Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
echo "Basename of /home/bozo/daily−journal.txt = `basename $a`"
echo "Dirname of /home/bozo/daily−journal.txt = `dirname $a`"
echo
echo "My own home is `basename ~/`." # Also works with just ~.
echo "The home of my home is `dirname ~/`." # Also works with just ~.
exit 0
split
Utility for splitting a file into smaller chunks. Usually used for splitting up large files in order to back
them up on floppies or preparatory to e−mailing or uploading them.
sum, cksum, md5sum
These are utilities for generating checksums. A checksum is a number mathematically calculated from
the contents of a file, for the purpose of checking its integrity. A script might refer to a list of
checksums for security purposes, such as ensuring that the contents of key system files have not been
altered or corrupted. For security applications, use the 128−bit md5sum (message digest checksum)
command.
bash$ cksum /boot/vmlinuz
1670054224 804083 /boot/vmlinuz
bash$ md5sum /boot/vmlinuz
0f43eccea8f09e0a0b2b5cf1dcf333ba /boot/vmlinuz
Note that cksum also shows the size, in bytes, of the target file.
Example 12−29. Checking file integrity
#!/bin/bash
# file−integrity.sh: Checking whether files in a given directory
# have been tampered with.
E_DIR_NOMATCH=70
E_BAD_DBFILE=71
dbfile=File_record.md5
# Filename for storing records.
set_up_database ()
{
echo ""$directory"" > "$dbfile"
# Write directory name to first line of file.
md5sum "$directory"/* >> "$dbfile"
# Append md5 checksums and filenames.
}
check_database ()
{
local n=0
local filename
local checksum
# −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− #
# This file check should be unnecessary,
Chapter 12. External Filters, Programs and Commands 187