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  • Re: Maybe off topic: Where is bash gesture ${X=Y} described ?

    From Greg Wooledge@21:1/5 to Thomas Schmitt on Wed Jul 24 16:00:01 2024
    On Wed, Jul 24, 2024 at 15:33:11 +0200, Thomas Schmitt wrote:
    GRUB's test scripts often show this gesture

    : "${TMPDIR=/tmp}"

    Well known and described is
    ${X:=Y}
    which assigns a default value to variable X if it is empty (man bash
    says: "If [...] unset or null").
    Also known and described is ":", the no-op command.
    So
    : "${TMPDIR:=/tmp}"
    would not be a riddle. But "=" instead ":=" causes a subtle change in behavior. A defined but empty variable does not get the default content:


    $ man bash
    [...]
    Parameter Expansion
    [...]
    When not performing substring expansion, using the forms documented be‐
    low (e.g., :-), bash tests for a parameter that is unset or null.
    Omitting the colon results in a test only for a parameter that is un‐
    set.

    ${parameter:-word}
    Use Default Values. [...]


    Everyone skips over the sentence that begins with "Omitting the colon".

    Every time we try to tell Chet, "Hey, man, please add examples that
    show BOTH syntaxes", he blows us off, because this is the way POSIX
    documents it. If it's good enough for POSIX, then it must be good
    enough for bash, right?[1]

    Ordinary users search for the SYNTAX. Then when they find a list of
    examples, they only read that list. They don't read the text BEFORE
    the list. They might read text AFTER the list, but even that's rare.

    This is why I created <https://mywiki.wooledge.org/BashSyntaxReference>.
    You can look up syntax there and see what it does.


    [1]Hmm, it looks like POSIX added a table:
    <https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html#tag_19_06_02>
    actually shows both forms, if you scroll down a bit.

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  • From Thomas Schmitt@21:1/5 to All on Wed Jul 24 15:40:01 2024
    Hi,

    maybe i am just too dumb to read the manual, or maybe i made an
    archeological discovery in the shells we use.

    I came to a strange shell gesture which i find neither in man bash nor
    man dash. Nevertheless it works the same in both shells.

    GRUB's test scripts often show this gesture

    : "${TMPDIR=/tmp}"

    Well known and described is
    ${X:=Y}
    which assigns a default value to variable X if it is empty (man bash
    says: "If [...] unset or null").
    Also known and described is ":", the no-op command.
    So
    : "${TMPDIR:=/tmp}"
    would not be a riddle. But "=" instead ":=" causes a subtle change in
    behavior. A defined but empty variable does not get the default content:

    $ unset x
    $ : ${x=YYY}
    $ echo $x
    YYY
    $ x=
    $ : ${x=YYY}
    $ echo $x

    $

    With ":=" the empty variable gets a new stuffing:

    $ : ${x:=YYY}
    $ echo $x
    YYY
    $

    The dash shell instead of bash shows the same behavior.

    Is this legacy behavior ?
    Are there shells which do ${X=Y} but not ${X:=Y} ?

    --------------------------------------------------------------------
    Reason why i ask:

    I came to that gesture while trying to find out why a run of GRUB's
    "make check" as superuser left a lot of empty directories in the
    /-directory, Most GRUB tests do early

    : "${TMPDIR=/tmp}"

    or use variable expansion with default value: ${TMPDIR:-/tmp}
    But tests/grub_cmd_cryptomount uses TMPDIR naively so that its test
    directories end up as
    /1718898505.LUKS1_test_cryptsetup_defaults
    ...
    /1718912880.LUKS2_test_with_second_key_slot_and_first_slot_using_different_password

    I would like to talk GRUB into using ${X:=Y} generally, because it does
    not matter whether TMPDIR is defined empty or is unset.
    So i need background knowledge about the currently used ${X=Y}.

    (One may say that is not a good idea to run a test as superuser.
    But most GRUB tests create and mount filesystems. So after a change in
    the tool grub-fstest i had to do it as superuser to check for new test
    failures caused by my change. It was only one and that was expected.)


    Have a nice day :)

    Thomas

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  • From tomas@tuxteam.de@21:1/5 to Greg Wooledge on Wed Jul 24 16:10:01 2024
    On Wed, Jul 24, 2024 at 09:50:36AM -0400, Greg Wooledge wrote:

    [...]

    Everyone skips over the sentence that begins with "Omitting the colon".

    Thanks for this one. I know /I/ did for long enough.

    Greg, this place would be a lot less helpful...

    Every time we try to tell Chet, "Hey, man, please add examples that
    show BOTH syntaxes" [...]

    ...and definitely more boring without you. Thanks for that :-)

    Cheers
    --
    t

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  • From Thomas Schmitt@21:1/5 to Greg Wooledge on Wed Jul 24 16:10:02 2024
    Hi,

    i wrote:
    maybe i am just too dumb to read the manual, or maybe i made an archeological discovery in the shells we use.

    Greg Wooledge wrote:
    Everyone skips over the sentence that begins with "Omitting the colon".

    Oh well. Too dumb and not alone.

    I did read the sentence immediately before "Omitting the colon".

    In man dash the statement comes after the list of modifications:

    In the parameter expansions shown previously, use of the colon in the
    format results in a test for a parameter that is unset or null; omission
    of the colon results in a test for a parameter that is only unset.

    If there would be written ":" instead of "colon" i might have passed the
    brain agility test.


    Have a nice day :)

    Thomas

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