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Using TMOUT as default for read bultin is unwise

From: Wheatley, Martin R
Subject: Using TMOUT as default for read bultin is unwise
Date: Wed, 14 Sep 2011 09:46:11 +0100 (BST)

Configuration Information [Automatically generated, do not change]:
Machine: sparc
OS: solaris2.7
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='sparc' 
-DCONF_OSTYPE='solaris2.7' -DCONF_MACHTYPE='sparc-sun-solaris2.7' 
-DCONF_VENDOR='sun' -DLOCALEDIR='/usr/local/depot/bash-3.2.17/share/locale' 
-DPACKAGE='bash' -DSHELL  -DHAVE_CONFIG_H -DSOLARIS   -I.  -I. -I./include 
-I./lib   -g -O2
uname output: SunOS gen-off-11 5.10 Generic_144488-17 sun4u sparc 
Machine Type: sparc-sun-solaris2.7

Bash Version: 3.2
Patch Level: 17
Release Status: release

        The contents of the environment variable TMOUT are used are the
        default timeout for the read builtin command when reading from
        a TTY or pipe AND as a shell command input idle time.

        This can lead to random failures of shell scripts

        Take the following example script snippet...

        ACQcommand | while read action
                executeTheAction "$action"

        Assume that ACQcommand generates output at around 90 second intervals
        and does so day-in/day-out until it needs to terminate because
        of an error condition.

        If a user has TMOUT=0 or TMOUT set to a value that is significantly
        greater than 90 in their profile then the above works fine.

        IF TMOUT is < 90 then the above fails repeatedly.

        If TMOUT=90 then the above sometimes works and sometimes fails.


        the -t option to read allows a specific timeout to be set to override
        the default timeout (as defined by TMOUT) BUT specifing "-t 0" does NOT 
        the timeout - which means that the only reliable way to have a read NOT 
        is to use...

                TMOUT=0 read junk

        Since a user may assert TMOUT to any value at any time then any script
        that uses 'read' can randomly fail - which must surely be undesirable.

        I think the TMOUT should not be overloaded and its use as a default 
        for the read builtin - especially for sub-shell pipelines is dangerous
        and should be discontinued otherwise all bash scripts that use the read 
        need to be modified to include TMOUT=0.

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