Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
To demonstrate just how versatile dd is, let's use it to capture keystrokes.
Example 12−41. Capturing Keystrokes
#!/bin/bash
# Capture keystrokes without needing to press ENTER.
keypresses=4
# Number of keypresses to capture.
old_tty_setting=$(stty −g)
# Save old terminal settings.
echo "Press $keypresses keys."
stty −icanon −echo
keys=$(dd bs=1 count=$keypresses 2> /dev/null)
# 'dd' uses stdin, if "if" not specified.
# Disable canonical mode.
# Disable local echo.
stty "$old_tty_setting"
# Restore old terminal settings.
echo "You pressed the \"$keys\" keys."
# Thanks, S.C. for showing the way.
exit 0
The dd command can do random access on a data stream.
echo −n . | dd bs=1 seek=4 of=file conv=notrunc
# The "conv=notrunc" option means that the output file will not be truncated.
# Thanks, S.C.
The dd command can copy raw data and disk images to and from devices, such as floppies and tape
drives (Example A−6). A common use is creating boot floppies.
dd if=kernel−image of=/dev/fd0H1440
Similarly, dd can copy the entire contents of a floppy, even one formatted with a "foreign" OS, to the
hard drive as an image file.
dd if=/dev/fd0 of=/home/bozo/projects/floppy.img
Other applications of dd include initializing temporary swap files (Example 29−2) and ramdisks
(Example 29−3). It can even do a low−level copy of an entire hard drive partition, although this is not
necessarily recommended.
People (with presumably nothing better to do with their time) are constantly thinking of interesting
applications of dd.
Example 12−42. Securely deleting a file
#!/bin/bash
Chapter 12. External Filters, Programs and Commands 209