Mailinglist Archive: opensuse-factory (602 mails)

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[opensuse-factory] Can we assume that /bin/sh is bash?
Hi Jan, all,

I'd like to carry our OBS dicussion to a wider audience
(https://build.opensuse.org/request/show/672510).

The question is whether we can assume that "/bin/sh" links to bash,
in particular whether rpm scriptlets without explicit -p interpreter
can be assumed to interpreted by bash.

I'm aware that, in principle, /bin/sh is supposed to be the Bourne
shell on Unix systems. But as a matter of fact, on current openSUSE, it
is not. Unless it's tampered with, /bin/sh is a symlink to /bin/bash.
bash is not started in full POSIX mode if invoked as /bin/sh, and even
if it's in POSIX mode, it supports some extensions over the POSIX shell
spec (e.g. the [[ ]] construct), which makes it behave differently than
another shell not supporting [[ ]] would (*). Problably there are more
differences, I can't claim to know them all.

Here are some arguments why I think it'd be reasonable to assume that
/bin/sh is bash on openSUSE:

1. patterns-base-minimal_base depends on bash, and the /bin/sh symlink
is a non-configurable part of the "bash" package.
2. we could handle /bin/sh via /etc/alternatives, but we don't.
3. our Wiki suggests testing failing scriplets using "bash -xv"
(https://en.opensuse.org/openSUSE:Packaging_scriptlet_snippets)
4. /bin/sh has pointed to bash for a long time (not sure how long
exactly).

FTR, Fedora basically guarantees (sh == bash) for the purpose of rpm
scriptlets (https://fedoraproject.org/wiki/Packaging:Scriptlets). So
Fedora <-> openSUSE portability may also be an issue to consider.

If we can't assume that /bin/sh is bash, what else can we assume? I
recall from earlier work that writing really 100% compatible shell code
for all kinds of environments is really hard. E.g., "[" isn't 100%
portable either, even though it's part of the POSIX "test" standard
(http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html).

We should have clear rules which syntax expressions can be used in rpm
scriptlets, and which can't. IMO we should define one of the various
existing shells as a reference by saying "if it's supported
by this shell, it's valid scriptlet code". That'd be easier (especially
for testing compliance) than referring to a spec. That reference shell
doesn't have to be bash, of course.

Thanks
Martin

(*) For example:

# ln -s /usr/bin/echo '/usr/bin/[['
# ls -l /bin/sh
lrwxrwxrwx 1 root root 4 Feb 8 10:38 /bin/sh -> bash
# /bin/sh --posix -c '[[ 2 = 1 ]] && echo yes'
# dash -c '[[ 2 = 1 ]] && echo yes'
2 -eq 1 ]]
yes

--
Dr. Martin Wilck <mwilck@xxxxxxxx>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)


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