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man page confusion about array with no members: var=()


From: idallen
Subject: man page confusion about array with no members: var=()
Date: Wed, 13 Jul 2016 16:36:00 -0400 (EDT)

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='x86_64' 
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='x86_64-pc-linux-gnu' 
-DCONF_VENDOR='pc' -DLOCALEDIR='/usr/share/locale' -DPACKAGE='bash' -DSHELL 
-DHAVE_CONFIG_H   -I.  -I../. -I.././include -I.././lib  -Wdate-time 
-D_FORTIFY_SOURCE=2 -g -O2 -fstack-protector-strong -Wformat 
-Werror=format-security -Wall
uname output: Linux idallen-oak 4.4.0-28-generic #47-Ubuntu SMP Fri Jun 24 
10:09:13 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 4.3
Patch Level: 46
Release Status: release

Description:
        The BASH man pags says "When there are no array members,
        address@hidden expands to nothing." and then later "An array variable
        is considered set if a subscript has been assigned a value.".

        The first sentence tells me that address@hidden is a valid use of a
        variable that exists but has no members, but the second sentence
        implies that if the array has no members it is considered "unset"
        and thus address@hidden would not be a valid use of the name.

        These are contradictory statements.  Using address@hidden with
        no array members generates an "unbound variable" error under
        "nounset"; it does not expand to "nothing".

        The reason address@hidden expands to nothing is not because it's
        a valid use of address@hidden, it's because any use of an unset
        variable expands to nothing, unless, as I do, you run with
        "nounset" enabled and it causes an error.

        To fit current behaviour, the first man page sentence above should
        be changed to say: When there are no array members, the array
        name is considered unset and address@hidden will expand to nothing or
        generate an "unbound variable" error under the "nounset" option.

        I'd prefer that BASH change the behaviour of var=() to not give
        an "unbound variable" error on address@hidden, but changing the man
        page is probably easier than changing the current behaviour.

Repeat-By:
        #!/bin/bash -u

        # make BASH complain about unset variables
        set -o nounset

        echo 'ONE: set var= and try $var and address@hidden - both work without 
error'
        unset var
        var=
        ( echo "ONE: [$var]" )
        ( echo "ONE: address@hidden" )

        echo 'TWO: set var=() and try again - both fail with "unbound variable"'
        unset var
        var=()
        ( echo "TWO: [$var]" )
        ( echo "TWO: address@hidden" )



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