Understanding Logic and Flow better Post: 302330369

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Top Forums Shell Programming and Scripting Understanding Logic and Flow better Post 302330369 by otheus on Wednesday 1st of July 2009 04:07:19 AM

07-01-2009

Registered User

Quote:

"Does anybody have some tips on understanding and mapping out the logic and flow of scripts"

First, you're rambling; get some sleep (if you can after 3 pots of coffee).

Second, "scripts" are usually executed in order the lines appear in the program, with the exception of things like loops. SH and derivatives (bash, zsh) handle functions, and these functions need to be defined before they are invoked. Other than that, it's pretty straight-forward.

*Sigh* then again, I've been coding since I was 10.

Third: You must understand Binary Logic (aka, Boolean Logic). The keys are understanding the basic operators: OR, AND, NOT. If you don't understand these correctly, you'll have a hard time. In most modern languages, including shell scripting, perl, and C, the operands can be expressions which means in the case of shell scripting, a program that returns an exit code.

So fourth, you have to understand that programs return exit codes. If the exit code is numerically 0, that means true. (0 for true is known sometimes as "negative logic").

If you are visually oriented, this tutorial might help.
I googled around and found this video which might help: YouTube - Prelude to Computer Science - 6: Binary Logic and YouTube - Prelude to Computer Science - 7: Logic Gates

otheus

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LEARN ABOUT LINUX

chsh

CHSH(1) 							   User Commands							   CHSH(1)

NAME

       chsh - change login shell

SYNOPSIS

       chsh [options] [LOGIN]

DESCRIPTION

       The chsh command changes the user login shell. This determines the name of the user's initial login command. A normal user may only change
       the login shell for her own account; the superuser may change the login shell for any account.

OPTIONS

       The options which apply to the chsh command are:

       -h, --help
	   Display help message and exit.

       -s, --shell SHELL
	   The name of the user's new login shell. Setting this field to blank causes the system to select the default login shell.

       If the -s option is not selected, chsh operates in an interactive fashion, prompting the user with the current login shell. Enter the new
       value to change the shell, or leave the line blank to use the current one. The current shell is displayed between a pair of [ ] marks.

NOTE

       The only restriction placed on the login shell is that the command name must be listed in /etc/shells, unless the invoker is the superuser,
       and then any value may be added. An account with a restricted login shell may not change her login shell. For this reason, placing /bin/rsh
       in /etc/shells is discouraged since accidentally changing to a restricted shell would prevent the user from ever changing her login shell
       back to its original value.

FILES

       /etc/passwd
	   User account information.

       /etc/shells
	   List of valid login shells.

       /etc/login.defs
	   Shadow password suite configuration.

SEE ALSO

       chfn(1), login.defs(5), passwd(5).

User Commands							    06/24/2011								   CHSH(1)