Basic about "shell" and shell script

[midnite]

As far as I understand (please correct me if wrong), different shells are different programs. They can be find in packages.

In most systems (including mine), /bin/sh -> bash. AFAIU sh and bash (as well as zsh) are different shells. I have chosen to use zsh as my daily user (not root). zsh is the shell used in the Gentoo version of SysRescueCD. It is more convenient than bash IMO. I installed app-shells/zsh from portage. How can I install /bin/sh? As it is not available in packages. /bin/sh is just a symlink pointing to /bin/bash. So /bin/sh is actually not present in my system.

AFAIU different shells are different [Wikipedia: Comparison of command shells]. And /bin/sh is kind of the ancestor of most shells. Most later shells are developed base on /bin/sh.

To write shell scripts as most portable as possible, we should use syntax of the /bin/sh. Even if I use #!/bin/sh but my system is symlinking it to /bin/bash, I cannot run (try and debug) my scripts in /bin/sh, is it?

As many people use the terms Bourne shell (the basic shell) and POSIX shell interchangeably, they are actually two entries (two rows in the table) in Wikipedia. Are they the same or actually different? Which one has the maximum compatibility?

https://www.geeksforgeeks.org/difference-between-sh-and-bash/

[Goverp]

On Gentoo (and I think most Linuxes) /bin/sh is a symbolic link to whichever shell you choose. On Gentoo you can set the link with "eselect sh".
That's sort of the "system" shell - anything shell script that starts "#!/bin/sh" will use it. If you want to code a script for a particular shell, you'd write "#!/bin/rsh" as the first line.
I'm too young to know the full history of Unix shells. If you want to code for "most" shells you should stick to POSIX compliant shells. Bash is extended way past the POSIX definitions; it has a setting somewhere to make it properly compliant. A good POSIX compliant shell for Gentoo is dash. It has the advantage of being very fast and reliable. A number of Linux distributions use dash as their "system" shell, and if you install dash and then "eselect sh set dash" it will be the system shell for your Gentoo system.

What you get at the command line is usually bash. It's specified in the /etc/passwd entry for each user, and the default entry is /bin/bash. It's more friendly than dash - its history commands are better, and dash doesn't do tab-completion.
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Greybeard

[eccerr0r]

The bourne shell describes a shell language. POSIX tried to standardize this language.

/bin/sh just needs to be a program that can execute bourne shell scripts, ideally understands the POSIX standard bourne shell. It does not matter what it is, just needs to execute it. Any shell that understands bourne shell will do as long as the consumers (all the scripts with #!/bin/sh as its first line) does not use extended feaures -- and this isn't always true.)

Bash does fine, along with ksh, ash, dash, etc., etc. as long as it's not csh, tcsh, or zsh. Technically csh also has a POSIX standard which tcsh tries to adhere to, but it sure won't run bourne shell scripts.

As for bourne shell compatibility, you just have to know your specific target. For the most part I don't see many people using strict bourne shell anymore as it's slow due to a lot of optimizations added in later shells that were not design issues in the original bourne shell.

Note that for so called "embedded" designs that end up with 1GB of memory, just use bash and all its bashisms. Unless your "embedded unix" is less than say 64MB or so, don't bother trying to not use bash.
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Intel Core i7 2700K/Radeon R7 250/24GB DDR3/256GB SSD
What am I supposed watching?

[midnite]

Thank you Greybeard and eccerr0r.

So I cannot literally emerge (install) /bin/sh?

I found a good online tool which checks my scripts against the POSIX standard.
https://www.shellcheck.net/

But ShellCheck is masked in Gentoo packages.
https://packages.gentoo.org/packages/dev-util/shellcheck
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- midnite.

[mv]

[eccerr0r]

Again there is no base bourne shell, nobody would use it.

- hell no, there is definitely no tab completion.
- It doesn't even have command line editing. Typo? Type it again from the beginning unless it was at the end of the line. If you already pressed enter...
- I believe the "! style history" was specified in the original bourne shell (which still exists in bash) but don't bother trying using your arrow keys to recall
- It has no job control. Can't suspend jobs. Forgot to make it asynchronous? Either abort it or wait till it's done before you can get your prompt back.
- more things would fork (as it was defined) so you pay your fork() penalty much more often.

