Reading an application's output in "real time"

simopal6 · 10-12-2007, 01:44 AM

Hello!
Suppose i have an application (in my case, wpa_supplicant), which, when executed, prints some output but never exits. Can i read the application's output as soon as it is printed?

I'm writing a shell script to manage wireless connections, and i need to use wpa_supplicant.. The problem is that wpa_supplicant never exits, so i don't know when authentication has been completed.

Any help?
Thank you!

juergen · 10-12-2007, 10:45 AM

You could try expect or Pexpect. They are made for this kind of task.

jlinkels · 10-12-2007, 08:27 PM

If you can redirect the output of wpa-supplicant to a file, or you pipe the output into the logger program within your script, do so. In another process you do a tail -f <filename> and presto.

jlinkels

ray_80 · 10-14-2007, 01:16 PM

Quote:

Originally Posted by jlinkels

If you can redirect the output of wpa-supplicant to a file, or you pipe the output into the logger program within your script, do so. In another process you do a tail -f <filename> and presto.

jlinkels

Good advice. Have you also tried strace?

man strace

Regards

simopal6 · 10-16-2007, 03:00 PM

Quote:

Originally Posted by jlinkels

If you can redirect the output of wpa-supplicant to a file, or you pipe the output into the logger program within your script, do so. In another process you do a tail -f <filename> and presto.

jlinkels

I've tried redirecting the output, and using cat to read it. The problem is that the output isn't flushed to file until the program exists. How can i force flushing?
Thanks!

jlinkels · 10-16-2007, 07:51 PM

Do you mean that wpa-supplicant doesn't write anything before the program exists? If so, there is nothing to do about it.

However, if wpa-supplicant writes any output, you can pipe it into logger. If wpa-supplicant write the interesting stuff to stderr, you can redirect stderr to stdout: 2>&1

Like:

Code:

$wpa-supplicant -options 2>&1 | logger

jlinkels

abencomoc90 · 11-21-2014, 10:08 PM

I got the same problem.... wpa_supplicant show log messages on the shell screen when you run it interactively, but their output were redirected don't flush the messages until end the process. I don't find a way to send the log messages to a file in realtime in order to do some script with those log messages... Any idea????

simopal6 · 11-22-2014, 05:54 AM

I had completely forgotten about the existence of this thread...

Anyway, you should probably follow jlinkels' advice, pipe wpa_supplicant's output to some program which reads from stdin. I have no idea if I did follow his suggestion at the time.
The first thing that comes to mind is calling the application like this:

Code:

$ wpa_supplicant -options | logger

where logger is a script which, for example processes its input (i.e., wpa_supplicant's output) line by line:

Code:

#!/bin/bash
while read line ; do
    ...process $line, e.g. grep, sed, tr, cut, whatever
done