33.5. A script calling itself (recursion)

Can a script recursively call itself? Indeed.

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-to-look-up" 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.

Caution

Too many levels of recursion can exhaust the script's stack space, causing a segfault.