Re: Bash bug?

From: Robert Nichols (rnicholsNOSPAM_at_comcast.net)
Date: 09/26/05

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    To: fedora-list@redhat.com
Date: Mon, 26 Sep 2005 01:31:02 -0500

    Zoltan Boszormenyi wrote:
    > Zoltan Boszormenyi írta:
    >
    >> Hi,
    >>
    >> I am trying something like the following,
    >> with a configuration file containing a token and
    >> a directory in a line, depending on the tokens,
    >> certain actions should be taken later. Validating
    >> the configuration file whether all the required/optional
    >> tokens are in the file should go like this:
    >>
    >> -----a.txt----------------------
    >> A directory1
    >> B directory2
    >> C directory3
    >> --------------------------------
    >>
    >> -----a.sh-----------------------
    >> #!/bin/bash
    >>
    >> HAS_A=0
    >> HAS_B=0
    >> HAS_C=0
    >> cat a.txt | while read i ; do
    >> if [ "`echo $i | awk '{ print $1 }'`" = "A" ]; then
    >> HAS_A=1
    >> fi
    >> if [ "`echo $i | awk '{ print $1 }'`" = "B" ]; then
    >> HAS_B=1
    >> fi
    >> if [ "`echo $i | awk '{ print $1 }'`" = "C" ]; then
    >> HAS_C=1
    >> fi
    >> echo "A: $HAS_A B: $HAS_B C: $HAS_C"
    >> done
    >> echo "Final A: $HAS_A B: $HAS_B C: $HAS_C"
    >> --------------------------------
    >>
    >> Result is:
    >>
    >> --------------------------------
    >> $ ./a.sh
    >> A: 1 B: 0 C: 0
    >> A: 1 B: 1 C: 0
    >> A: 1 B: 1 C: 1
    >> Final A: 0 B: 0 C: 0
    >> --------------------------------
    >>
    >> It seems to be a bug to me, the envvars lose their values
    >> they gained in the loop. It's an ancient bug I must add,
    >> I just rechecked it and bash in RedHat 7.1 behaves the same.
    >> How can I preserve the variables' values? Putting "export"
    >> in front of every assignments doesn't help.
    >>
    >> Best regards,
    >> Zoltán Böszörményi
    >
    >
    > I just tried to narrow the test for the bug:
    >
    > ---b.sh-------------------------
    > #!/bin/bash
    >
    > X=0
    > A=2
    > while [ $A -gt 0 ]; do
    > X=1
    > A=$(($A - 1))
    > echo "A: $A X: $X"
    > done
    > echo "Final A: $A X: $X"
    > --------------------------------
    >
    > Result:
    >
    > --------------------------------
    > $ ./b.sh
    > A: 1 X: 1
    > A: 0 X: 1
    > Final A: 0 X: 1
    > --------------------------------
    >
    > The variables here keep their values. Hm. Then the bug may come
    > from the "cat a.txt | while read i; do" construct, maybe another
    > sub-shell is opened for this loop. But then putting "export"
    > in front of the assignments inside the loop should solve this but
    > it doesn't. It still seems to be a bug to me somewhere.

    Yes, the "cat a.txt | while read i; do ..." construct does cause
    the "while" loop to be executed in a subshell. Everything is working
    as it should. The only bug is in your understanding of how processes
    and environments work. Each process has its very own environment, and
    exporting a variable causes a child process's initial enviromnent to
    receive a copy of that variable. Nothing the child subsequently does
    to that copy has any effect on the parent process.

    Hopefully you've got a copy of the bash FAQ installed on your system
    (/usr/share/doc/bash-3.0/FAQ on mine). Take a look at answer E4. 

    -- 
Bob Nichols         Yes, "NOSPAM" is really part of my email address.
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