There are some shells like ash and dash that sort of tries to at least add readline(or at least a homemade subset) and job control features in and then some, to make it somewhat "functional" instead of "braindead".

Anyway, shellcheck is simply keyworded ~ unstable, not masked - you can just accept that, and it should merge fine in Gentoo just fine.

---

And hmm... looks like zsh will support csh-like syntax and bourne-like syntax. What is it trying to do...
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Intel Core i7 2700K/Radeon R7 250/24GB DDR3/256GB SSD
What am I supposed watching?

[mv]

[midnite]

[figueroa]

My opinion, it would be a favor to you and your system to re-emerge bash. It is widely assumed across the Linuxverse that bash exists. On most Debian variants, /bin/sh is a symlink to dash, though bash is usually the main shell. I think dash is a bit of a red herring. Having bash on board would give you a less breakable and more normal system.
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Andy Figueroa
hp pavilion hpe h8-1260t/2AB5; spinning rust x3
i7-2600 @ 3.40GHz; 16 gb; Radeon HD 7570
amd64/23.0/split-usr/desktop (stable), OpenRC, -systemd -pulseaudio -uefi -wayland

[mv]

[Zucca]

If I write a shell script to other than bash, I'll make it run on busybox sh. I also tend to test them on mksh (ksh -like shell).
Busybox sh has just enough features but is also enough limited, so that all scripts I've written work on most shells (those which feel like POSIX, so excludes fish for example) then.

If you require arrays, then busybox sh (and POSIX too) wouldn't cut it, unless you use push from fellow member mv. ;)
But then again, there's awk in almost every *NIX system. I write inline awk code for, more compllex things, inside my shell scripts and make them into shell functions.

There's also another topic about which package owns /bin/sh.
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..: Zucca :..

[xanderal]

Ok, I'm running both Linux and BSD systems, so portability is very important for me and something along those lines seems to be your use case as well, so here are my two cents:

When scripting on e.g. OpenBSD stuff is really easy because /bin/sh is actually a proper POSIX-compliant executable, so that's close to foolproof to work with (sadly I'm not aware of any Linux distro that makes it that easy for us).
But in general I use #!/bin/sh and test it with shellcheck(1). That has support for sh/bash/dash/ksh but is not perfect. Nevertheless it should flag most blunders.
I don't know how far you want to take your portability. If it's only about your stuff running in different shells on a Linux system with GNU core utils, then this should suffice.

But if you're looking into making it run with busybox or other core utils (like e.g. the different BSD ones), then you might have to consider some more stuff:
Not every flag you like is supported, so read manpages (well written ones include a STANDARDS section that among other things tells you which flags are extensions) and other proper reference material (stack overflow is not a reference).

Just an example: Say you want to replace some string, what do you use? 'sed -i'? Nope, you don't, at least not always. It works with GNU and OpenBSD, but e.g. on FreeBSD this might not work.
Or to quote from the OpenBSD manpage on sed(1):

[Zucca]

[xanderal]

[Zucca]

@xanderal, so far I only miss arrays in busybox shell, but there are ways to overcome that limitation if one really needs arrays.
busybox awk works just fine. I just need to avoid some functions like gensub. I've written my custom initramfs scripts to be run with with busybox.
I also looked at toybox to replace busybox, but it doesn't implement sh and awk yet (or they reside in 'pending' directory).
One quite frustrating, but manageable, incompability between GNU, busybox and toybox was mktemp. With careful choosing of arguments it "beacame" portable too. Then there's date. It's a same story, but you can also use awk for most date realted things.

So it's not like busybox is ever too limited, but the portability is the tricky part in my opinion. Since shell scripting isn't just the syntax, but the command line tools too.
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..: Zucca :..

[Genone]

[mv]

[xanderal]

[Zucca]

[palak231]

Hi this is Palak Sharma.
Well while writing any shell ( https://www.techgeekbuzz.com/blog/shell-commands-in-linux/ ) programs we use /bin/sh and /bin/bash. And there is not too much different in between both of them. You can understand different in between both of them with the help of example by clicking here.

https://stackoverflow.com/questions/5725296/difference-between-sh-and-bash

Hope it will helps you.
Thanks

[xanderal]

Hi Palak,