How to check for a numerically named file and, if it exists, copy it with a new name using the next sequential number

Question

First I want to apologize for my lack of knowledge; this is quite literally my first ever bash script and my Linux knowledge is basically nonexistent. Explanatory answers would be much appreciated; I'd like to learn the why not just the how.

I am trying to write a script to automate a tedious process that has to be done multiple times a day. In short, this script should prepare a set of files based on certain requirements. The script must either move a template file in and assign it a version number, or copy an existing file using a new version number, based on what is found inside of one particular directory (eg create if empty, copy if there's something there).

As an example the file might be named

YYYYMMDD_##_username_testfile.json

where the ## (version number) should be 01 for the first creation, then 02 for the copy, 03 for the copy after that, etc. A single directory could contain upwards of 10+ of these files, all with different dates and sequential version numbers, as well as other files using the same format but ending in something other than testfile.json

I can't seem to find a way to have the script look in the directory "test" for the file ending in testfile.json that has the highest version number, and if found extract the version number as a variable/parameter I can pass back into the cp sequence.

At this point I'm not even sure what command to be using; the internet has offered everything from grep to awk to listing and sorting...my head's spinning!

Any help would be appreciated. I've stripped all identifying info out, please let me know if you need more information.

Answer 1

Lets break it down into a number of steps.

We are going to write a function that will return the next number. We are going to aim for simplicity, rather than maximal efficiency

You need to give it a directory name

You need to give it the testfile.json part of the filename

We need to find all the files in that directory which end with that ending.

Parameters are passed into a function using $1, $2 etc.

getnum(){
  ls "$1" | grep "$2\$"
}

Use cut to pull out just the second field, using _ as the delimiter. (Note if I was going this I would use sed instead of grep and cut, others might use awk, but we are going for simplicity here).

getnum() {
 ls "$1" | grep "$2\$" | cut -d_ -f 2
}

Use sort to sort them numerically

getnum() {
  ls "$1" | grep "$2\$" | cut -d_ -f 2 | sort -n
}

Use head to get just the first value

 getnum() {
  ls "$1" | grep "$2\$" | cut -d_ -f 2 | sort -n | head -1
 }

Wrap this up so we get the value in a local variable

getnum() {
      local oldnum=$(ls "$1" | grep "$2\$" | cut -d_ -f 2 | sort -n | head -1)
}

If there were no values then set the old value to 0. Print out a 2 digit number that is one more that the old one.

 getnum() {
  local oldnum=$(ls "$1" | grep "$2\$" | cut -d_ -f 2 | sort -n | head -1)
  if [ "$oldnum" = "" ]
  then
      oldnum=0
  fi
  printf "%02d\n" $(($oldnum+1))
 }

Note that I could have omitted the test for oldnum not being set, as $((oldnum+1)) would expand to $((+1)) in that case. I can also use expressions like $((${oldnum:-0}+1)), consult the manuals for more information.

You could then use function like this

  dir=somewhere/logs
  ext=testfile.json
  cp template "$dir/$(date +%Y%m%d_)$(getnum "$dir" "$ext")_$USER_$ext"