LinuxQuestions.org - [SOLVED] Functionality differences between Bash and sh

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Functionality differences between Bash and sh

Hi All,

Need some explanation on Functionality differences between Bash and sh(better be with examples)

Thanks

BASH, csh, zsh, DASH, etc are all different shells. sh is a symlink to your preferred shell. Have a look ay /bin (in my case):

Code:

ls -l /bin

-rwxr-xr-x 1 root root 811156 Apr 10  2010 bash

.

.

.

lrwxrwxrwx 1 root root      9 Jun  4  2012 sh -> /bin/bash

If you have a shell script that starts:

Code:

#! /bin/sh

then whatever shell is symlinked to /bin/sh is used to run the script.

However, if you have a shell script that starts:

Code:

#! /bin/bash

then the BASH shell is used to run the script. Simalarly:

Code:

#! /bin/csh

then the csh is used to run the script.

Hope it starts you off.

A little excerpt from the Bash manual page:

Code:

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.

Quote:

Originally Posted by towheedm (Post 4870496)

BASH, csh, zsh, DASH, etc are all different shells. sh is a symlink to your preferred shell. Have a look ay /bin (in my case):

Code:

ls -l /bin

-rwxr-xr-x 1 root root 811156 Apr 10  2010 bash

.

.

.

lrwxrwxrwx 1 root root      9 Jun  4  2012 sh -> /bin/bash

If you have a shell script that starts:

Code:

#! /bin/sh

then whatever shell is symlinked to /bin/sh is used to run the script.

However, if you have a shell script that starts:

Code:

#! /bin/bash

then the BASH shell is used to run the script. Simalarly:

Code:

#! /bin/csh

then the csh is used to run the script.

Hope it starts you off.

one Question-
if we execute a script which begins with /bin/sh in a bash shell what will be the effect?

Quote:

Originally Posted by charithsrng (Post 4871948)

one Question-
if we execute a script which begins with /bin/sh in a bash shell what will be the effect?

Actually that is the correct way of executing ANY script. /bin/sh will provide so called expected behavior. If you run script as /bin/bash, that means your script is using Bash-specific features and might not be portable.

Quote:

Originally Posted by wigry (Post 4871950)

actually that is the correct way of executing any script. /bin/sh will provide so called expected behavior. If you run script as /bin/bash, that means your script is using bash-specific features and might not be portable.

cleared..thanks a lot

This is not entirely germane to this post but I have a post here:

http://www.linuxquestions.org/questi...1/#post4871241

(which I'm inclined to think might have been better on the 'Linux-General' forum). It's a sort of "I understand that I can do this & this but I know that doesn't work - why not?" question. I have only ever used bash. Might, I wonder, some other shell give different results.

ming

The shebang line merely tells the shell which interpreter to use to run the script. So even from BASH:

Code:

#! /bin/csh

will use the csh as the command interpreter.
Similarly:

Code:

#! /bin/sh

will tell the shell to use whatever command interpreter is symlinked to /bin/sh.
And even:

Code:

#! /usr/bin/perl

will tell the shell to use PERL as the command interpreter.

Note also that you can completely leave out the shebang line. When you do this, you can run the script with:

Code:

sh myscript

to use the command interpreter symlinked to /bin/sh, or

Code:

bash myscript

to use the BASH command interpreter to run the script.
Note also that if you leave out the shebang line, you do not need to set the executable bit on the script.

@towheedm - Thanks for that. I'l play around with it a bit.

On most Linux systems /bin/sh is symlinked to either bash or dash, but that doesn't matter. When you run a script as sh, then it will be parsed according to POSIX-defined specifications (with a few legacy bourne-compatible modifications also thrown in), no matter what the actual shell doing the parsing is.

(The POSIX standard defines everything necessary for a minimally compatible unix-like operating system, and one of its parts is a basic shell that supports certain commands and syntax. So every POSIX OS provides some form of /bin/sh, even if the actual shell is bash or ksh, or whatever.)

What this means for any particular script depends on what POSIX says, and what the actual shell is.

If the code being parsed is explicitly defined by POSIX, then it will be treated as such. syntax or options that are incompatible with POSIX will be rejected with errors.

If the code in question is undefined by POSIX, that usually means that it's left up to the individual shell implementation to decide what to do. In practice what this means is that if the shell doing the parsing understands the syntax, then it will usually continue to work. So most bash constructs will continue to work if the reading shell is bash, or possibly ksh.

For example, if you try to use an array (an undefined feature) in your sh script, and the parser is bash or ksh, then it will continue to work, at least according to that shell's understanding. But if the parser is dash, it will not, because dash does not support arrays at all.

So it's very important, if true portability is required, that you stick with the POSIX syntax; only that is guaranteed to always run*. The dash shell in particular is one that follows POSIX very carefully, and does not have many extensions, if any. so if you want to know if your script will run as sh, try using #!/bin/dash as your shebang.

This page has a good rundown of bash syntax constructs and their POSIX equivalents:
http://mywiki.wooledge.org/Bashism

*Of course even then you still have to deal with differences between platforms and the commands they have available. But that's a different question.

Quote:

Originally Posted by David the H. (Post 4872966)

Thanks for the heads up.