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Korn Shell Basics




This is an ancient post with no relevance to modern systems.

The Korn shell has been available on SCO systems since Xenix 2.3.4, so you can usually depend on finding it. It's not something you have to add, like bash or tcsh, and it's usually available on any modern Unix you are likely to work on. Personally, I prefer Bash (very similar in features to ksh), but you may not have it easily available. If that's the case, ksh is better than sh.

If you are using Linux, I'd stick with Bash.

The Korn shell has some neat features that make common tasks easier. For example, it has command recall and editing, built in math, and extensive string manipulation. Because of that, some people write fairly extensive programs using ksh.

I wouldn't. The Korn shell is nice, and has some useful features, but if you are writing real programs, or anything much more complex than what you'd type at the shell prompt, Perl is a far better choice. That doesn't mean ksh is useless; on the contrary, it is very useful and has some nice features which I will cover below. But it does mean that I'm not going to bother with the pattern matching or string handling stuff: Perl does it better.

The simplest way to get started with Korn shell is simply to type

ksh
set -o vi
PS1='$PWD $ '
 

I'll explain later how to set up so that you always have a Korn shell, and how to get the "set -o vi" to happen automatically. For now, notice that your prompt has been changed to include your current directory, but nothing else looks any different. Issue some normal commands, like "lc" or "df -v". Everything works as it always did.

Try a "cd" to somewhere else. Notice that your prompt changes to reflect the directory you are currently in. Nothing so great so far, except maybe that setting the prompt to be the current directory is a little easier than in sh or csh.

Ok, time for something more impressive. Try:

echo $((3*4+8*5))

Unless your computer has invented a new way of doing math, you should have gotten "52" as a response. Isn't that easier than "expr 3 \* 4 + 8 \* 5"? No backslashes, no worrying about spacing. You can also do this:

((ans=3*4+8*5))
echo $ans

Before we go on with math, let's try something else. If you haven't done the math examples I showed, do them both first. Then hit the ESC key, then "/", and then type "echo" and press RETURN (<ESC>/echo<ENTER>). You should see the "echo $ans" on your screen. Hit "/" again, and the "echo $((3*4+8*5))" pops up. Now hit "k" (lowercase, please). You should see whatever command you typed just before that. Hit "j", and you are back at "echo $((3*4+8*5))" again. "k" moves up toward previous commands, "j" moves down. All this gets started by hitting ESC. If you have a line on the screen and press ENTER, the command gets executed. If you press DEL (or ctrl-C if that's your "intr" character), it doesn't. In either case, you are back at a normal shell after that, and "k" or "j" don't work to recall lines.

Before leaving this, let's try one more thing:

for i in a b c d e
 

After typing this and pressing ENTER, you'll get a ">" prompt. Go ahead and keep typing, pressing ENTER after each line. You'll get a ">" prompt after each line but the last. I'll explain that in a minute. For now, just continue:

do
echo $i
done
 

After typing "done" and pressing ENTER, you get

a
b
c
d
e
 

on your screen. If you don't already understand the ">" prompt, think of it is the ksh's (and Bourne sh, too) way of saying "I understand what you said, but I need more". The shell understands what "for i in a b c d e" means, and tells you so with ">". As long as you keep typing things it understands, the shell won't complain. If you typed "doit" instead of "do" on the second line, you'd get an unfriendly message.

But the point of this was not to explain secondary prompts, or even "for" loops. Hit "ESC", and then "k", and you should see

for i in a b c d e^Jdo^Jecho $i^Jdone
 

You can press ENTER to execute this again, or you can press "v" and something really useful happens: your text gets called up into vi for editing!

If you don't know vi, check the VI Primer for a painless introduction.

To re-execute after making changes (or not), simply do ":wq", or to quit without executing, type ":q!". One of the things you could do while in the editor is type ":w /tmp/this_is_it" to put your little program into a file for later reuse.

The "vi" editing works even without pressing "v". If you recall a command, you can use space to move to the right, "x" to delete a character, "a" to append, and so on. You should see "man ksh" for the full details, but one more useful hint is that you don't have to press ENTER and recall a line to fix a mistake. As soon as you realize the error, hit ESC and you are in editing mode.

The ksh can also use emacs style editing if you prefer. You'd set that with "set -o emacs". Although I personally much prefer the power of vi, the emacs mode is probably more intuitive for the inexperienced user. Use CTRL-P and CTRL-N to move through the stack, CTR-R to search. See the "man ksh" for more details.

Now back to math again. The Korn shell has only integer math, but it does have modulo division:

# echo $((52 % 3))
1
#
 

and bit operators (left and right shift, AND,OR, Exclusive OR). These have some use at the command line for quick sanity checks on masks, for example. What I find even more useful is that ksh can handle base conversions:

typeset -i2 a
typeset -i8 b
typeset -i16 c
typeset -i36 d
a=254
b=254
c=254
d=254
echo $a $b $c $d
 

gets you:

2#11111110 8#376 16#fe 36#72
 

Conversely, you can do:

typeset -i10 e
e=2#11100011
echo $e
 

and get 227. This sort of thing is very useful at the command line. Of course, things like:

echo $((8#376 / 2))
 

work as they should, also.

You might be wondering now how to set up so that you are always using Korn shell. There are a couple of ways to do that. One is to have set yourself up that way to begin with; to have specified ksh when your account was created. If that wasn't done, it's not too late.

If you can change your account using "usermod" or "scoadmin" or "sysadmsh" (on older systems) or "chsh" (Linux, BSD), do so. That will get you going with ksh as your default shell. However, you still have more to do. If you can't change your default shell (you aren't the administrator, for example), you can still get 99% of the work done automatically.

First, you need to add this to the end of your .profile:

if [ -f $HOME/.kshrc -a -r $HOME/.kshrc ]; then
        ENV=$HOME/.kshrc                # set ENV if there is an rc file
        export ENV
fi
 

Second, you need to create a $HOME/.kshrc.

Use vi, or:

cat > $HOME/.kshrc
:
if [ -z "$VISUAL" -a -z "$EDITOR" ]; then
        set -o vi
fi
PS1='$PWD $ '

(type control-D to end the cat)
 

If you were able to change your default shell, you now have everything you need. If not, just add:

/bin/ksh
 

to the end of your .profile (after the ENV lines you already added).

Or, simply type "ksh" when you want it. Because "ENV" has been exported, the shell will read the settings in .kshrc and everything will work as you want it.

Command line recall

Add these to your .kshrc:

  set -o emacs
  alias __A=$(print '\0020')    # ^P = up    = previous command
  alias __B=$(print '\0016')    # ^N = down  = next command
  alias __C=$(print '\0006')    # ^F = right = forward a character
  alias __D=$(print '\0002')    # ^B = left  = back a character
  alias __H=$(print '\0001')    # ^A = home  = beginning of line
 



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---December 27, 2004







Sat Mar 19 17:36:57 2005: 202   debdutta


Hello,
Thanks for all the information. Only one problem i am facing now i.e
if i want to logout from the system them i have to typed two exit command. probably 1st exit to comeout from the ksh environment and 2nd one to logout from the system.
My Mail address is [email protected]
Thanks & Regards.
Debashis Dutta



Sat Mar 19 19:19:14 2005: 204   TonyLawrence

gravatar
Read again. You can set this as your login shell if you want to. This method is just for those who don't want to.




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