Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
#!/bin/bash
# Generates a log file in current directory
# from the tail end of /var/log/messages.
# Note: /var/log/messages must be world readable
# if this script invoked by an ordinary user.
# #root chmod 644 /var/log/messages
LINES=5
( date; uname −a ) >>logfile
# Time and machine name
echo −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− >>logfile
tail −$LINES /var/log/messages | xargs | fmt −s >>logfile
echo >>logfile
echo >>logfile
exit 0
# Exercise:
# −−−−−−−−
# Modify this script to track changes in /var/log/messages at intervals
#+ of 20 minutes.
# Hint: Use the "watch" command.
Example 12−5. copydir, copying files in current directory to another, using xargs
#!/bin/bash
# Copy (verbose) all files in current directory
# to directory specified on command line.
if [ −z "$1" ] # Exit if no argument given.
then
echo "Usage: `basename $0` directory−to−copy−to"
exit 65
fi
ls . | xargs −i −t cp ./{} $1
# This is the exact equivalent of
# cp * $1
# unless any of the filenames has "whitespace" characters.
expr
exit 0
All−purpose expression evaluator: Concatenates and evaluates the arguments according to the
operation given (arguments must be separated by spaces). Operations may be arithmetic, comparison,
string, or logical.
expr 3 + 5
returns 8
expr 5 % 3
returns 2
expr 5 \* 3
returns 15
Chapter 12. External Filters, Programs and Commands 154