Dear bash(1) team,
I have recently identified a failure mode in bash(1) scripts that has
plagued us with random process failures for over 20 years!
The read(1) built-in command has the -t option to specify a timeout on
the read and it appears that the TMOUT variable is also honoured as an
alternative if it is defined (and this is where I think there is an issue).
>From the bash(1) manual entry...
The time-out is applied if read(1) is reading from a terminal,
pipe or other special file.
Since TMOUT can be inheritted from a much 'higher' process a script has
no knowledge about whether it has been set or not nor whether the value
is appropriate for the script.
Because the read(1) can time-out if the input is from a pipe then the
following can randomly fail in the "while read l" command if the input
generation command takes longer than TMOUT seconds to geneate any output.
someCommandThatTakesManyManySecondsBeforeGeneratingAnyOutput | while
read l
do
echo "$(date) $l"
done
The issue can be demonstrated using...
sh -c "sleep 10 ; date" | while TMOUT=10 read l ; do echo l=$l ; done
If TMOUT is -gt 10 then the "echo l=" output is seen; if TMOUT is -lt
10 then the "echo l=" output is not seen and since TMOUT can be
specified by a parent process rather than on the command line then
random failures occur.
I can (and now do) 'unset TMOUT' where I know my scripts are
susceptible to this issue but would ask that you consider whether TMOUT
should ever be considered when reading from a pipe.
I would argue that read(1) from a pipe should not time-out unless the
code explicitly specifies a time-out with the -t option.
Regards
Martin Wheatley
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Martin Wheatley
Solaris Systems Administrator (JET)
Computing Division
Providing services on behalf of United Kingdom Atomic Energy Authority
Culham Campus, Abingdon, OX14 3DB, UK
Tel. +44 1235 464784
Mob.
Email: [email protected]
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