On a linux box, there is a simple way to search for some text inside a file using regular expressions in the command line
So, the short answer, in Linux, you can simply execute this line
grep -R -B3 -A4 "MYTEXT" /var/ > /root/findingres.txt
this will show you 3 lines before the text and 4 lines after the string and the file name, it will search in the /var/ folder, and will store the searching results to /root/findingres.txt
The above is a simple example, you can use regular expressions to find more complicated stuff
Also, if you want to search the entier hard drive for a string, and you want to search only in one type of file that you want to specify using a file extension you can use the following
grep -R --include=*.txt "MYTEXT" /etc/
Now, if you want to search for a file by file name,
find / -type f -name "myfile.txt"
would look for an exact file name, if you are looking to find a file using wildcards, for example, any file that ends in a certain extension you would
find / -type f -iname "*.psd"
If you want to include directories in your search, remove the -type flag