Dump php system command output to a file

Question

In the php system command we use the following

system("ffmpeg -i test.avi -ab 56 -ar 44100 -b 200 -r 15 -s 320x240 -f flv output_file.flv 2>&1 &"). 

Please explain the usage of the above mentioned system command. What does '2>&1 &' stand for ? I want to dump the process details to a file how do I do that ?

Thank you very much.

Answer 1

2>&1 redirects «stderr» to «stdout», & at the end makes the command run in background.

To make it complete it should be

«command» 2>&1 > /tmp/somefile.log &

Without redirecting «stdout» to a file (or to /dev/null) running in background from system() doesn't make much sense, as your command would get killed as soon as PHP terminates (e.g. reaching time limit).

From system() manual:

Note: If a program is started with this function, in order for it to continue running in the background, the output of the program must be redirected to a file or another output stream. Failing to do so will cause PHP to hang until the execution of the program ends.