Linux Know-How: Script Recycling with Variables

Script Recycling with Variables

Apart from making the script more readable, variables will also enable you to faster apply a script in another environment or for another purpose. Consider the following example, a very simple script that makes a backup of franky's home directory to a remote server:

#!/bin/bash

# This script makes a backup of my home directory.

cd /home

# This creates the archive
tar cf /var/tmp/home_franky.tar franky > /dev/null 2>&1

# First remove the old bzip2 file.  Redirect errors because this generates some if the archive
# does not exist.  Then create a new compressed file.
rm /var/tmp/home_franky.tar.bz2 2> /dev/null
bzip2 /var/tmp/home_franky.tar

# Copy the file to another host - we have ssh keys for making this work without intervention.
scp /var/tmp/home_franky.tar.bz2 bordeaux:/opt/backup/franky > /dev/null 2>&1

# Create a timestamp in a logfile.
date > /home/franky/log/home_backup.log
echo backup succeeded > /home/franky/log/home_backup.log

First of all, you are more likely to make errors if you name files and directories manually each time you need them. Secondly, suppose franky wants to give this script to carol, then carol will have to do quite some editing before she can use the script to back up her home directory. The same is true if franky wants to use this script for backing up other directories. For easy recycling, make all files, directories, usernames, servernames etcetera variable. Thus, you only need to edit a value once, without having to go through the entire script to check where a parameter occurs. This is an example:

#!/bin/bash

# This script makes a backup of my home directory.

# Change the values of the variables to make the script work for you:
BACKUPDIR=/home
BACKUPFILES=franky
TARFILE=/var/tmp/home_franky.tar
BZIPFILE=/var/tmp/home_franky.tar.bz2
SERVER=bordeaux
REMOTEDIR=/opt/backup/franky
LOGFILE=/home/franky/log/home_backup.log

cd $BACKUPDIR

# This creates the archive
tar cf $TARFILE $BACKUPFILES > /dev/null 2>&1

# First remove the old bzip2 file.  Redirect errors because this generates some if the archive
# does not exist.  Then create a new compressed file.
rm $BZIPFILE 2> /dev/null
bzip2 $TARFILE

# Copy the file to another host - we have ssh keys for making this work without intervention.
scp $BZIPFILE $SERVER:$REMOTEDIR > /dev/null 2>&1

# Create a timestamp in a logfile.
date > $LOGFILE
echo backup succeeded > $LOGFILE

	Large directories and low bandwidth
	The above is purely an example that everybody can understand, using a small directory and a host on the same subnet. Depending on your bandwidth, the size of the directory and the location of the remote server, it can take an awful lot of time to make backups using this mechanism. For larger directories and lower bandwidth, use rsync to keep the directories at both ends synchronized.