Moving onto where we require a greater degree of control, we can use the elif keyword. Unlike else, elif requires an additional condition to be tested for each elif. In this way, we can provide for different circumstances. We can add in as many elif conditions as required. The following shows a pseudo-code:
if condition; then statement elif condition; then statement else statement fi exit 0
A script may make life easier for the operator by providing a simplified selection for a more complex code. Even though the script becomes gradually more complex to meet the requirements, to the operator the execution is greatly simplified. It is our job to enable users to run more complex operations easily from the command line when creating scripts. Often, this will necessitate the addition of more complexity to our scripts; however, we will be rewarded with the reliability of the scripted application.
We can revisit the script that we created to run the earlier backup. This script, $HOME/bin/backup.sh, prompts the user for the file type and the directory to store the backup. The tools used for the backup are find and cp.
With this new found knowledge, we can now allow the script to run the backup using the command tar and the level of compression selected by the operator. There is no requirement, to select the file type as the complete home directory will be backed up with the exclusion of the backup directory itself.
The operator can select the compression based on three letters H, M, and L. The selection will affect the options passed to the tar command and the backup file created. The selection of high will be using bzip2 compression, medium using gzip compression and low creating an uncompressed tar archive. The logic exists in the extended if statement that follows:
if [ $file_compression = "L" ] ; then tar_opt=$tar_l elif [ $file_compression = "M" ]; then tar_opt=$tar_m else tar_opt=$tar_h fi
Based on the user selection, we can configure the correct options for the tar command. As we have three conditions to evaluate the if, elif, and else statements are appropriate. To see how the variables are configured we can view the following extract from the script:
tar_l="-cvf $backup_dir/b.tar --exclude $backup_dir $HOME" tar_m="-czvf $backup_dir/b.tar.gz --exclude $backup_dir $HOME" tar_h="-cjvf $backup_dir/b.tar.bzip2 --exclude $backup_dir $HOME"
The complete script can be created as $HOME/bin/backup2.sh and should read as the following code:
#!/bin/bash # Author: @theurbanpenguin # Web: www.theurbapenguin.com read -p "Choose H, M or L compression " file_compression read -p "Which directory do you want to backup to " dir_name # The next lines creates the directory if it does not exist test -d $HOME/$dir_name || mkdir -m 700 $HOME/$dir_name backup_dir=$HOME/$dir_name tar_l="-cvf $backup_dir/b.tar --exclude $backup_dir $HOME" tar_m="-czvf $backup_dir/b.tar.gz --exclude $backup_dir $HOME" tar_h="-cjvf $backup_dir/b.tar.bzip2 --exclude $backup_dir $HOME" if [ $file_compression = "L" ] ; then tar_opt=$tar_l elif [ $file_compression = "M" ]; then tar_opt=$tar_m else tar_opt=$tar_h fi tar $tar_opt exit 0
When we execute the script we need to select H, M, or L in upper-case as this is how the selection is made within the script. The following screenshot shows the initial script execution where the selection for M has been made:
