Bash - Expanding variables for command arguments leads to quoting hell - bash

Question

So, I'm scripting an rsync command. I've been fiddling around for the entire day. It's time to ask for help.
My issue appears to be quoting, but it's not obvious to me exactly what's going wrong. This snippet:
#!/bin/bash
sudo -v
COMMAND=`basename ${0}`
SRC='/etc'
SRC=`realpath ${SRC}` # make relative paths absolute
SRC=${SRC}/ # force rsync to disallow symlinks to the parent path
if [[ ! -d ${SRC} ]];
then
echo Use ${COMMAND} to backup directories only
exit 1
fi
BACKUP_DIR='/tmp/backup prep'
TMP_DIR=${BACKUP_DIR}/tmp
DEST=${BACKUP_DIR}/`basename ${SRC}`
LOG_DIR=${TMP_DIR}
LOG_FILE=${LOG_DIR}/${COMMAND}-`date +%Y-%b-%d-%H-%M-%S-%N`.log
for DIR in "${BACKUP_DIR}" "${TMP_DIR}" "${LOG_DIR}"
do
if [[ ! -d "'${DIR}'" ]];
then
echo Creating "'${DIR}'"
sudo mkdir "${DIR}"
fi
done
RSYNC_OPTS=""
#--dry-run, -n
RSYNC_OPTS=${RSYNC_OPTS}" --dry-run"
# --recursive, -r recurse into directories
RSYNC_OPTS=${RSYNC_OPTS}" --recursive"
#--filter=RULE, -f add a file-filtering RULE
RSYNC_OPTS=${RSYNC_OPTS}" --filter='dir-merge,p- .gitignore'"
# --checksum, -c skip based on checksum, not mod-time & size
RSYNC_OPTS=${RSYNC_OPTS}" --checksum"
echo "rsync ${RSYNC_OPTS} '${SRC}' '${DEST}'" | sudo tee "${LOG_FILE}"
echo --------
echo --------
echo
echo
set -x
sudo rsync "${RSYNC_OPTS} '${SRC}' '${DEST}'"
Produces this:
Creating '/tmp/backup prep'
mkdir: cannot create directory ‘/tmp/backup prep’: File exists
Creating '/tmp/backup prep/tmp'
mkdir: cannot create directory ‘/tmp/backup prep/tmp’: File exists
Creating '/tmp/backup prep/tmp'
mkdir: cannot create directory ‘/tmp/backup prep/tmp’: File exists
rsync --dry-run --recursive --filter='dir-merge,p- .gitignore' --checksum '/etc/' '/tmp/backup prep/etc'
--------
+ sudo rsync ' --dry-run --recursive --filter='\''dir-merge,p- .gitignore'\'' --checksum '\''/etc/'\'' '\''/tmp/backup prep/etc'\'''
rsync: [sender] change_dir "/home/aaron/bin/ --dry-run --recursive --filter='dir-merge,p- .gitignore' --checksum '/etc/' '/tmp/backup prep" failed: No such file or directory (2)
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1333) [sender=3.2.3]
And appears to do nothing.
The thing is, if I sudo the echoed command line, sudo rsync --dry-run --recursive --filter='dir-merge,p- .gitignore' --checksum '/etc/' '/tmp/backup prep/etc'
Everything seems to work as expected.
Shell expansion gets me every bloody time. It'd be nice if there were a command that'd show you what the hell is going on.

