Ttyjfyk

When you run jobs like the example below, they're killed when you logout:

$ ./job.sh &

However, when you execute them as in the below example, redirecting stdout to /dev/null and stderr to stdout as well as putting the job in the background, they're not killed when you log out. It's somewhat like nohup, except that the output goes to /dev/null instead of a file.

$ ./job.sh > /dev/null 2>&1 &

I noticed that some daemons work like that and then I got curious to know the reason why they aren't killed at logout.

Can someone explain me why?

EDIT 1:

As suggested by @Patrick on his comment, I tried the same with the simplest job ever and the result is the same: the job does not die when I log out.

# while true; do sleep 1; done > /dev/null 2>&1 &
[1] 4320
# logout

luis-macbook:~ luis$
luis-macbook:Downloads luis$ ssh server
luis.alen@server's password: 
# jobs
#
# ps -ef | grep 4320
root 4320 1 0 01:17 ? 00:00:00 -bash

EDIT 2:

As requested by Patrick again, I did the same test without redirecting the streams and, to my surprise, the process didn't die. Now I'm completely confused... I swear I remember processes put in background dying when you logged out. Am I absolutely wrong?

# while true; do sleep 1; done &
[1] 20720
# logout
luis-macbook:~ luis$ ssh server
# jobs
#
# ps -ef | grep 20720
root 20720 1 0 23:31 ? 00:00:00 -bash

As for daemons doing it, that's because they want any output or error messages they might produce to be discarded no matter how you redirect a process's input and output streams, it will still be SIGHUP'd if it's attached to a session and that session is closed to leave processes running.

To leave processes running, there are few approaches:

1. detach them from the session — daemons do this by forking a new process and then exiting the original process; now the new process has no parent and is adopted by init
   you can also accomplish that using the bash internal command disown




2. use nohup to block the process from receiving the SIGHUP when the session dies; the process doesn't get SIGHUP, doesn't exit, its parent dies and init adopts it




3. attach it to a session that won't die — use screen

Please recheck your work on statement "Jobs aren't killed at logout when executed with > /dev/null 2>&1 &"

It's the behavior of jobs on bash as far as I know, I did a test here and it kept running with those streams opened as well:

sergiopa@sergiopa:~/Downloads$ find / -print >/dev/null 2>&1 &
[1] 14152
sergiopa@sergiopa:~/Downloads$ jobs
[1]+ Running find / -print > /dev/null 2>&1 &
sergiopa@sergiopa:~/Downloads$ ps -ef | grep find
sergiopa 14152 13913 10 15:47 pts/18 00:00:01 find / -print
sergiopa 14195 13913 0 15:48 pts/18 00:00:00 grep --color=auto find
sergiopa@sergiopa:~/Downloads$ 

sergiopa@sergiopa:~/Downloads$ lsof -c find
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
find 14152 sergiopa cwd DIR 252,1 4096 4808 /usr/src/linux-headers-2.6.35-22/drivers/rapidio
find 14152 sergiopa rtd DIR 252,1 4096 2 /
find 14152 sergiopa txt REG 252,1 141980 391686 /usr/bin/find
find 14152 sergiopa mem REG 252,1 1421892 26489 /lib/libc-2.12.1.so
find 14152 sergiopa mem REG 252,1 118084 26452 /lib/ld-2.12.1.so
find 14152 sergiopa mem REG 252,1 149392 26485 /lib/libm-2.12.1.so
find 14152 sergiopa mem REG 252,1 30684 26469 /lib/librt-2.12.1.so
find 14152 sergiopa mem REG 252,1 121578 26490 /lib/libpthread-2.12.1.so
find 14152 sergiopa mem REG 252,1 2768240 391861 /usr/lib/locale/locale-archive
find 14152 sergiopa 0u CHR 136,18 0t0 21 /dev/pts/18 <<<<
find 14152 sergiopa 1w CHR 1,3 0t0 4293 /dev/null <<<<
find 14152 sergiopa 2w CHR 1,3 0t0 4293 /dev/null <<<<
find 14152 sergiopa 3r DIR 252,3 4096 16777344 /home/sergiopa/Downloads
find 14152 sergiopa 4r DIR 252,3 4096 16777344 /home/sergiopa/Downloads
find 14152 sergiopa 5r DIR 252,1 4096 4808 /usr/src/linux-headers-2.6.35-22/drivers/rapidio
sergiopa@sergiopa:~/Downloads$ 

See lines with '<<<<', those are STDIN,STDOUT and STDERR.
Closed the shell but find is still running.