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Re: "here strings" and tmpfiles

From: L A Walsh
Subject: Re: "here strings" and tmpfiles
Date: Sun, 07 Apr 2019 13:06:21 -0700
User-agent: Thunderbird

On 3/20/2019 5:19 AM, Greg Wooledge wrote:
> On Wed, Mar 20, 2019 at 07:49:34AM +0700, Robert Elz wrote:
>> However, using files for here docs makes here docs unusable in a shell
>> running in single user mode with no writable filesystems (whatever is
>> mounted is read only, until after file system checks are finished).
> Meanwhile, proposals based around /dev/fd/* would also make here docs
> unusable in a shell running early in the boot process, before all
> file systems are mounted.
> Just like that one time L. Walsh tried to write a bash boot script that
> used <() to populate an array, and it failed because she was running
> it too early in the boot sequence, and /dev/fd/ wasn't available yet.

    /dev/fd was available, and so was /proc that it symlinked to.
What wasn't available was "/tmp" being mounted as a writeable
file system. I.e. -- exactly the case we are talking about being
a problem *AGAIN*.

    Various boot processes use /dev and /proc before any file systems
are mounted.  Requiring a mounted, writeable file system to run a shell
script during boot was the reason I had problems.

> So, my counterpoints are:
> 1) Leave it alone.  It's fine.
    No, it's not -- it's been biting people for the past 4
years or omre.
> 2) Don't use bash for scripts that run early in the boot sequence.
    unacceptable as bash is used as *THE* defacto linux shell.
> 3) Whatever features you *do* use in boot scripts, make sure they're
>    available at the point in the boot sequence when the script runs.
    Pipes are available in the OS before any user scripts are run.

> 4) Whatever features you use in scripts *in general*, make sure you
>    understand how they work.
    No... Do you understand how your TV works to watch it?   Or
your microwave, in order to heat food.   This attitude is why so
many people have resisted using computers -- because programmers who
made "friendly user interfaces", were outnumbered in the 1990's by
those who got liberal arts degrees and thought that qualified them
as a software programmer.   They often could write programs that
worked, but required more support and user training because most
of them don't know how to design something friendly. 

    The features should behave according to the documents.  That's
why in some cases, I've tried to get wording improved - like the
person recently who couldn't find documentation for '+=-?' along
side ':+ := := :?', because it was buried in a passing sub-clause
in a prior section (I never could find it either and assumed it
was some old shell practice that will be supported to the end of
time,  but is no longer 'in favor', like $[integer exp] vs. using
$((integer exp)).
> Even if Chet changed how here docs work in bash 5.1, nobody would
> be safe to use those features in their "I'm feeding a password with
> a here string" scripts for at least 20 years, because there will
> still be people running older versions of bash for at least that long.
    So untrue.  If the system boots on bash5.1, because that's what
ships on linux 5.x from vendors, then that's what will be there.
We aren't porting OS-boot scripts from linux to machines that can't
run current software requirements.

    Your script doesn't have to support Bourne Shell 1.0.  It might
have to support some posix implementation -- but bash doesn't even
support aliases working in interactive mode by default -- as required
to be posix compatible.  That means anyone relying on aliases to work
because they are using the posix requirements as a minimum, will be
surprised when a user uses bash and aliases are broken (don't work,
not enabled) by default -- they work in any posix compatible shell
which bash claims to be, but disassociates its posix mode from
some ancient-no alias mode such that toggling the posix bit resets
multiple features and doesn't save and restore those feature when
toggling it back.  It's like it's posix mode was designed to
not play well with normal bash function -- like it was designed to
be broken.

    If you enter an optional mode, that an later exit, it's a
basic computer software 'given', that the previous mode should be
restored, by default.  Global effects are generally considered
a poor practice because of the tendency to cause unexpected effects
"at a distance" (far from the point they were changed).

> Thus, leave it alone.

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