Update file UNIX_HPC_cheat_sheet.md

exercises/UNIX_HPC_cheat_sheet.md

@@ -44,6 +44,7 @@ wc foo.txt             # List number of lines, words and characters in the file
 wc -l foo.txt          # Counts the lines
 cut -d [delimiter] -f [field] foo.txt     # Cuts out the specified fields that are separated by the specified delimiter out of each line of a file
 touch foo.txt           # Create empty file if it does not exist or change "modified by" date of existing file    
+grep pattern foo.txt    # search for pattern in file
 ```
 
 ## Input and Output