How to back up using shell scripts

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In general, a shell script configures which directories to backup, and passes those directories as arguments to the tar utility, which creates an archive file. The archive file can then be moved or copied to another location. The archive can also be created on a remote file system such as a Network File System (NFS) mount.

The tar utility creates one archive file out of many files or directories. tar can also filter the files through compression utilities, thus reducing the size of the archive file.

In this guide, we will walk through how to use a shell script for backing up files, and how to restore files from the archive we create.

The shell script

The following shell script uses the basic backup shell script from our Reference section. It uses tar to create an archive file on a remotely mounted NFS file system. The archive filename is determined using additional command line utilities. For more details about the script, check out the example Reference page.

#!/bin/bash
####################################
#
# Backup to NFS mount script.
#
####################################
    
# What to backup. 
backup_files="/home /var/spool/mail /etc /root /boot /opt"
    
# Where to backup to.
dest="/mnt/backup"
    
# Create archive filename.
day=$(date +%A)
hostname=$(hostname -s)
archive_file="$hostname-$day.tgz"
    
# Print start status message.
echo "Backing up $backup_files to $dest/$archive_file"
date
echo
    
# Backup the files using tar.
tar czf $dest/$archive_file $backup_files
    
# Print end status message.
echo
echo "Backup finished"
date
    
# Long listing of files in $dest to check file sizes.
ls -lh $dest

Running the script

Run from a terminal

The simplest way to use the above backup script is to copy and paste the contents into a file (called backup.sh, for example). The file must be made executable:

chmod u+x backup.sh

Then from a terminal prompt, run the following command:

sudo ./backup.sh

This is a great way to test the script to make sure everything works as expected.

Run with `cron`

The cron utility can be used to automate use of the script. The cron daemon allows scripts, or commands, to be run at a specified time and date.

cron is configured through entries in a crontab file. crontab files are separated into fields:

# m h dom mon dow   command

Where:

m: The minute the command executes on, between 0 and 59.
h: The hour the command executes on, between 0 and 23.
dom: The day of the month the command executes on.
mon: The month the command executes on, between 1 and 12.
dow: The day of the week the command executes on, between 0 and 7. Sunday may be specified by using 0 or 7, both values are valid.
command: The command to run.

To add or change entries in a crontab file the crontab -e command should be used. Also note the contents of a crontab file can be viewed using the crontab -l command.

To run the backup.sh script listed above using cron, enter the following from a terminal prompt:

sudo crontab -e

Note:
Using sudo with the crontab -e command edits the root user’s crontab. This is necessary if you are backing up directories only the root user has access to.

As an example, if we add the following entry to the crontab file:

# m h dom mon dow   command
0 0 * * * bash /usr/local/bin/backup.sh

The backup.sh script would be run every day at 12:00 pm.

Note:
The backup.sh script will need to be copied to the /usr/local/bin/ directory in order for this entry to run properly. The script can reside anywhere on the file system, simply change the script path appropriately.

Restoring from the archive

Once an archive has been created, it is important to test the archive. The archive can be tested by listing the files it contains, but the best test is to restore a file from the archive.

To see a listing of the archive contents, run the following command from a terminal:
```
tar -tzvf /mnt/backup/host-Monday.tgz
```
To restore a file from the archive back to a different directory, enter:
```
tar -xzvf /mnt/backup/host-Monday.tgz -C /tmp etc/hosts
```
The -C option to tar redirects the extracted files to the specified directory. The above example will extract the /etc/hosts file to /tmp/etc/hosts. tar recreates the directory structure that it contains. Also, notice the leading “/” is left off the path of the file to restore.
To restore all files in the archive enter the following:
```
cd /
sudo tar -xzvf /mnt/backup/host-Monday.tgz
```
Note:
This will overwrite the files currently on the file system.