Shell Programming and Scripting

HI,
I would like to ask You about some good books or links where I can find information about shells, theoretical information.
I will be grateful if You can help me

And I have question about zsh loop

trivial script:

Code:

#!/bin/zsh
 for i in {1..100000}
 do
 echo $i;
 done

exec time is 10 sec
but for 500000 loops
exec time is 300 sec

Why for 500000 loops execution time is 300 seconds, it's only 5x 100000 loops it should be about 50 sec
In other shells it's OK

1. there is not a lot of 'shell theory'. Production shops almost always choose a POSIX-compliant shell: probably one of: ksh (ksh88 ksh93 zsh) bash. The reason for this is portability.

read ISO/IEC subsection 3 ISO/IEC 9945
This tells you how shells are 'meant to be' I dunno if that is theory or not.

2.
a. terminal i/o is slow, net traffic botlenecks may also be involved
try this on two large-ish files, use different files to circumvent file caching:

Code:

time cat bigfile1 
time cat bigfile2 > somefile

b. the primary cause of loops bogging is process creation. You are executing echo.
if it is /usr/bin/echo and not the shell builtin echo, every loop execs a whole new child process. This is a huge amount of overhead.

Code:

which echo

tells you whether the echo you ran is builtin or not.


We need more information on the platform and OS to give you anything more useful

When I type

Code:

which echo

it is /usr/bin/echo
Can I choose which echo would be executing ?

And why in different shells everything is OK ?

---------- Post updated at 01:09 PM ----------

Actually, I just now noticed the "zsh" part of the subject, checked and saw that zsh (which I have zero experience with) does have a "which" builti-in. My apologies, jim_mcnamara, for jumping the gun. I had assumed you were referring to the common (though I don't think posix standardized) which(1) executable found on most BSD and Linux systems.

I will leave my original post (which follows) as is, as I think it may prevent misunderstandings among those who (like me) do not use zsh.


---------- Posted at 12:35 PM ----------


That's incorrect. which(1) will search $PATH directories for an executable. If it finds anything, it will always be an external executable.

Use type to see what is actually being run.

Code:

$ which echo
/bin/echo
$ type echo
echo is a shell builtin
$ type -a echo
echo is a shell builtin
echo is a tracked alias for /bin/echo

The first result from the type built-in is what the shell will actually use. There are ways to use something other than the first found option, read your shell's man page.

Also, on ksh variants, instead of type use whence (although it may have type aliased to whence -v already as a convenience):

Code:

$ ksh --version
  version         sh (AT&T Labs Research) 1993-12-28 p

$ alias type
type='whence -v'

Regards,
Alister

Sorry for my ignorance,
and thanx for informations.

BTW.

Code:

% which echo
echo: shell built-in command
% type -a echo
echo is a shell builtin
echo is /bin/echo

Which versions of zsh are you using?

Code:

print $ZSH_VERSION

I also had such experience with old zsh versions (on Solaris < 10 default distributions, for example).

Could you please also post the versions and the timings of your tests with the other shells?

zsh version:

Code:

4.3.10

ksh version

Code:

Version JM 93t+ 2009-05-01

ksh
100000 loops

Code:

1.8 sec

ksh
500000 loops

Code:

9.4 sec