Book and Links about Shells; and zsh question Post: 302405840

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Top Forums Shell Programming and Scripting Book and Links about Shells; and zsh question Post 302405840 by jim mcnamara on Saturday 20th of March 2010 09:33:59 AM

03-20-2010

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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

jim mcnamara

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LEARN ABOUT SUNOS

endusershell

getusershell(3C)					   Standard C Library Functions 					  getusershell(3C)

NAME

       getusershell, setusershell, endusershell - get legal user shells

SYNOPSIS

       #include <unistd.h>

       char *getusershell(void);

       void setusershell(void);

       void endusershell(void);

DESCRIPTION

       The  getusershell()  function  returns  a  pointer  to  a  legal  user  shell  as defined by the system manager in the file /etc/shells. If
       /etc/shells does not exist, the following locations of the standard system shells are used in its place:

       /bin/bash		  /bin/csh
       /bin/jsh 		  /bin/ksh
       /bin/pfcsh		  /bin/pfksh
       /bin/pfsh		  /bin/sh
       /bin/tcsh		  /bin/zsh
       /sbin/jsh		  /sbin/pfsh
       /sbin/sh 		  /usr/bin/bash
       /usr/bin/csh		  /usr/bin/jsh
       /usr/bin/ksh		  /usr/bin/pfcsh
       /usr/bin/pfksh		  /usr/bin/pfsh
       /usr/bin/sh		  /usr/bin/tcsh
       /usr/bin/zsh		  /usr/xpg4/bin/sh

       The getusershell() function opens the file /etc/shells, if it exists, and returns the next entry in the list of shells.

       The setusershell() function rewinds the file or the list.

       The endusershell() function closes the file, frees any memory used by getusershell() and setusershell(), and rewinds the file /etc/shells.

RETURN VALUES

       The getusershell() function returns a null pointer on EOF.

BUGS

       All information is contained in memory that may be freed with a call to endusershell(), so it must be copied if it is to be saved.

SunOS 5.10							    30 Aug 2004 						  getusershell(3C)