grep

• The grep command is used to search for strings inside of files.
• The string it searches for can also be a regular expression.
• Any line in the file that matches the search string will be printed.

• The basic syntax for calling grep is:

   grep regex file

• It can take multiple files as input too; just add the filenames to the end. It will print each line of all the files that match.

• If you do not pass in a file name, it gets its input from standard input. This is useful for piping things to grep.

• Some useful options (there are more):

      -i   ignore case
      -l   just list the files that have lines that match
      -v   prints the lines that do not match the search string
  

• Here are some examples

  grep '^[ 	]*$' file1   # prints all the lines that are, or look blank
             	             # (that's a space and a tab inside the square
             	             # brackets).
  
  grep -i unix file1       # prints all the lines in file1 that have the 
                           # word unix, in any case word (i.e. unix or
                           # UNIX or uNIx, ...).
  
  grep -l -v unix *        # prints the names of the files in the current
                           # directory that do not have the word 
                           # unix in them.
  

• What will the following do?

  
  cut -f2,4 -d: /etc/passwd | grep -i cmu
  
  grep -i cmu /etc/passwd | cut -f2,4 -d: 
  
  grep '^[    ]*$' file1  | wc -l     # (that's a space and a tab inside the square
             	                    # brackets).
  
  

• Remember that regular expressons are different than file-matching metacharacters!