Example 33-8. A (useless) script that recursively calls itself
#!/bin/bash
# recurse.sh
# Can a script recursively call itself?
# Yes, but is this of any practical use?
# (See the following.)
RANGE=10
MAXVAL=9
i=$RANDOM
let "i %= $RANGE" # Generate a random number between 0 and $RANGE - 1.
if [ "$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
# Note:
# ----
# This script must have execute permission for it to work properly.
# This is the case even if it is invoked by an "sh" command.
# Explain why.
Example 33-9. A (useful) script that recursively calls itself
#!/bin/bash
# pb.sh: phone book
# Written by Rick Boivie, and used with permission.
# Modifications by ABS Guide author.
MINARGS=1 # Script needs at least one argument.
DATAFILE=./phonebook
# A data file in current working directory
#+ named "phonebook" must exist.
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"
# 'grep' prints an error message if $DATAFILE not present.
else
( shift; "$PROGNAME" $* ) | grep $1
# Script recursively calls itself.
fi
exit 0 # Script exits here.
# Therefore, 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 Francisco, 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
$bash pb.sh Roe Sam
Sam Roe 956 E. 8th St., New York, NY 10009 (212) 444-5678
# When more than one argument is passed to this script,
#+ it prints *only* the line(s) containing all the arguments.
Example 33-10. Another (useful) script that recursively calls itself
#!/bin/bash
# usrmnt.sh, written by Anthony Richardson
# Used with permission.
# usage: usrmnt.sh
# description: mount device, invoking user must be listed in the
# MNTUSERS group in the /etc/sudoers file.
# ----------------------------------------------------------
# This is a usermount script that reruns itself using sudo.
# A user with the proper permissions only has to type
# usermount /dev/fd0 /mnt/floppy
# instead of
# sudo usermount /dev/fd0 /mnt/floppy
# I use this same technique for all of my
#+ sudo scripts, because I find it convenient.
# ----------------------------------------------------------
# If SUDO_COMMAND variable is not set we are not being run through
#+ sudo, so rerun ourselves. Pass the user's real and group id . . .
if [ -z "$SUDO_COMMAND" ]
then
mntusr=$(id -u) grpusr=$(id -g) sudo $0 $*
exit 0
fi
# We will only get here if we are being run by sudo.
/bin/mount $* -o uid=$mntusr,gid=$grpusr
exit 0
# Additional notes (from the author of this script):
# -------------------------------------------------
# 1) Linux allows the "users" option in the /etc/fstab
# file so that any user can mount removable media.
# But, on a server, I like to allow only a few
# individuals access to removable media.
# I find using sudo gives me more control.
# 2) I also find sudo to be more convenient than
# accomplishing this task through groups.
# 3) This method gives anyone with proper permissions
# root access to the mount command, so be careful
# about who you allow access.
# You can get finer control over which access can be mounted
# by using this same technique in separate mntfloppy, mntcdrom,
# and mntsamba scripts.
Too many levels of recursion can exhaust the
script's stack space, causing a segfault.
