How is the match evaluated in parameter expansion in bash?

Question

I read "Linux Bible 10th Edition", chapter 7: Writing Simple Shell Scripts, at 149 page, "Parameter expansion in bash" paragraph.

• ${var:-value} — If variable is unset or empty, expand this to value.
• ${var#pattern} — Chop the shortest match for pattern from the front of var’s value.
• ${var##pattern} - Chop the longest match for pattern from the front of var’s value.
• ${var%pattern} — Chop the shortest match for pattern from the end of var’s value.
• ${var%%pattern} - Chop the longest match for pattern from the end of var’s value.

Please, explain to me, how the longest/shortest match is calculated? From right to the left or vice versa?

Also, explain please, how code below is working and how output is calculated

MYFILENAME=”/home/digby/myfile.txt”—Sets the value of MYFILENAME 
FILE=${MYFILENAME##*/}—FILE becomes "myfile.txt" 
DIR=${MYFILENAME%/*}—DIR becomes "/home/digby" 
NAME=${FILE%.*}—NAME becomes "myfile" 
EXTENSION=${FILE##*.}—EXTENSION becomes "txt"

Thank you.

Answer 1

Let's start with the "#" case which chops from the front. Consider a string such as myfile.tar.gz. If you match this against the string *. (which means "zero or more characters" followed by a full stop) then there are two possible matches: myfile. and myfile.tar.. The shortest match is:

${MYFILENAME#*.} # ie myfile.

so the result is what is left: tar.gz. The longest match is:

${MYFILENAME##*.} # ie myfile.tar.

so the result is what is left: gz.

Now consider the "%" case. Here matching is on the right of the string. This time match with .*. We are looking for a dot with some (or none) characters after it. The shortest match is .gz and the longest .tar.gz so we get:

${MYFILENAME%.*}  # => myfile.tar
${MYFILENAME%%.*} # => myfile

Your other examples are variations on a theme to illustrate this:

MYFILENAME=”/home/digby/myfile.txt”—Sets the value of MYFILENAME 
FILE=${MYFILENAME##*/}—FILE becomes "myfile.txt" 
DIR=${MYFILENAME%/*}—DIR becomes "/home/digby" 
NAME=${FILE%.*}—NAME becomes "myfile" 
EXTENSION=${FILE##*.}—EXTENSION becomes "txt"

interpreted:

1. The longest string matching */ from the front is /home/digby/
2. The shortest string matching /* at the end is /myfile.txt
3. The shortest string matching .* at the end is .txt
4. The longest string matching "*.at the front ismyfile.`