Tar (child) error

There are very few files that absolutely must be different between two machines, and need to be regenerated when cloning:

• The host name /etc/hostname.
• The SSH host keys: /etc/ssh_host_*_key* or /etc/ssh/ssh_host_*_key* or similar location.
• The random seed: /var/lib/urandom/random-seed or /var/lib/random-seed or similar location. (/var/lib/systemd/random-seed on systems using systemd)

Anything else could be identical if you have a bunch of identical machines.

A few files are typically different on machines with different hardware:

• /etc/fstab, /etc/crypttab, /etc/mdadm.conf, and bootloader configuration files (if located in /etc — some distributions put them in /boot) if disks are partitioned differently.
• /etc/X11/xorg.conf, if present, if the machines have different graphics cards.
• Modules to load or blacklist in /etc/modules, /etc/modprobe.conf, /etc/modprobe.d/ and /etc/modutils/.

In addition, some network configuration may need to change, in particular:

• If you have static IP addresses, they need to be diversified per machine. The location of IP configuration varies between distribution (e.g. /etc/network/interfaces on Debian, /etc/sysconfig/network on Red Hat).
• /etc/hosts often contains the host name.
• Mail configuration often contains the host name: check /etc/mailname.

There's no general answer to “what are the files in /etc folder (…) are unique for each computer” because the whole purpose of /etc is to store files that can be customized on each computer. For example, if you have different accounts on different machines, then obviously you can't share the account database — and if you want to be able to share the account database, then you'll end up with the same accounts.

Generally speaking, don't try to share /etc by default unless you have a set of machines with the same software configuration — same installed software, same accounts, etc. If you do share /etc, you'll need to blacklist a few files from sharing as indicated above.

If you have machines with different configurations, then whitelist what you synchronize. Treat files in /etc as distinct on different machines, like files in /var. Synchronize only the ones that you've decided should apply everywhere.

One possible way to manage synchronization is to keep machine-specific files in a different directory, e.g. /local/etc, and make symbolic links like /etc/fstab -> ../local/etc/fstab. This still requires a largely homogeneous set of machines in terms of software as different distributions put files in different locations. Or, conversely, keep only the machine-specific files in /etc and all generic files elsewhere — but typical distributions don't accommodate this well.

You obviously can't do a live test of the restoration of the system configuration of one system on a different system. To test the restoration of your backups, fire up a virtual machine that emulates the hardware configuration sufficiently well (in particular, with a similar disk layout).

The issue is that ~-expansion is not performed upon variable expansion (when you refer to that $SRCDIR unquoted) nor inside double quotes (when you assign that SRCDIR variable).

In leaving $SRCDIR unquoted, you're invoking the split+glob operator. That is the string that is stored in that scalar $SRCDIR variable (~/PI/tutos/* ~/scripts/*) is first split according to $IFS (blanks by default), and then each word undergoes globbing, that is are treated as patterns that expand to the list of matching files.

Because ~ is not expanded to your home directory there, that ~ is just treated like any other character, so it's just looking for files in the ~/PI/tutos directory, where ~ would be a directory in the current directory, which in your case doesn't exist.

Best here would be to make $SRCDIR an array and have the globs expanded at the time of the assignment:

SRCDIR=(~/PI/tutos/* ~/scripts/*) # ~ and globs expanded at this point
tar cvpzf "$FILENAME" "${SRCDIR[@]}"

Note that applying the split+glob operator on $FILENAME doesn't make sense, so we're disabling it by quoting $FILENAME.

Note that if ~/PI/tutos/* doesn't match, it will be left as-is, so you'd still get an error from tar. To avoid that, you could do:

shopt -s nullglob # remove non-matching globs
SRCDIR=(~/PI/tutos/* ~/scripts/*)
if ((${#SRCDIR[@]} != 0)); then
  tar cvpzf "$FILENAME" "${SRCDIR[@]}"
fi

You may be tempted to do:

SRCDIR="$HOME/PI/tutos/* $HOME/scripts/*"
tar cvpzf "$FILENAME" $SRCDIR

As variables (such as $HOME) are expanded within double quotes, but I would advise against it as that wouldn't work properly if $HOME contains glob characters or characters of $IFS.

~s are expanded in variable assignments when not quoted and when at the start or following : (that's so that it works in assignments of variables like $PATH, $LD_LIBRARY_PATH... such as PATH=~/bin:~/sbin). So you may be tempted to do:

SRCDIR=~/PI/tutos/*:~/scripts/* # ~ (not globs) expanded here
IFS=:
tar cvpzf "$FILENAME" $SRCDIR

But that's the same as above, that won't work properly if $HOME contains $IFS characters (this time :, so very unlikely as /etc/passwd that defines your home directory is a colon-separated table) or glob characters.