Advanced Bash−Scripting Guide

Advanced Bash−Scripting Guide
# ==> Comments added by author of this document.
HERE=`uname −n` # ==> hostname
THERE=bilbo
echo "starting remote backup to $THERE at `date +%r`"
# ==> `date +%r` returns time in 12−hour format, i.e. "08:08:34 PM".
# make sure /pipe really is a pipe and not a plain file
rm −rf /pipe
mkfifo /pipe # ==> Create a "named pipe", named "/pipe".
# ==> 'su xyz' runs commands as user "xyz".
# ==> 'ssh' invokes secure shell (remote login client).
su xyz −c "ssh $THERE \"cat >/home/xyz/backup/${HERE}−daily.tar.gz\" < /pipe"&
cd /
tar −czf − bin boot dev etc home info lib man root sbin share usr var >/pipe
# ==> Uses named pipe, /pipe, to communicate between processes:
# ==> 'tar/gzip' writes to /pipe and 'ssh' reads from /pipe.
# ==> The end result is this backs up the main directories, from / on down.
# ==> What are the advantages of a "named pipe" in this situation,
# ==> as opposed to an "anonymous pipe", with |?
# ==> Will an anonymous pipe even work here?
exit 0
+
Stephane Chazelas contributed the following script to demonstrate that generating prime numbers does not
require arrays.
Example A−17. Generating prime numbers using the modulo operator
#!/bin/bash
# primes.sh: Generate prime numbers, without using arrays.
# Script contributed by Stephane Chazelas.
# This does *not* use the classic "Sieve of Eratosthenes" algorithm,
#+ but instead uses the more intuitive method of testing each candidate number
#+ for factors (divisors), using the "%" modulo operator.
LIMIT=1000 # Primes 2 − 1000
Primes()
{
(( n = $1 + 1 )) # Bump to next integer.
shift
# Next parameter in list.
# echo "_n=$n i=$i_"
if (( n == LIMIT ))
then echo $*
return
fi
Appendix A. Contributed Scripts 389