>   ``Add this directory iff it's not already in the path''

Very useful.  My /etc/profile has had an add_path() function for
years.  It does not check for the directory being in PATH because in
/etc/profile you _know_... the add_path() and friends used in
/etc/ksh.kshrc though do test for presence in PATH.

I recently extracted the functions into a separate file:

:
# NAME:
#	add_path.sh - add dir to path
#
# DESCRIPTION:
#	These functions originated in /etc/profile and ksh.kshrc, but
#	are more useful in a separate file.
#
# SEE ALSO:
#	/etc/profile
#	
# AUTHOR:
#	Simon J. Gerraty <sjg@zen.void.oz.au>

# RCSid:
#	$Id: add_path.sh,v 1.1 1995/09/30 12:45:23 sjg Exp $
#
#	@(#)Copyright (c) 1991 Simon J. Gerraty
#
#	This file is provided in the hope that it will
#	be of use.  There is absolutely NO WARRANTY.
#	Permission to copy, redistribute or otherwise
#	use this file is hereby granted provided that 
#	the above copyright notice and this notice are
#	left intact. 

# is $1 missing from $2 (or PATH) ?
no_path() {
	eval "case :\$${2-PATH}: in *:$1:*) return 1;; *) return 0;; esac"
}
# if $1 exists and is not in path, append it
add_path () {
  [ -d ${1:-.} ] && no_path $* && eval ${2:-PATH}="\$${2:-PATH}:$1"
}
# if $1 exists and is not in path, prepend it
pre_path () {
  [ -d ${1:-.} ] && no_path $* && eval ${2:-PATH}="$1:\$${2:-PATH}"
}
# if $1 is in path, remove it
del_path () {
  no_path $* || eval ${2:-PATH}=`eval echo :'$'${2:-PATH}: | 
    sed -e "s;:$1:;:;g" -e "s;^:;;" -e "s;:\$;;"`
}

# we use this all the time too
Which() {
        for d in `IFS=:; echo ${2-$PATH}`
	do
                test -x $d/$1 && { echo $d/$1; break; }
	done
}

>   ``Add this directory iff it [and every parent directory] are protected
>     from writing by anyone but root or bin''

This is a good idea too, you could come up with a variation on the
above without too much effort.

--sjg