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A Real Bourne Shell for Linux?

Posted by Cliff on from the is-bash-really-all-that-different dept.

the_code_poet asks: "I'm a lead developer for a software development company, and one of my responsibilities has been writing an installer for our product (of which Linux is one of the platforms). In keeping with UNIX tradition, the installer is written in shell (thats /bin/sh), but as many of you know there is no Bourne shell for Linux - only bash. This has caused inconsistencies (mostly bugs in bash) when writing a generic UNIX sh script that works fine on commerical *NIX's." For a semi-complete list of differences between bash and sh, you will want to check out section C1 of the Bourne Again Shell FAQ. To be honest, I have yet to run into much trouble with a script starting with #!/bin/sh with /bin/bash, and I've been using the latter for years. If any of you have had problems related to this, please tell us what the problem was and how you solved it. Also: would anyone out there be interested in writing a real Bourne Shell for Linux?

"On every distro I've ever seen /bin/sh is just a soft link to /bin/bash. If bash is invoked with sh as its name (argv[0]) then its supposed to act like Bourne - but that just doesnt happen (for example: export FOO=bar is *not* valid Bourne shell syntax, you must say FOO=bar; export FOO)

Do you think that the startup scripts for most distributions would break because, even though they say #!/bin/sh at the top, they REALLY mean #!/bin/bash?

Given that there is no real Bourne shell for Linux, and that bash has an exhorbitant file size. Quoting bash's man page, here: '...it's too big and too slow' for something that is to be used as the defacto-standard shell for scripting, do you think its a worthy venture to set out to write a small, tight, pure Bourne shell?

*asbestos disclaimer*: This has nothing to with Bash as an interactive user shell and has nothing to do with a holy war over who's favorite shell is better than whomever's."

While doing a small bit of research on this question, I noted there was another Bourn-compatible shell out there called "ash", yet it's billed as doing "some things better and some things worse than bash". Does anyone use it, and find it better than bash for their shell scripting needs?

6 of 388 comments (clear)

  1. Easy. by Flarners · · Score: 5, Informative
    1. Download FreeBSD source.
    2. cd /usr/src/bin/sh; make; make install
    AFAIK the BSD Bourne shell is more or less the same as the real Bourne shell.
    --
    "The problem with the French is that they don't have a word for 'entrepeneur'." -George W. Bush
  2. Slackware uses ash for toolset by dangermouse · · Score: 5, Informative
    Slackware uses ash because (a) it's tiny and (b) it's restrictively Bourne-compatible, which means you won't write some tool that is accidentally dependant on bash.

    Basically, Slackware uses ash for development for the exact reasons you're looking for a Bourne-compatible shell.

    Have fun.

  3. bash as /bin/sh by zdzichu · · Score: 4, Informative

    "On every distro I've ever seen /bin/sh is just a soft link to /bin/bash."
    You haven't seen Polished Linux Distribution. PLD's /bin/sh is not bash, and all 'bashisms' are removed from distribution's scripts - it's all plain /bin/sh.

    (Plus, PLD is fully IPV6 ready :)

    --
    :wq
  4. Bourne shell or POSIX shell? by psamuels · · Score: 5, Informative
    Do you think that the startup scripts for most distributions would break because, even though they say #!/bin/sh at the top, they REALLY mean #!/bin/bash?

    Many would. However, note that POSIX requires /bin/sh to be a POSIX shell, whose specs are derived mostly from the Korn Shell - so ksh is a valid POSIX shell, as is Bash, modulo a few features. Plain-vanilla Bourne shell is not.

    AIX has the same dilemma: to be POSIX-compliant they have /bin/sh -> ksh and /bin/bsh is the real Bourne. HP-UX 11 has /bin/sh distinct from ksh, but it too allows things like 'export FOO=BAR'. I don't know if a real Bourne shell exists on HP-UX.

    For a POSIX shell without the bash overhead, use ash, which is distributed with many (most?) Linux distributions. ash is the NetBSD /bin/sh, and at least on Debian the installation gives you the /bin/sh symlink option as well. Let me tell you, ash is much faster than bash in the autoconf-generated-configure "benchmark".

    Since many Debian people use ash for /bin/sh, packages regularly have bugs filed -- and fixed -- vis-a-vis #!/bin/sh vs. #!/bin/bash. I don't know how careful other distros are about this sort of thing. Note that even many Debian scripts would fail if you found a real Bourne shell for /bin/sh rather than a POSIX-compliant shell.

    --
    "How can you claim that you are anti-crack, while still writing a window manager?" — Metacity README
  5. Re:It's a non-issue. by pete-classic · · Score: 5, Informative

    What you are saying is valid, but it misses the point.

    Even if every valid bourne script runs perfectly under bash there is still a problem.

    That problem is that an interpreter enforces valid syntax. It would be nice if everyone who wanted to write a portable (read bourne) script spent a month studying and was totally zen-ed out on the subject. But that 'aint the real world.

    I personally could benefit from a real sh on Linux, since it is the only *NIX I have, and I can't test scripts for sh compatibility without it.

    To be more specific, I wrote a bash script that was included in an OSS project. The maintainer wanted me to change the first line from "#!/bin/bash" to "#!/bin/sh" "for compatibility." I told him that I wasn't comfortable with this since the script wasn't tested on sh. He said he tested it and it worked fine. I asked him if he really tested it with sh or with "/bin/sh -> /bin/bash". Turned out it was the latter. Apparently he made the change anyway.

    A while later we got a bug report on the list to the effect "your script is not a valid sh script." Luckily the guy sent a diff, and I tested it with bash. I guess it works with sh now too, but I still don't know for sure.

    See how it isn't a non-issue, at least for me?

    -Peter

  6. Linux Standard Base by kingdon · · Score: 5, Informative

    Others have addressed the various issues about what is a "real" Bourne shell and bash extensions and all that. Anyway, the Linux Standard Base has a section on shells. In a nutshell, bash 2.x was the most POSIX-compliant of the shells that the LSB tested (and no, I don't know exactly what versions or which shells or the like), with pdksh getting an honorable mention. And there were two ways in which bash was not POSIX-compliant and concerning which the LSB therefore diverges from POSIX (whether $0 is the full pathname or just the basename, and what happens if you try to use "." to run a script without the read bit set). I don't know whether a future version of POSIX is planning to change the specification, or whether this is likely to remain a divergence for the foreseeable future or what. In any event, these two issues shouldn't be hard to deal with in writing scripts.