bin/sh & bin/bash

jonette20 · 08-10-2009, 03:24 PM

Hi,

I noticed on my system that some scripts have bin/sh and others bin/bash.
Does this make a difference?
I have Ubuntu 8.04 installed.

Thanks

joeBuffer · 08-10-2009, 03:39 PM

/bin/sh is usually (always, as far as I know ... but I could be wrong) a link to another file. Using Slackware, I know it is a link to /bin/bash, using Ubuntu 9.04 it is a link to /bin/dash (the Debian Almquist shell).

Code:

joebuffer@ubuntu:~$ ls -l /bin/sh
lrwxrwxrwx 1 root root 4 2009-08-07 17:32 /bin/sh -> dash

i92guboj · 08-10-2009, 04:44 PM

"sh" is the name of the original Bourne Shell. Nowadays, most shells try to be backwards compatible with it, so, at least theoretically, most actual shells should be able to run an old script written for "sh" without needing any modification at all. "sh" is old and not too featureful, so nowadays linux distros use anything else (bash is very famous, but other common ones are ksh, csh, zsh, dash, ash...).

/bin/sh is usually a symlink to any of these (I am not sure all of them could comply, particularly csh), so if in your system /bin/sh is a symlink to /bin/bash, there's no difference in using either of them. However, if you aim for the widest portability, you want to use #!/bin/sh in your scripts. Of course, that means that you then shouldn't use any bash-only feature (bash is a lot more complete than the original sh). All the unix alikes will always have a substitute for sh, but not all of them will always have bash installed, or any other shell by that matter.

That's why the #!/bin/sh bang is still used, and will continue to be so.

Uncle_Theodore · 08-10-2009, 05:07 PM

Quote:

Originally Posted by i92guboj

if in your system /bin/sh is a symlink to /bin/bash, there's no difference in using either of them.

Well, not quite. Although /bin/sh is a symlink to /bin/bash, when bash is called as /bin/sh, its behavior is somewhat different than when it's invoked as /bin/bash. It's right there in man bash:

Quote:

If bash is invoked with the name sh, it tries to mimic the startup
behavior of historical versions of sh as closely as possible, while
conforming to the POSIX standard as well.

But otherwise you're correct, /bin/sh is added for backward compatibility and maximum portability.

i92guboj · 08-10-2009, 05:17 PM

You are quite right, in my aim to simplify the issue I omitted info so that part of my post wasn't totally correct. However, even when invoked that way, all the bashishms will still work, you can try this one which contains bash specific code that would fail to run on the genuine bourne sh

Code:

#!/bin/sh
foo="bar"
echo "${foo/r/ll}"

joeBuffer · 08-10-2009, 05:32 PM

I've read this was true, also, but thought saying it was a link or symlink would be easier, and I haven't really read about the details of using sh instead of bash and what differences it makes. I do remember reading that it becomes POSIX, but not fully from what I remember.