Changes between Initial Version and Version 1 of Internal/OpenFlow/miscUnix

Timestamp:

Feb 19, 2012, 11:51:52 PM (13 years ago)

Author:

akoshibe

Comment:

—

Internal/OpenFlow/miscUnix

@@ +1 @@
+= *Nix related tidbits... =
+This is a page on various tools and techniques related to *nix administration, programming, disaster recovery - basically, odds and ends that come in handy once every so often. 
+Anything based on a link is reiterated just for completeness, and just in case a link dies. 
+
+== Redirecting the output of a live process. ==   
+source: http://etbe.coker.com.au/2008/02/27/redirecting-output-from-a-running-process/
+
+Say that you have a running process whose outputs are either being piped to /dev/null or a terminal that you can't get a hold of, but you'd like to be able to see its output. Conversely, you may want to redirect something's output to /dev/null. You can force the process to change its output using `gdb`, the GNU debugger. This is a powerful tool that allows you to attach to a live process and inspect and manipulate the contents of its address space.
+
+Here I have a script that I had backgrounded but is still outputting to my terminal (annoying). I will redirect the program's output to a file. 
+
+1. Find the PID of your process. Mine is called arrival_collector.rb, so I look for it using `ps`.  
+{{{
+# ps -ef | grep collector
+root     17793 11276  0 18:39 pts/1    00:00:00 ruby arrival_collector.rb -r a,h,ee,f,wknd1,wknd2 -s scott,busch_a,pubsafs,foodsci,scott,scott -v
+}}}
+
+/proc/ shows that its STDOUT is my terminal, /dev/pts/1:
+{{{
+# ls -l /proc/17793/fd
+total 0
+lrwx------ 1 root root 64 2012-02-19 18:45 0 -> /dev/pts/1
+lrwx------ 1 root root 64 2012-02-19 18:45 1 -> /dev/pts/1
+lrwx------ 1 root root 64 2012-02-19 18:40 2 -> /dev/pts/1
+lrwx------ 1 root root 64 2012-02-19 18:45 3 -> socket:[1566783742]
+}}}
+
+2. Attach to the process with GDB. 
+{{{
+# gdb -p [PID] [path/to/executable]
+}}}
+
+It wil spew a bunch of output but eventually give you a prompt:
+{{{
+# gdb -p 17793 /opt/grailrtls/grail3_ruby/grail3protocols/arrival_collector.rb
+GNU gdb (GDB) 7.2-ubuntu
+Copyright (C) 2010 Free Software Foundation, Inc.
+...
+Loaded symbols for /lib/libnss_files.so.2
+0xb78c7424 in __kernel_vsyscall ()
+(gdb) 
+}}}
+
+3. Close STDOUT:
+{{{
+(gdb) p close(1)
+$1 = 0
+}}}
+
+4. Point STDOUT to a file. Like in the link, I will use creat() to point STDOUT to a log file in /tmp/. creat() takes the path to the file and its permissions as the two arguments. 
+{{{
+(gdb) p creat("/tmp/output.log", 0600)
+$2 = 1
+}}}  
+
+5. Exit gdb. Choose yes when asked if you want to quit.
+{{{
+(gdb) quit
+A debugging session is active.
+
+        Inferior 1 [process 17793] will be detached.
+
+Quit anyway? (y or n) y
+Detaching from program: /usr/bin/ruby1.8, process 17793
+}}}
+
+Now when you check /proc/ you should see that your STDOUT is directed att the file you created:
+{{{
+# ls -l /proc/17793/fd
+total 0
+lrwx------ 1 root root 64 2012-02-19 18:45 0 -> /dev/pts/1
+lrwx------ 1 root root 64 2012-02-19 18:45 1 -> /tmp/output.log
+lrwx------ 1 root root 64 2012-02-19 18:40 2 -> /dev/pts/1
+lrwx------ 1 root root 64 2012-02-19 18:45 3 -> socket:[1566783742]
+}}} 
+
+you can confirm this with `tail -f` (or the fact that your program has stopped outputting to terminal).