Dirname

dirname is a standard UNIX computer program. When dirname is given a pathname, it will delete any suffix beginning with the last slash ('/') character and return the result. dirname is described in the Single UNIX Specification and is primarily used in shell scripts.

The Single UNIX Specification for dirname is.

dirname will retrieve the directory-path name from a pathname ignoring any trailing slashes

Since dirname accepts only one operand, its usage within the inner loop of shell scripts can be detrimental to performance. Consider

The above excerpt would cause a separate process invocation for each line of input. For this reason, shell substitution is typically used instead

or if relative pathnames need to be handled as well

Note that these handle trailing slashes differently than dirname.

...
Wikipedia