I have a backup script that backs up home directories and /etc. I have got the script to exit if the 'tar' command returns a nonzero code. At random it seems to return a code of 2, but it seems to back up all files.

Could this be due to files being deleted or changed while the tar command is in progress? Should I ignore these errors?

Tar exit code 2 means "failed read" as far as I am able to determine.

Since you are backing up /home and /etc, I would suggest that there are files/folders in /etc that you as user do have read access to. The failed reads may record the file name, but not the contents.

Try running the script as root to see if the error repeats.

I am running it as root.

I wonder if files in the maildirs in the home directories are changing while the backup is in progress - hence the error code.

Document of tar
http://www.gnu.org/software/tar/manu.../Synopsis.html

Possible exit codes of GNU tar are summarized in the following table:

0
‘Successful termination’.

1
‘Some files differ’. If tar was invoked with ‘--compare’ (‘--diff’, ‘-d’) command line option, this means that some files in the archive differ from their disk counterparts (see section Comparing Archive Members with the File System). If tar was given ‘--create’, ‘--append’ or ‘--update’ option, this exit code means that some files were changed while being archived and so the resulting archive does not contain the exact copy of the file set.

2
‘Fatal error’. This means that some fatal, unrecoverable error occurred.

Maybe a way to combat this could be having the script COPY the 2 directorys to a temporary directory, then tar up the files, then delete the temp files. Of course this method would require more time and hard drive space, but the principle is there.