Shell Programming and Scripting

Hi. I have been running some scripts ok with no extension on the name, and they work fine. What difference does it make if I call them whatever.sh? And I have some scripts starting #!/bin/bash - which debian recognises as shell scritps, even without the .sh ending - and some which don't. I'm sure this is all explained in a handy page somewhere but I can't find it. Please could someone point me in the right direction. Thanks.

The ending doesn't matter to the OS. It is just for humans to get a quick idea, what this file is about.
The shebang (#!/.....) tells the shell which interpreter to use for the script. This is rather important while the ending is just a help.

Naming your shellscripts with extension .sh (or .bash or .ksh,...) simply helps you to know that the file is actually a script without looking at its content.
Self documenting filenames make maintenance of a system easier.

Windows operating system which mostly require file name extensions for applications/utilities to function properly.

Linux and Unix-like operating systems does not require file name extensions and it doesn't make any difference.

As cero mentioned it is used for easily identifying the file type.

You can run the file command to determine file type.

There are three sets of tests, performed in this order: filesystem tests, magic number tests, and language tests to classify the file.

Check the man page for further reference:

Code:

man file

Thanks for all the answers. So I get that the extension is purely for human use. What difference does the shebang make? My scripts have been running ok without it. Does that mean that the system guessed right which interpreter to use? Is it good practice to always put it in? I am calling functions such as mkdir and rsync - does it make a difference if I put in the shebang or not? (I have read man file but I am not a lot wiser on this point as a result.)

The system doesn't guess at all -- it assumes /bin/sh if you don't tell it.

Which works great if your script is a garden-variety bourne shell. For Perl, it's wrong.

Yes, especially if you're using shell features like arrays which may not always be present in the garden-variety /bin/sh... /bin/sh may be a full-featured BASH on your system, but on others it may be a mouldy old shell with no features but pure Bourne.

So use #!/bin/sh for pure bourne scripts, #!/bin/ksh for ksh scripts, #!/bin/bash for bash scripts, and so forth. Use the exact right shell and people will know which it needs. Better they get a 'no such file' error than your script misbehaves and does unintended things.

These are not functions, they are external programs. As such they don't care at all which shell you use, any shell has access to them and they all run outside it.

Shell builtins on the other hand can vary between shells. /bin/sh may not have pushd/popd, but bash will.

Thank you. This is getting much clearer. So is it good practice to put in /bin/sh for the kind of script I am running, or just leave it open as I have been doing? Is the bourne shell the 'vanilla' shell that ought to be called when calling other programs as I am doing?