34.4. Recursion

#!/bin/bash
# recurse.sh

#  Can a script recursively call itself?
#  Yes, but is this of any practical use?
#  (See the following script.)

RANGE=10
MAXVAL=9

i=$RANDOM
let "i %= $RANGE"  # Generate a random number between 0 and $MAXVAL.

if [ "$i" -lt "$MAXVAL" ]
then
  echo "i = $i"
  ./$0             #  Script recursively spawns a new instance of itself.
fi                 #  Each child script does the same, until
                   #+ a generated $i equals $MAXVAL.

#  Using a "while" loop instead of an "if/then" test causes problems.
#  Explain why.

exit 0

#!/bin/bash
# pb.sh: phone book

# Written by Rick Boivie, and used with permission.
# Modifications by document author.

MINARGS=1     # Script needs at least one argument.
DATAFILE=./phonebook
PROGNAME=$0
E_NOARGS=70   # No arguments error.

if [ $# -lt $MINARGS ]; then
      echo "Usage: "$PROGNAME" data"
      exit $E_NOARGS
fi      


if [ $# -eq $MINARGS ]; then
      grep $1 "$DATAFILE"
else
      ( shift; "$PROGNAME" $* ) | grep $1
      # Script recursively calls itself.
fi

exit 0        #  Script exits here.
              #  It's o.k. to put non-hashmarked comments
              #+ and data after this point.

# ------------------------------------------------------------------------
# Sample "phonebook" datafile:

John Doe        1555 Main St., Baltimore, MD 21228          (410) 222-3333
Mary Moe        9899 Jones Blvd., Warren, NH 03787          (603) 898-3232
Richard Roe     856 E. 7th St., New York, NY 10009          (212) 333-4567
Sam Roe         956 E. 8th St., New York, NY 10009          (212) 444-5678
Zoe Zenobia     4481 N. Baker St., San Franciso, SF 94338   (415) 501-1631
# ------------------------------------------------------------------------

$bash pb.sh Roe
Richard Roe     856 E. 7th St., New York, NY 10009          (212) 333-4567
Sam Roe         956 E. 8th St., New York, NY 10009          (212) 444-5678

$bash pb.sh Roe Sam
Sam Roe         956 E. 8th St., New York, NY 10009          (212) 444-5678

#  When more than one argument passed to script,
#+ prints *only* the line(s) containing all the arguments.