I just finished reading a fascinating thread about a parameter substitution problem in bash titled "Yet Another Bash Quotes Within Quotes Issue".

I'm new to learning Unix/Linux (and this group) and am working on a simple script. Filing this thread under "Obscure Scripting Features that May Bite You on the Ass" I read through for future reference.

The problem involved getting a script to generate an e-mail message titled "Error from daily_file_backup" to root. This person wanted to use:

MAIL="/bin/mailx -s Error from $(basename $0) root"
/bin/umount /data_backup||{ echo "backup unmount failed"|$MAIL;exit 1;

with quotes around "Error from $(basename $0)" so that $0 would expand into daily_file_backup.

After a lot of back and forth, the solution was to use:

basename ${0/-//}

or "basename ${0/-//}", it isn't clear which.

Then the thread died without an explanation why this would work!!!!!!!!!!!!!!!!!!!!!!!!!

The user was happy, but WTF? I've tried all kinds of bash tutorials, but this type of construction doesn't appear, except that "/" and "//" are used for substring replacement. That's as far as I get.

Rather that posting this to the original thread--which appears to have died quite a few months ago--I'm posting this new thread.

Without the full context of the original, specifically the exact name of the script as contained in $0, it's hard to know exactly what it's supposed to do. But you're right in that it is the substring replacement. Specifically, it replaces the first instance of "-" with "/". Then basename strips the rest of the filepath off to give you just the name of the script itself.

Note also that quotation marks inside subshell constructions like $() are treated as separate from the ones in the rest of the command.

Run your script with sh -x yourscript

It will become obvious what is interpreted in the wrong way. Then you can start working on a solution.

jlinkels

Just got an e-mail reminder on the thread (blush). I appreciate the quick assistance, but I haven't had time to follow through on the suggestions.