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Full Discussion: Help in background process
Top Forums Shell Programming and Scripting Help in background process Post 302148874 by gus2000 on Tuesday 4th of December 2007 01:46:01 AM
Old 12-04-2007
Note that this will not work:

Code:
sub.ksh & >/tmp/logfile 2>&1

but this will:

Code:
sub.ksh >/tmp/logfile 2>&1 &

Redirection can get tricky, which is why I add this code to any of my scripts that run in the background or unattended:

Code:
exec >/tmp/${0##*/}.log 2>&1

After this line, all output (stdout and stderr) will go to a file in /tmp using the program basename with a ".log" extension. Don't try this from the command line!

UPDATE: I actually use a base set of code for background jobs:

Code:
# Send all output to a logfile and supress input
typeset LOG="/tmp/${0##*/}.out"
mv $LOG ${LOG}.old >/dev/null 2>&1
[[ -t 1 ]] && echo "Writing to logfile '$LOG'."
exec > $LOG 2>&1
exec < /dev/null 2<&1

Note that the shell expression "${0##*/}" is the same as "basename $0". Also, the "[[ -t ]]" should return TRUE when the process is attached to a terminal. In this case, it just prints a message about the logfile (instead of having the user wonder if the script is even running).

I also close stdin since an unattended script should never need input. For instance, if you have "grep $FILENAME" or "cat $FILENAME" and a bug has left $FILENAME unset, then the script would normally hang waiting for input from stdin. If that input is already /dev/null then the script can continue.
Last edited by gus2000; 12-05-2007 at 04:19 PM..
 

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LEARN ABOUT DEBIAN

bootlogd

BOOTLOGD(8)						Linux System Administrator's Manual					       BOOTLOGD(8)

NAME

       bootlogd - record boot messages

SYNOPSIS

       /sbin/bootlogd [-c] [-d] [-r] [-s] [-v] [ -l logfile ] [ -p pidfile ]

DESCRIPTION

       Bootlogd  runs in the background and copies all strings sent to the /dev/console device to a logfile. If the logfile is not accessible, the
       messages will be kept in memory until it is.

OPTIONS

       -d     Do not fork and run in the background.

       -c     Attempt to write to the logfile even if it does not yet exist.  Without this option, bootlogd will wait for the  logfile	to  appear
	      before attempting to write to it.  This behavior prevents bootlogd from creating logfiles under mount points.

       -r     If there is an existing logfile called logfile rename it to logfile~ unless logfile~ already exists.

       -s     Ensure  that the data is written to the file after each line by calling fdatasync(3).  This will slow down a fsck(8) process running
	      in parallel.

       -v     Show version.

       -l logfile
	      Log to this logfile. The default is /var/log/boot.

       -p pidfile
	      Put process-id in this file. The default is no pidfile.

BUGS

       Bootlogd works by redirecting the console output from the console device.  (Consequently bootlogd requires PTY support in the  kernel  con-
       figuration.)   It  copies that output to the real console device and to a log file.  There is no standard way of ascertaining the real con-
       sole device if you have a new-style /dev/console device (major 5, minor 1) so bootlogd parses the kernel  command  line	looking  for  con-
       sole=... lines and deduces the real console device from that.  If that syntax is ever changed by the kernel, or a console type is used that
       bootlogd does not know about then bootlogd will not work.

AUTHOR

       Miquel van Smoorenburg, miquels@cistron.nl

SEE ALSO

       dmesg(8),fdatasync(3).

								   Jul 21, 2003 						       BOOTLOGD(8)
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