topic Re: Problems with nohup in Operating System - HP-UX

Problems with nohup

Brian Atkins — Fri, 13 Jul 2001 14:06:30 GMT

When attempting to nohup a script, the script apepars to start, but hangs. After entering the command, nohup reports back with the PID, but doesn't return to a command line. If I touch , I receive a second messsage that the script has stopped.

# nohup /root/bin/ett_load.ksh &
[1] 2496
# Sending output to nohup.out

(hangs so I touch )

[1] + Stopped (tty output) nohup /root/bin/ett_load.ksh &

The man pages state that nohup will fail if the user does not have sufficient permission to write to the current directory or to their home directory. However, this is not the case, especially since the script requires to run as root.

I am certain that the problem is not with the script itself as I have tried it without nohup and it functions as advertised.

The only item that I think might be affecting it is that the script su's to several users to perform different functions.

Re: Problems with nohup

James R. Ferguson — Fri, 13 Jul 2001 14:15:52 GMT

Hi Brian:

The 'su' within your script may be causing the problem. If the script is running as root, then no password is necessary to do the 'su'. If you are not running as root, and issue an 'su' then the process is attempting to read from stdin. Since redirection to 'su' isn't allowed, the script will hang.

...JRF...

Re: Problems with nohup

Brian Atkins — Fri, 13 Jul 2001 14:26:33 GMT

James,

When the script is being executed I am logged in as root and the script is being run as root. The su's in question are to oracle and another user.

Brian

Re: Problems with nohup

Robin Wakefield — Fri, 13 Jul 2001 14:46:54 GMT

What happens if you redirect stdout and stderr, even though nohup should handle it:

nohup {script} >/tmp/stdout 2>/tmp/stderr &

Robin.

Re: Problems with nohup

A. Clay Stephenson — Fri, 13 Jul 2001 14:51:39 GMT

Hi Brian,

One other thing that could cause this is if somewhere in your script (and especially if your are executing .profile's) there may be a stty tostop which will block when background jobs try to produce output.

Clay

Re: Problems with nohup

Brian Atkins — Fri, 13 Jul 2001 14:54:30 GMT

I tried that one, too, thinking that there might be some sort of contengency with permissions or another nohup, so I did:

# nohup /root/bin/ett_load.ksh 1>/tmp/output 2>/tmp/errors &

I did this hoping at least I might get something that would ive me some clue as to what was wrong. No dice.

By the way, system logs don't report anything unusual either.

Re: Problems with nohup

Laurent Paumier — Fri, 13 Jul 2001 15:01:38 GMT

Try redirecting stdin from /dev/null. If there are commands playing with the terminal (such as stty), that might help.

Re: Problems with nohup

Brian Atkins — Fri, 13 Jul 2001 15:11:06 GMT

If I understood you correctly, I did the following:

# nohup /root/bin/ett_load.ksh < /dev/null &

But, unfortunately got the same error...

Re: Problems with nohup

Brian Atkins — Fri, 13 Jul 2001 15:19:53 GMT

Clay,

I checked stty setting in the .profile for the 2 users referenced in the script. What I came up with was:

stty erase "^H" kill "^U" intr "^C" eof "^D"
stty hupcl ixon ixoff

I'm not sure exactly what the second line does, but I'm pretty sure that the firstline simply gives the user the frequently used keyboard functions (i.e., backspace key, esc key). I did try commenting them out and re-running the script. No dice.

Nohup 10
SysAdm 0

Re: Problems with nohup

James R. Ferguson — Fri, 13 Jul 2001 15:19:54 GMT

Hi Brian:

This is interesting. Here's a simple script that executes fine in the foreground but hangs for SIGTTOU (background process attempting write):

#!/usr/bin/sh
su - anyuser -c "echo `date`"
#.end.

Call the script "my.sh" and:

./my.sh #...works as expected...
./my.sh > /dev/null 2>&1 #...works...
./my.sh > /dev/null 2>&1 & #...hangs...

Therefore...?

...JRF...

Re: Problems with nohup

Brian Atkins — Fri, 13 Jul 2001 15:26:34 GMT

James,

So what you're tring to impress upon me is the fact that:

1) The problem is not with nohup, but with the process running in the background.
2) The background process is attempting to write to the terminal, but can't and therefore hangs.

