shell - Day 3

Goals

• do shell arithmetic
• use subroutines
• control variable scope
• reuse components
• getopts

POST

1. log into your Solaris machine
2. write a script called day3post.sh
3. the script should find each .sh file in your directory and back it up:
   • store the path to your backup dir in a variable for ease, resuse, and accuracy.
   • the backed up file should have a date-based extension of some kind. For this exercise, filenames should have no spaces.
   • bonus: keep only the 3 most recent backups of any particular file...

use math

use exitcodes

Intro to scope

• parents spawn children
• children inherit environment by value
• children cannot affect the namespace of the parent
• children cannot affect the pwd of the parent

subroutines

subname () {

	#	receive parms by position $1, $2, etc	
	#	or by $* or "$@"		
	return returnvalue

}

Note this difference

• $* each arg separately
• "$*" all args together
• $@ same as $*
• "$@" correctly parses quoted args

variable scope

• save positional vars ASAP
• may be able to localize the variable with local
• if not, maintain separate namespace

component reuse

getopts

• getopts is meant to be run in a loop
• a case construct is used used to associate each switch value with a variable.

#set some default vars here

#start looping to assign input
while getopts n:r:s: option
	do
	case "$option"
	
		in
			n)	name="$OPTARG";;
			r)	rank="$OPTARG";;
			s)	serialnumber="$OPTARG";;
			*)	echo "error in options processing";;
	esac
	done

echo "name is $name";
echo "rank is $rank";
echo "serial is $serialnumber";

Homework

• restricted shell
• subshells and inline commands
• other shells (esp. ksh, but also bash, csh)

Remember, your login is based on your machine's hostname, not on any other number.
~/[initials] refers to the subdirectory under your homedir, named after your initials. Everything except for .dotfiles will be stored in your ~/[initials] directory.