Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
Another application of /dev/zero is to "zero out" a file of a designated size for a special purpose,
such as mounting a filesystem on a loopback device (see Example 13−6) or securely deleting a file
(see Example 12−42).
Example 29−3. Creating a ramdisk
#!/bin/bash
# ramdisk.sh
# A "ramdisk" is a segment of system RAM memory
#+ that acts as if it were a filesystem.
# Its advantage is very fast access (read/write time).
# Disadvantages: volatility, loss of data on reboot or powerdown.
# less RAM available to system.
#
# What good is a ramdisk?
# Keeping a large dataset, such as a table or dictionary on ramdisk
#+ speeds up data lookup, since memory access is much faster than disk access.
E_NON_ROOT_USER=70
ROOTUSER_NAME=root
MOUNTPT=/mnt/ramdisk
SIZE=2000
BLOCKSIZE=1024
DEVICE=/dev/ram0
# Must run as root.
# 2K blocks (change as appropriate)
# 1K (1024 byte) block size
# First ram device
username=`id −nu`
if [ "$username" != "$ROOTUSER_NAME" ]
then
echo "Must be root to run \"`basename $0`\"."
exit $E_NON_ROOT_USER
fi
if [ ! −d "$MOUNTPT" ]
then
mkdir $MOUNTPT
fi
# Test whether mount point already there,
#+ so no error if this script is run
#+ multiple times.
dd if=/dev/zero of=$DEVICE count=$SIZE bs=$BLOCKSIZE # Zero out RAM device.
mke2fs $DEVICE
# Create an ext2 filesystem on it.
mount $DEVICE $MOUNTPT
# Mount it.
chmod 777 $MOUNTPT
# So ordinary user can access ramdisk.
# However, must be root to unmount it.
echo "\"$MOUNTPT\" now available for use."
# The ramdisk is now accessible for storing files, even by an ordinary user.
# Caution, the ramdisk is volatile, and its contents will disappear
#+ on reboot or power loss.
# Copy anything you want saved to a regular directory.
# After reboot, run this script again to set up ramdisk.
# Remounting /mnt/ramdisk without the other steps will not work.
exit 0
Chapter 29. Of Zeros and Nulls 318