3
2
How can I redirect the stdout of a program to a file when it's run in the background?
I have a program that generates output every second. When I run it normally and redirect to a file, the output is in that file as expected:
#./program > file.txt
#cat file.txt
output
output
output
#
When I try to do the same thing in the background, the file will remain empty:
#./program > file.txt &
#cat file.txt
#
Rauffle
Posted 2012-07-19T19:32:58.537
Reputation: 544
3
What about sh -c './program > file.txt; cat file.txt' & ?
Xiè Jìléi
Posted 2012-07-19T19:32:58.537
Reputation: 14 766
This does not work for me – singpolyma – 2015-12-29T19:08:47.110
http://stackoverflow.com/a/11337310/8611 with tee worked. Not sure why buffering is different in background than foreground – singpolyma – 2015-12-29T19:28:20.403
2
When redirecting stdout to a text file, you are changing the stream type from console to file. The console output is unbuffered, while the file output is buffered. Until the program flushes the stdout device (file), the text will continue to buffer until the buffering threshold is reached. If the text "output\n" is printed every 2 seconds, and the threshold is 4,096 bytes, it would take nearly 20 minutes to see any output from the program in the file.
user251069
Posted 2012-07-19T19:32:58.537
Reputation: 21
Not the way I expect it to be. What program are you running? – slhck – 2012-07-19T19:44:16.887
The program itself is irrelevant but is a simple executable that checks something every second and writes the current value to stdout. – Rauffle – 2012-07-19T19:48:41.620
1Is it also empty when the program is finished? – Bernhard – 2012-07-19T19:53:19.687
@Rauffle I guess the program itself may be not irrelevant. Normally, you should have the content in your file. Are you sure the program outputs to STDOUT and not e.g. to STDERR? – speakr – 2012-07-19T19:55:28.013
Does the
jobscommand show that your program is running? It may be stopped for some reason. – garyjohn – 2012-07-19T19:57:50.487Tried with another program that prints to stdout and it's the same thing. If I direct stdout to a file but run it in the foreground the file will have content. If I run it in the background, the file remains empty even after the program finishes running. I'm rather new to CentOS, is this perhaps some kind of 'feature' of the OS? – Rauffle – 2012-07-19T20:00:24.507
1Can you give us more detail on the actual program you are running? Ordinarily, running in the background should make no difference. – chepner – 2012-07-19T20:11:32.260
Maybe your program needs to take input from StdIn and crashed because it's not connected? – billc.cn – 2012-07-19T23:06:19.100
1What about
sh -c './program > file.txt; cat file.txt' &? – Xiè Jìléi – 2012-07-21T14:21:09.913@Xie I've since found a workaround to achieve what I wanted, but this worked (modified to "sh -c './program > file.txt' &"). Want to submit it as an answer? – Rauffle – 2012-07-25T17:40:12.023
@Rauffle Done! :) – Xiè Jìléi – 2012-07-26T00:39:13.907