Get stream from executing `tail -f app.log | grep error`?

[Alexey Petrushin]

Hi, I'd like to provide administrative app with a page when you can see live logs. To do so I need to execute `tail -f app.log | grep error` and get the output chunk by chunk (streaming) as it get added to log file. But the problem is:

1. child_process.exec doesn't have streaming, it return only when the child process is terminated

2. child_process.spawn does have streaming, but it can execute only commands, it can't execute complex expressions like `tail -f app.log | grep error`

Is there an easy way to solve it?

[Aria Stewart]

Why not tail yourself — fs.createReadStream, and pipe it through a filter in javascript?

[greelgorke]

`tail -f app.log | grep error`

there is a .pipe() hidden here. Oh wait, here it is:

var spawn = require('child_process').spawn,
    tail  = spawn('tail', ['-f','app.log']),
    grep  = spawn('grep', ['error']);

tail.stdout.pipe(grep.stdin)
grep.stdout.pipe(whateverStream)

of course its all untested, but just to give a hint