grep finds a string in a specified list of file(s).
grep [-in] expression [filename...]where expression is a literal which may contain metacharacters (in which case, the literal must be in quotes), and filename... may be one or more files.
Some useful options are:
For example, to find, in every file ending with .dat, every occurrence of Silicon Graphics, use
grep -i "Silicon Graphics" *.dat
Or, to find the string Claude Cantin in files personnel and payroll.list, use
grep "Claude Cantin" personnel payroll.list
Finally, to find all occurences of prog2 to prog9 in all the files of the directory, use
grep "prog[2-9]" *