System/child_process.spawn not working as expected

Add-ons Development
1

I am having some issues getting spawn() from the system/child_process SDK module to execute a command successfully on Centos 6.6. I am calling it as

let result = child_process.spawn(command, args, options)

Watching in the debugger, I see that after this step that result contains properties killed: false, exitCode: 0, and a plausible value for the pid. However, I can’t see any evidence that the command was actually executed. I tried running a command that would touch a file and no file was created. I tried running /bin/uname and catching stdout as illustrated in the example here: https://developer.mozilla.org/en-US/Add-ons/SDK/Low-Level_APIs/system_child_process but my callback was not called. I tried making the command /bin/sleep and the argument 60, but when I checked with ps the pid shown in result was not associated with any active process. If I change command to something invalid, result changes to having an exitCode of -1 and the console shows a “File not found” error, so spawn is doing something.

I tried using child_process.spawn on Arch Linux and it worked for me there. What could be different? What could I do to debug this? I’m not seeing any error message.

2

Have you tried running exec() instead? Also it might be good to run Firefox from the command line to see its output. There might be clues there about this problem.

3

Tell us what is behind vars command, args and options.

4

Hi!! , I have tried with the exec command always returns me: Cannot open display. Is possible may have to enable the graphical mode? You know how?

5

What are you trying to run with exec?

6

I’m trying to run gnome terminal or any application with GUI (firefox , gedit , etc …). In Windows 7 I have been able to run cmd from the Firefox extension, but in linux not work … I think it’s a problem with the configuration of X11. Any help?

7

Execute child process with options argument which contains DISPLAY environment variable.
child_process.exec(command, { env: DISPLAY: displayEnv });.

displayEnv is what you get in $DISPLAY in terminal.
But it can not be accessed from add-on, so needs user’s input (e.g. in Add-ons Manager).

system/child_process - Mozilla | MDN

  • in node.js, spawn() and exec() inherit the
    environment variables from the parent process, by default. The SDK
    versions don’t: so when you specify a command you must pass in a
    complete path to the command or use the env option to set up the child process environment.

Also, maybe xhost + need to be set by user.

8

You can get the $DISPLAY environement variable with the add-on SDK using the env proxy of the sdk/system module. However you have to fetch each value explicitly, the values cannot be enumerated.