How to back up using shell scripts
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.
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:
This is a great way to test the script to make sure everything works as expected.
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 files are separated into fields:
# m h dom mon dow command
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
Using sudo with the
crontab -ecommand 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
# m h dom mon dow command 0 0 * * * bash /usr/local/bin/backup.sh
backup.sh script would be run every day at 12:00 pm.
backup.shscript 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
tarredirects the extracted files to the specified directory. The above example will extract the
tarrecreates 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
This will overwrite the files currently on the file system.
For more information on shell scripting see the Advanced Bash-Scripting Guide
The CronHowto Wiki Page contains details on advanced cron options.
See the GNU tar Manual for more tar options.
The Wikipedia Backup Rotation Scheme article contains information on other backup rotation schemes.
The shell script uses tar to create the archive, but there many other command line utilities that can be used. For example:
cpio: used to copy files to and from archives.
dd: part of the coreutils package. A low level utility that can copy data from one format to another.
rsync: a flexible utility used to create incremental copies of files.