Pattern substitution

bash-pattern-subst

$ VAR='XOXOXOX'
$ echo ${VAR/X/.}    ## first occurence only (anywhere)
.OXOXOX
$ echo ${VAR//X/.}   ## all occurences
.O.O.O.

$ VAR='OXOXOX'
$ echo ${VAR/#X/.}   ## first occurence (only if beginning match)
OXOXOX

$ VAR='XOXOXO'
$ echo ${VAR/$X/.}   ## first occurence (only if ending match)
XOXOXO

## Real file example:
# printf-style value substituition
$ file='/some/path'
$ format='cat %f'
$ echo ${format//%f/$file}
cat /some/path