Refreshing variable just before invoking it? - bash

Question

I have some easy backup script to tar, xz and save to SMB share some folders and want to use variable ($invocation) for a part of the command.
Unfortunately, when script runs it remembers that the variable $location is empty and a result of it is no backup
(xz: /mnt/backupMatrix//2020-06-05T00-27.tar.xz: No such file or directory).
I do know why it's not working, but how to do it correctly?
How to "reload" variable $invocation (with freshly set $location) just before it's launch, WITHOUT hardcoding whole invocation to the function?
Code below.
#! /bin/bash
invocation="xz --threads=8 -z -9 -v -v - > /mnt/backupMatrix/$location/$(date +%Y-%m-%dT%H-%M).tar.xz"
function mcuser2
{
#backup mcuser2
location="mcuser2"
echo "setting var location to: $location"
echo "Stopping minecraft2.service"
sudo systemctl stop minecraft2.service
echo "Stopped minecraft2.service"
echo "Starting taring mcuser2"
# --exclude='/home/mcuser2/world'
tar cpf - --exclude='/home/mcuser2/plugins/dynmap/web/tiles' /home/mcuser2/ | $invocation
echo "Completed taring mcuser2"
echo "starting minecraft2.service"
sudo systemctl start minecraft2.service
echo "started successfully minecraft2.service"
}
case "$1" in
mcuser2)
mcuser2
;;
esac

Answer 1

Since you set location unconditionally in the function, the simplest approach is to move that line from the function and place it before you set invocation.
An alternative is to use more variables:
: "${XZ:=xz}"
: "${XZOPTIONS:=--threads=8 -z -9 -v -v -}"
: "${TSTAMP:=$(date +%Y-%m-%dT%H-%M)}"
: "${BASEDIR:=/mnt/backupMatrix}"
: "${TAR:=tar}"
: "${TAROPTIONS:=-cpf -}"
STARTDIR="/home/mcuser2" # $HOME?
EXCLUDE="--exclude=$STARTDIR/plugins/dynmap/web/tiles"
function mcuser2
{
#backup mcuser2
location="mcuser2"
echo "setting var location to: $location"
echo "Stopping minecraft2.service"
sudo systemctl stop minecraft2.service
echo "Stopped minecraft2.service"
echo "Starting tarring mcuser2"
$TAR $TAROPTIONS - "$EXCLUDE" "$STARTDIR" |
$XZ $XZOPTIONS > "$BASEDIR/$location/$TSTAMP.tar.xz"
echo "Completed tarring $STARTDIR"
echo "starting minecraft2.service"
sudo systemctl start minecraft2.service &&
echo "started successfully minecraft2.service"
}
Warning: untested shell scripting.
I'd probably not include the full path in the archive (GNU tar drops the leading slash anyway); I'd cd to the start directory and specify . as the directory to be backed up — possibly use -C "$STARTDIR" on the tar command line. That would require me to alter the exclude option too, I expect, using a relative path instead of an absolute path.
You could also think of using shell arrays for the options lists.
I'm not convinced I'd use a function in this script; it doesn't really seem to help very much.