Bash parameter expansion

Have you ever seen ## (hash hash/pound pound) or %% (percent percent) inside a bash script and wondered what it means? It's a form of parameter expansion that allows you to manipulate your strings using regexes.

All the following commands work with a variable called $string.
'pattern' means bash pattern, or can also be an ordinary string:

• String length: ${#string}
• Extract a substring: ${string:position}
• Extract a substring, specifying length: ${string:position:length}
• Delete shortest match of substring from the beginning of string: ${string#substring}
• Delete shortest match of substring from the end of string: ${string%substring}
• Delete longest match of substring from the beginning of string: ${string##substring}
• Delete longest match of substring from the end of string: ${string%%substring}
• Find and replace first substring: ${string/pattern/replacement}
• Find and replace all substrings: ${string//pattern/replacement}

Thanks to TheGeekStuff