stderr, stdout, stdin , how to output to linux file

http://tldp.org/HOWTO/Bash-Prog-Intro-HOWTO-3.html

There are 3 file descriptors, stdin, stdout and stderr (std=standard).

Basically you can:

1. redirect stdout to a file
2. redirect stderr to a file
3. redirect stdout to a stderr
4. redirect stderr to a stdout
5. redirect stderr and stdout to a file
6. redirect stderr and stdout to stdout
7. redirect stderr and stdout to stderr

I  stderr to a file. 

scrapy crawl XXX 2> nohup.txt

or 

scrapy crawl XXX 2>> nohup.txt

">>"  means append.