pm2 can't start express project with option --log

use relative path

vagrant@vagrant-ubuntu-trusty-64:~/app$ ./node_modules/pm2/bin/pm2 start index.js --name example --log ~/logs/

[PM2] Starting /home/vagrant/app/index.js in fork_mode (1 instance)
[PM2] Done.
┌──────────┬────┬──────┬─────┬────────┬─────────┬────────┬─────┬─────┬──────────┐
│ App name │ id │ mode │ pid │ status │ restart │ uptime │ cpu │ mem │ watching │
└──────────┴────┴──────┴─────┴────────┴─────────┴────────┴─────┴─────┴──────────┘
 Use `pm2 show <id|name>` to get more details about an app

use absolute path

vagrant@vagrant-ubuntu-trusty-64:~/app$ ./node_modules/pm2/bin/pm2 start index.js --name example --log /home/vagrant/logs/

[PM2] Starting /home/vagrant/app/index.js in fork_mode (1 instance)
[PM2] Done.
┌──────────┬────┬──────┬─────┬────────┬─────────┬────────┬─────┬─────┬──────────┐
│ App name │ id │ mode │ pid │ status │ restart │ uptime │ cpu │ mem │ watching │
└──────────┴────┴──────┴─────┴────────┴─────────┴────────┴─────┴─────┴──────────┘
 Use `pm2 show <id|name>` to get more details about an app

after set path mode 777

drwxrwxrwx 2 vagrant vagrant 4096 Dec  2 08:17 logs/

vagrant@vagrant-ubuntu-trusty-64:~/app$ ./node_modules/pm2/bin/pm2 start index.js --name example --log ~/logs
[PM2] Starting /home/vagrant/app/index.js in fork_mode (1 instance)
[PM2] Done.
┌──────────┬────┬──────┬─────┬────────┬─────────┬────────┬─────┬─────┬──────────┐
│ App name │ id │ mode │ pid │ status │ restart │ uptime │ cpu │ mem │ watching │
└──────────┴────┴──────┴─────┴────────┴─────────┴────────┴─────┴─────┴──────────┘
 Use `pm2 show <id|name>` to get more details about an app
vagrant@vagrant-ubuntu-trusty-64:~/app$ ./node_modules/pm2/bin/pm2 start index.js --name example --log /home/vagrant/logs/
[PM2] Starting /home/vagrant/app/index.js in fork_mode (1 instance)
[PM2] Done.
┌──────────┬────┬──────┬─────┬────────┬─────────┬────────┬─────┬─────┬──────────┐
│ App name │ id │ mode │ pid │ status │ restart │ uptime │ cpu │ mem │ watching │
└──────────┴────┴──────┴─────┴────────┴─────────┴────────┴─────┴─────┴──────────┘
 Use `pm2 show <id|name>` to get more details about an app

Reproduce the reported behavior with ./node_modules/pm2/bin/pm2, index.js, and the --log option using both relative and absolute log-directory paths. Trace how the CLI handles those paths and establish the expected result; the issue is complete when both forms reliably produce the intended logs, with regression coverage if an existing test entry point is found.