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finding if a file exists and take decisions	
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Ambreesh Khurana posed "Out of 100 available directories, check if
a file x exists in it. If it does, execute a command y, otherwise
execute another command z."

There are, I suspect, at least half a dozen ways, of  solving this
problem.  I will start using "find" because it is the best tool for
file searching. "find" allows a variety of searches, including recursive
searches.

$ ls 
FileExists.txt

$ find . -name FileExists.txt
./FileExists.txt

$ find -name Khurana
                       #returns null string named file does not exist

We are now set

$ if [[ $(find . -name Khurana) ]];then; echo "file found"; else; echo "not found"; fi

Note that the "test" construct has to have one or more space on
either side of  "[[" and one or more space on the left side of "]]".
Sorry, this is Unix arcana.