To search for files based on their contents, you can use the grep
program. For example, to find out which C source files in the current
directory contain the string thing, you can do:
grep -l thing *.[ch]
If you also want to search for the string in files in subdirectories,
you can combine grep with find and xargs, like
this:
find . -name '*.[ch]' | xargs grep -l thing
The -l option causes grep to print only the names of files
that contain the string, rather than the lines that contain it. The
string argument (thing) is actually a regular expression, so it
can contain metacharacters. This method can be refined a little by
using the -r option to make xargs not run grep if
find produces no output, and using the find action
-print0 and the xargs option -0 to avoid
misinterpreting files whose names contain spaces:
find . -name '*.[ch]' -print0 | xargs -r -0 grep -l thing
For a fuller treatment of finding files whose contents match a pattern,
see the manual page for grep.