#!/bin/bash
# May need to be invoked with  #!/bin/bash2  on older machines.
#
#  Random password generator for Bash 2.x +
#+ by Antek Sawicki <tenox@tenox.tc>,
#+ who generously gave usage permission to the ABS Guide author.
#
# ==> Comments added by document author ==>


MATRIX="0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"
# ==> Password will consist of alphanumeric characters.
LENGTH="8"
# ==> May change 'LENGTH' for longer password.


while [ "${n:=1}" -le "$LENGTH" ]
# ==> Recall that := is "default substitution" operator.
# ==> So, if 'n' has not been initialized, set it to 1.
do
        PASS="$PASS${MATRIX:$(($RANDOM%${#MATRIX})):1}"
        # ==> Very clever, but tricky.

        # ==> Starting from the innermost nesting...
        # ==> ${#MATRIX} returns length of array MATRIX.

        # ==> $RANDOM%${#MATRIX} returns random number between 1
        # ==> and [length of MATRIX] - 1.

        # ==> ${MATRIX:$(($RANDOM%${#MATRIX})):1}
        # ==> returns expansion of MATRIX at random position, by length 1. 
        # ==> See {var:pos:len} parameter substitution in Chapter 9.
        # ==> and the associated examples.

        # ==> PASS=... simply pastes this result onto previous PASS (concatenation).

        # ==> To visualize this more clearly, uncomment the following line
        #                 echo "$PASS"
        # ==> to see PASS being built up,
        # ==> one character at a time, each iteration of the loop.

        let n+=1
        # ==> Increment 'n' for next pass.
done

echo "$PASS"      # ==> Or, redirect to a file, as desired.

exit 0