Bash - shorten script with function - bash

Question

In my script I need to get the highest number of a file two times, so i wanted to create a function. This is the command in the script:
First time:
highest=$( ls $path.bak.* | sort -t"." -k2 -n | tail -n1 | sed -r 's/.*\.(.*)/\1/')
Second time:
newhighest=$(ls $path.bak.* | sort -t"." -k2 -n | tail -n1 | sed -r 's/.*\.(.*)/\1/')
Now my question:
How can I shorten this with a function?
Here my Input-Files:
test.bak.1
test.bak.2
test.bak.3
test.bak.4
test.bak.5
test.bak.6
test.bak.7
test.bak.8
test.bak.9
test.bak.10
test.bak.11
Expected return: 11

Answer 1

Written out for readability:
#!/usr/bin/env bash
# ^^^^ - Ensure that this script is run with bash, not /bin/sh
# Enable "extended globs", so we can exclude names that don't end with digits
shopt -s extglob
# since your files are test.bak.*
path=test
get_highest() {
# set the function's argument list
set -- "$path".bak.+([[:digit:]])
# if we have just one valid filename, we know the glob expanded successfully.
# otherwise, no such files exist, so exit the function immediately
[[ -e $1 || -L $1 ]] || return 1
# stream our list of extensions into sort, and let awk find the highest number
printf '%s\n' "${###*.}" | awk '$0>last{last=$0}END{print last}'
}
highest=$(get_highest) || { echo "No backup files found" >&2; exit 1; }
new_highest=$(get_highest) || { echo "No backup files on 2nd pass" >&2; exit 1; }
Note:
Expansions need to be quoted; "$path"/*, not $path/*, or else path="Directory With Spaces/test" would look for files in Spaces/test, after emitting Directory and With as results.
ls should never be used programatically.
extglob syntax allows regex-like capabilities for matching groups of files, letting us assert here that we only consider filenames that end in .bak. followed by a digit.
In general, you should write your scripts to be easy to read and understand as a higher priority than writing them to be short. Your future self (and others who need to maintain code in the future) will thank you.
Because newlines can contain in filenames, they're unsafe to use to separate filenames in a stream; only the NUL character is safe for this use when names are not otherwise quoted or escaped. Thus, when emitting a stream of arbitrary names they should be formatted with the string %s\0 and sorted with the -z argument. However, we're only printing the numeric extensions here, making newlines safe.