Se eu entendi sua pergunta corretamente, você quer substituir o início do caminho. Você pode fazer assim:
file='/Volumes/temp/folder/file.txt'
echo ${file/#\/Volumes/cifs://servername}
Isso é descrito em man bash
:
${parameter/pattern/string}
Pattern substitution. The pattern is expanded to produce a pat‐
tern just as in pathname expansion. Parameter is expanded and
the longest match of pattern against its value is replaced with
string. If pattern begins with /, all matches of pattern are
replaced with string. Normally only the first match is
replaced. If pattern begins with #, it must match at the begin‐
ning of the expanded value of parameter. If pattern begins with
%, it must match at the end of the expanded value of parameter.
[...]