The value that you are assigning to directoryname is dir /s c:\abc.exe.
this value is then substituted for %directoryname% in your cmd line, which executes the command dir/s..., showing you the location(s) of abc.exe in the familiar dir format.
If what you want is just the directoryname in directoryname, then you need
for /f "delims=" %%a in ('dir /s /B /a-d "c:\abc.exe"') do set "directoryname=%%~dpa"
which will first execute the dir command, then process each line of output from that command and assign it in its entirety to %%a.
The dir command shown would "display" the matching names found in the nominated directory (c:\) and its subdirectories (/s) in basic form (/b) - that is, names only, no size or date or report-headers or report-footers, and a-d without directorynames (should they match the "mask" abc.exe)
The delims= option to the for /f command instructs that the entire line as output by the command in single-quotes, be assigned to %%a.
When the result is assigned to the variable directoryname, only the Drive and Path parts are selected by using the ~dp prefix the the a.
Note that only the very last name found will be assigned to the variable as any earlier assignment will be overwritten by a succeeding assignment.