Answer 1

Here is my finished script. Many thanks to the commentors on my question. With your hints I was able to make progress.
For what it's worth, the objective was to periodically back up a /etc that is using etckeeper and keep the .git repo, but ignore stuff in .gitignore
Some observations:
When building a list of arguments for a command, an array is totally the way to go.
When using the array, the shell beautifully quotes values with spaces. Yo can see this by using set -x in the script below.
Using set -x is really helpful.
When you're struggling to get something working shellcheck is an amazing diagnostic tool.
#!/bin/bash
COMMAND=$(basename "${0}")
SRC="/etc"
BACKUP_DIR="/root/backup prep"
TMP_DIR="${BACKUP_DIR}/tmp"
LOG_DIR="${TMP_DIR}"
if ! sudo -v
then
echo ${COMMAND} quitting...
exit 1
fi
SRC=$(realpath "${SRC}") # make relative paths absolute
SRC="${SRC}"/ # force rsync to disallow symlinks to the parent path
# https://www.shellcheck.net/wiki/SC2086
# Note that $( ) starts a new context, and variables in it have to
# be quoted independently
LOG_FILE="${LOG_DIR}/${COMMAND}-$(date +%Y-%b-%d-%H-%M-%S-%N).log"
for DIR in "${BACKUP_DIR}" "${TMP_DIR}" "${LOG_DIR}"
do
if sudo test ! -d "${DIR}";
then
echo Creating "${DIR}"
sudo mkdir "${DIR}"
fi
done
RSYNC_OPTS=()
# ------------------------------------
# -------------Testing----------------
# ------------------------------------
#
#--dry-run, -n
#RSYNC_OPTS+=("--dry-run")
# ------------------------------------
# -------------Verbosity--------------
# ------------------------------------
#
# --verbose, -v increase verbosity
#RSYNC_OPTS+=("--verbose")
# --progress show progress during transfer
#RSYNC_OPTS+=("--progress")
# --itemize-changes, -i output a change-summary for all updates
#RSYNC_OPTS+=("--itemize-changes")
# --stats give some file-transfer stats
#RSYNC_OPTS+=("--stats")
# --human-readable, -h output numbers in a human-readable format
RSYNC_OPTS+=("--human-readable")
# --log-file=FILE log what we're doing to the specified FILE
RSYNC_OPTS+=("--log-file=${LOG_FILE}")
# --log-file-format=FMT log updates using the specified FMT
# ------------------------------------
# -------------Permissions------------
# ------------------------------------
#
# --owner, -o preserve owner (super-user only)
RSYNC_OPTS+=("--owner")
# --group, -g preserve group
RSYNC_OPTS+=("--group")
# --perms, -p preserve permissions
RSYNC_OPTS+=("--perms")
# --xattrs, -X preserve extended attributes
RSYNC_OPTS+=("--xattrs")
# --acls, -A preserve ACLs (implies --perms)
RSYNC_OPTS+=("--acls")
# ------------------------------------
# -------------Times------------------
# ------------------------------------
#
# --times, -t preserve modification times
RSYNC_OPTS+=("--times")
# --crtimes, -N preserve create times (newness)
# rsync: This rsync does not support --crtimes (-N)
# RSYNC_OPTS+=("--crtimes")
# --atimes, -U preserve access (use) times
RSYNC_OPTS+=("--atimes")
# --open-noatime avoid changing the atime on opened files
RSYNC_OPTS+=("--open-noatime")
# ------------------------------------
# -------------Where------------------
# ------------------------------------
#
# --mkpath create the destination's path component
RSYNC_OPTS+=("--mkpath")
# --recursive, -r recurse into directories
RSYNC_OPTS+=("--recursive")
# --links, -l copy symlinks as symlinks
RSYNC_OPTS+=("--links")
# Rsync can also distinguish "safe" and "unsafe" symbolic links. An
# example where this might be used is a web site mirror that wishes to
# ensure that the rsync module that is copied does not include symbolic
# links to /etc/passwd in the public section of the site. Using
# --copy-unsafe-links will cause any links to be copied as the file they
# point to on the destination. Using --safe-links will cause unsafe
# links to be omitted altogether. (Note that you must specify --links
# for --safe-links to have any effect.)
# --copy-unsafe-links where the link would point outside of the new tree, copy the file
RSYNC_OPTS+=("--copy-unsafe-links")
# --hard-links, -H preserve hard links
RSYNC_OPTS+=("--hard-links")
# --one-file-system, -x don't cross filesystem boundaries
RSYNC_OPTS+=("--one-file-system")
# ------------------------------------
# -------------Exclusions-------------
# ------------------------------------
#
#--filter=RULE, -f add a file-filtering RULE
RSYNC_OPTS+=("--filter=dir-merge,p- .gitignore")
# --delete-excluded also delete excluded files from dest dirs
RSYNC_OPTS+=("--delete-excluded")
#--delete-after receiver deletes after transfer, not during
RSYNC_OPTS+=("--delete-after")
# --force force deletion of dirs even if not empty
RSYNC_OPTS+=("--force")
# --update, -u skip files that are newer on the receiver
#RSYNC_OPTS=${RSYNC_OPTS}" --update")
# ------------------------------------
# -------------Misc-------------------
# ------------------------------------
#
# --protect-args, -s no space-splitting; wildcard chars only
# This option sends all filenames and most options to the remote rsync
# without allowing the remote shell to interpret them.
RSYNC_OPTS+=("--protect-args")
# --checksum, -c skip based on checksum, not mod-time & size
RSYNC_OPTS+=("--checksum")
# --temp-dir=DIR, -T create temporary files in directory DIR
RSYNC_OPTS+=("--temp-dir=${TMP_DIR}")
#echo sudo rsync "${RSYNC_OPTS[#]}" "'${SRC}'" "'${DEST}'"
#echo
#set -x
sudo rsync "${RSYNC_OPTS[#]}" "${SRC}" "${DEST}"