Bash script to obtain the newest file X in a folder and create a new variable called X+1 - bash

Question

I am trying to create a loop in Bash script for a series of data migrations:
At the beginning of every step, the script should get the name of the newest file in a folder
called "migrationfiles/ and store it in the variable "migbefore" and create a new variable called "migbefore+1":
Example: if the "migrationfiles/" folder contains the following files:
migration.pickle1 migration.pickle2 migration.pickle3
The variable "migbefore" and migafter should have the following value:
migbefore=migration.pickle3
migafter=migration.pickle4
At the end of every step, the function "metl", which is in charge of making the data migration, uses the file "migbefore" to load the data and creates 1 new file called "migafter" and stores it in the "migrationfiles/" folder, so in this case, the new file created will be called:
"migration.pickle4"
The code I pretend using is the following:
#!/bin/bash
migbefore=0
migafter=0
for y in testappend/*
for x in migrationfiles/*
do
migbefore=migration.pickle(oldest)
migafter=migbefore+1
done
do
metl -m migrationfiles/"${migbefore}"
-t migrationfiles/"${migafter}"
-s "${y}"
config3.yml
done
Does anyone know how I could make the first loop (The one that searches for the newest file in the "migrationfiles/" folder) and then assigns the name of the variable "migafter" as "migbefore+1"?

Answer 1

I think this might do what you want.
#!/bin/bash
count=0
prefix=migration.pickle
migbefore=$prefix$((count++))
migafter=$prefix$((count++))
for y in testappend/*; do
echo metl -m migrationfiles/"${migbefore}" \
-t migrationfiles/"${migafter}" \
-s "${y}" \
config3.yml
migbefore=$migafter
migafter=$prefix$((count++))
done

Answer 2

Copy with Numbered Backups
It's really hard to tell what you're really trying to do here, and why. However, you might be able to make life simpler by using the --backup flag from the cp command. For example:
cp --backup=numbered testappend/migration.pickle migrationfiles/
This will ensure that you have a sequence of migration files like:
migration.pickle
migration.pickle.~1~
migration.pickle.~2~
migration.pickle.~3~
where the older versions have larger ordinal numbers, while the latest version has no ordinal extension. It's a pretty simple system, but works well for a wide variety of use cases. YMMV.

Answer 3

# configuration:
path=migrationfiles
prefix=migration.pickle
# determine number of last file:
last_number=$( find ${path} -name "${prefix}*" | sed -e "s/.*${prefix}//g" | sort -n | tail -1 )
# put together the file names:
migbefore=${prefix}${last_number}
migafter=${prefix}$(( last_number + 1 ))
# test it:
echo $migbefore $migafter
This should work even if there are no migration files yet. In that case, the value of migbefore is just the prefix and does not point to a real file.