# a single first slash means first occurrence
# format=${haystack/needle/replacement}
# if you use a double first slash it means ALL occurrences
# format=${haystack//needle/replacement}
email=${file/.txt-marked/}
In the example above we are searching in the variable $file for ".txt-marked" and replacing with nothing (the empty part after the last / means we are replacing with nothing or actually removing it).
How to replace all occurrences?
email=${file//.txt-marked/}
The only difference is where the first slash goes. If you use double slashes it means to replace ALL occurences.