn your child process without spawn ENOENT
being thrown.