iperf daemon not outputting anything to log file

Question

According to documentation, iperf (v2) server can be run in daemon mode and send its output to a log file like this: iperf -s -D > iperflog

I'm running that command exactly, as a regular user, from my home directory on CentOS, but the log file is always empty.

If I run it without -D, it behaves as expected, creating a log file like this:

------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 85.3 KByte (default)
------------------------------------------------------------
[  4] local x.x.x.x port 5001 connected with x.x.x.x port 50928
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0- 3.0 sec   100 MBytes   279 Mbits/sec

Is this a bug in iperf, or do I not understand output redirection, or what?

Answer 1

Was having this issue after trying the exact same thing. As novice mentions in the comments, there is an -o flag which should output to file, however I found this didn't work under linux despite being in the man page. Ultimately what I needed was for iperf to continue running after ssh session interruption. In the end I used the following command.

nohup iperf -s > iperflog 2>&1 &

To break that down a little, we're running iperf with the server flag and redirecting stdout to the file iperflog. 2>&1 is redirecting stderr to stdout to ensure all output is captured in this file, including errors. The & tells the shell to run this command in the background. This will still result in the process being terminated when the session closes as it will be sent a SIGHUP. nohup fixes this by ignoring that signal and allowing the process to continue running.

You can then run tail -f iperflog to view the output in the shell as it would normally appear.

This is not ideal as it does require a shell where as the previous command shouldn't, but this got me where I needed.