Re: Ignoring trap command for a grep function

From: Henry Townsend (henry.townsend_at_not.here)
Date: 07/14/05

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    Date: Wed, 13 Jul 2005 19:06:14 -0400

    Chris F.A. Johnson wrote:
    > On 2005-07-13, sharma.raghvendra@gmail.com wrote:
    >
    >>Hi,
    >>
    >>We have a trap statement for every non zero returning command/function.
    >
    >
    > Don't.

    I don't make a habit of disagreeing with CFAJ but must make an exception
    in this case. Using a trap on ERR in shell programming turns it into an
    exception-throwing language like Java[*]; you can catch the exceptions
    (non-zero exit statuses) that you care about as special cases and all
    others result in fatal errors. It's as robust a shell programming style
    as I know. If using ksh or bash you can add useful data such as

            trap 'echo Failed at $0:$LINENO >&2' ERR

    >>Are their any possibilities ? We want to avoid changing the design
    >>involving the use of trap in and around the trap statement.
    >
    >
    > The design is inappropriate for your script; change it.

    I always tell people to add a new shell builtin to their lexicon: "||:"
    (without the quotes). Of course it's really two old builtins jammed
    together to mean "ignore preceeding exit status". In other words
    '>/dev/null' means "throw away stdout", '2>/dev/null' means "throw away
    stderr", and '||:' means "throw away return code". So you take your grep
    command, and any others for which you don't want to fail on failure, and
      stick ||: on the end:

            grep foobar /etc/motd ||:

    [*] The merits of exceptions vs manual error checking can be and have
    been debated in other places; I simply note that the exception style
    cannot be said to be "wrong", though some may not be used to or prefer
    it. I happen to think you can't write a large reliable shell script
    without it. 

    -- 
Henry Townsend
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