So, somewhere I need to redirect how it is writing or where it is writing to...

Re: Problems with nohup

Tracey — Fri, 13 Jul 2001 15:29:18 GMT

Does anything get created in the output file? Try putting: set -x as the first line of the script. This will echo to the output file each line in the main script as it executes. That way at least you might be able to find out which line it is failing on.

Re: Problems with nohup

Brian Atkins — Fri, 13 Jul 2001 15:35:42 GMT

Tracy,

I tried the set -x and got the same output in the nohup.out file. Nothing.

I don't think that the script is the problem. It really would appear to be something more like the ability of nohup (or &) to write output...

Re: Problems with nohup

James R. Ferguson — Fri, 13 Jul 2001 15:46:23 GMT

Hi Brian:

OK. You must be doing 'su - user'. Do a simple 'su user' and then all should be OK. Clay had it nailed. The script I posted was an effort to dissect things down to a simple test case. The SIGTTOU is the tell-all.

The 'su - user' causes "user"'s profile to be read and output. Usually this is done to get environmental variables. Instead, put the variables you need in your script and do a simple "su".

Regards!

...JRF...

Re: Problems with nohup

A. Clay Stephenson — Fri, 13 Jul 2001 15:50:45 GMT

Hi Brian,

James' test made it possible to nail this.
What is going on is then the stty's and the tset's in the standard .profile are blocking when in background.

I was able to add this block of code around the stty's ands tset's and that fixed the problem.

if [ -t 0 -a -t 1 -a -t 2 ]
then
do tset and stty commands
fi

This tests if stdin,stdout, and stderr is a terminal. If so, do the stty/tset stuff otherwise bypass the stty/tset stuff.

If you will modify the .profiles of each user you su - to, I think you will be fixed.

Regards and an interesting problem, Clay

Re: Problems with nohup

A. Clay Stephenson — Fri, 13 Jul 2001 15:53:26 GMT

Hi again Brian,

If you have any tabs commands in your .profile then the same rules apply.

Regards, Clay

Re: Problems with nohup

Deshpande Prashant — Fri, 13 Jul 2001 15:55:59 GMT

HI
Can you try redirecting output in script to log file before you start your actual commands.

exec >LOGFILE 2>&1
...

Thanks.
Prashant Deshpande.

Re: Problems with nohup

Laurent Paumier — Fri, 13 Jul 2001 16:00:17 GMT

If it's just the commands in user's .profile that hang, you could run the command in the foreground and, after a few seconds, ^Z and bg... Not a clean solution, but maybe a workaround the problem.

Re: Problems with nohup

Brian Atkins — Fri, 13 Jul 2001 17:13:35 GMT

Man, this is a lot tougher than I originally thought. Alas, no joy. Clay, Laurent, and Prashant, I still cannot get it to work correctly. I have attempted all three of your suggestions, but still get the same results. Here is an excerpt from one of the .profiles, after the mod Clay suggested, that are referenced by the script:

INTER=$(set -o |grep interactive | awk '{print $2}')
if [ "$INTER" = "on" ]
then
# Set up the terminal:
if [ "$TERM" = "" ]
then
eval ` tset -s -Q -m ':?hp' `
else
eval ` tset -s -Q `
fi

if [ -t 0 -a -t 1 -a -t 2 ]
then
stty erase "^H" kill "^U" intr "^C" eof "^D"
stty hupcl ixon ixoff
tabs
fi
fi

Hope I did it correctly.

Re: Problems with nohup

A. Clay Stephenson — Fri, 13 Jul 2001 17:22:39 GMT

Hi Brian,

Put the if [ -t 0 ... ]
around the tset code as well. It too will block.

Also, do a ps while you are blocked and you should see the process in question.

I would first try James's little script as an su - username -c command to see if you get the .profiles fixed before moving on to more your obviously more complicated version.

This is exactly why it is better to set up Oracle or other environments in a location like /usr/local/bin/oraenv.sh and then include them
in your .profiles via . /usr/local/bin/oraenv.sh.

You could then simply do an su username -c somescript and somescript also includes /usr/local/bin/oraenv.sh.

I think you can get your existing profiles to work,it's just more difficult this way.

Clay