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Full Discussion: Double square brackets question
Top Forums Shell Programming and Scripting Double square brackets question Post 302359865 by Scrutinizer on Wednesday 7th of October 2009 03:16:33 PM
Old 10-07-2009
High King Nothing,

man ksh:
Code:
[[ expression ]]
              Evaluates expression and returns a zero exit status when expression is true.  
See Conditional Expressions below, for a description of expression.

Code:
expression1 && expression2
              True, if expression1 and expression2 are both true.

The && construct makes use of the fact that in order to find out whether both expressions are true, first the left side gets evaluated. Because of efficiency the right side only gets evaluated if the left side is true. If the left side is false it does not matter what value the right side evaluates to, so it never gets evaluated. Thus this happens to work out like an if then construction. The end value of the && construct then never gets used. I personally prefer the if then else way of testing conditions since it usually provides cleaner, more legible code.

I don't know why the examples you provide don't work with ksh. They sure work in my ksh (does /usr/bin/ksh exist on your system?). The first example strikes me as strange because of the parentheses, which mean the trap statement is run in a separate environment, so it will never output the statements in the current environment. If you use { .. ;} instead it will work:

Code:
[[ $mode =  "INTERACTIVE" ]] && {trap 'rm -rf ${WORK_DIR}/*.$$; echo "\n\nInterrupted !!\n\n"; exit 4' 1 2 3 15;}

 

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LEARN ABOUT OSX

platform::shell

platform::shell(n)					       Tcl Bundled Packages						platform::shell(n)

__________________________________________________________________________________________________________________________________________________

NAME

       platform::shell - System identification support code and utilities

SYNOPSIS

       package require platform::shell ?1.1.4?

       platform::shell::generic shell
       platform::shell::identify shell
       platform::shell::platform shell
_________________________________________________________________

DESCRIPTION

       The platform::shell package provides several utility commands useful for the identification of the architecture of a specific Tcl shell.

       This  package  allows the identification of the architecture of a specific Tcl shell different from the shell running the package. The only
       requirement is that the other shell (identified by its path), is actually executable on the current machine.

       While for most platform this means that the architecture of the interrogated shell is identical to the architecture of  the  running  shell
       this  is  not generally true. A counter example are all platforms which have 32 and 64 bit variants and where a 64bit system is able to run
       32bit code. For these running and interrogated shell may have different 32/64 bit settings and thus different identifiers.

       For applications like a code repository it is important to identify the architecture of the shell which will  actually  run  the  installed
       packages, versus the architecture of the shell running the repository software.

COMMANDS

       platform::shell::identify shell
	      This command does the same identification as platform::identify, for the specified Tcl shell, in contrast to the running shell.

       platform::shell::generic shell
	      This command does the same identification as platform::generic, for the specified Tcl shell, in contrast to the running shell.

       platform::shell::platform shell
	      This command returns the contents of tcl_platform(platform) for the specified Tcl shell.

KEYWORDS

       operating system, cpu architecture, platform, architecture

platform::shell 						       1.1.4							platform::shell(n)
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