bash create dir with sequential numbers

Question

I am creating a script to run on OS X which will be run often by a novice user, and so want to protect a directory structure by creating a fresh one each time with an n+1 over the last:

target001 with the next run creating target002

I have so far:

lastDir=$(find /tmp/target* | tail -1 | cut -c 6-)
let n=$n+1
mkdir "$lastDir""$n"

However, the math isn't working here.

Answer 1

No pipes and subprocesses:

targets=( /tmp/target* )                # all dirs in an array
lastdir=${targets[@]: (-1):1}           # select filename from last array element
lastdir=${lastdir##*/}                  # remove path
lastnumber=${lastdir/target/}           # remove 'target'
lastnumber=00$(( 10#$lastnumber + 1 ))  # increment number (base 10), add leading zeros

mkdir /tmp/target${lastnumber: -3}      # make dir; last 3 chars from lastnumber

A version with 2 parameters:

path='/tmp/x/y/z'                       # path without last part
basename='target'                       # last part

targets=( $path/${basename}* )          # all dirs in an array
lastdir=${targets[@]: (-1):1}           # select path from last entry
lastdir=${lastdir##*/}                  # select filename
lastnumber=${lastdir/$basename/}        # remove 'target'
lastnumber=00$(( 10#$lastnumber + 1 ))  # increment number (base 10), add leading zeros

mkdir $path/$basename${lastnumber: -3}  # make dir; last 3 chars from lastnumber