Problems understanding how set works

Question

Can someone please help me understand the following code segment?

set -- ${BACKUPDIR}/backup-???.tgz
lastname=${!#}
backupnr=${lastname##*backup-}
backupnr=${backupnr%%.*}
backupnr=${backupnr//\?/0}
backupnr=$[10#${backupnr}]

I found it in this backup script https://wiki.ubuntuusers.de/Skripte/inkrementelles_Backup/ where it is used to get the number of the filename of the last stored backup. The filenames of the backups are "backup-ccc.tgz" where ccc is a three digit number that is incremented every time a new backup is created.

I do understand lines three and four they cut off the rest of the filename so just the needed part with the number is left. What I don't get is how the first two lines are working. I read the manpage of set but now I'm confused more then I had been before. What does the command set -- filename do? Also what exactly is the option ${!#} for, that is being stored in lastname. I think ${} is to dereference a given parameter, but which parameter does the option !# point to?

Secondly I don't get why I need the command in line 5. As I understand this is used to replace all question marks with an 0 that are stored in the variable backupnr, but why is this done? If there hasn't been an error in the commands before backupnr should be a three digit number, if there has been an error and the content of the variable is corrupt, you can't know what is the actual value of backupnr. So if I want to scan for corruption why don't I have to scan for everything thats not a number, but just for a question mark? Is this maybe somehow related to whats happening in the first two lines?

Answer 1

set is often used to set the positional parameters ($1, $2, $3, etc., which are usually provided as arguments to the script). By using these parameters, we can get a simple way to count things via the $# variable, which stores the number of set positional parameters.

? is a wildcard matching any single character, so ??? matches any combination of three characters.

Combined: set -- ${BACKUPDIR}/backup-???.tgz sets every file named backup-xxx.tgz, where xxx are any three characters, as the parameters.

${!#}, uses variable indirection:

If the first character of parameter is an exclamation point (!), it introduces a level of variable indirection. Bash uses the value of the variable formed from the rest of parameter as the name of the variable; this variable is then expanded and that value is used in the rest of the substitution, rather than the value of parameter itself. This is known as indirect expansion.

Since $#, which is what you get when you remove the !, is the count of parameters (and the number of the last parameter), ${!#} is a simple way to get the last argument.

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As for command 5, what happens when no matching files are present?

$ bash -c 'set -- backup-???.tgz; echo "$@"'
backup-???.tgz

The shell leaves that word alone, so now we have just a literal backup-???.tgz. The author handles that edge case by replacing the ? with 0, to mark the first backup file.