Basics of GREP Command In Unix/Linux

grep stands for : g/re/p (globally search a regular expression and print)

SYNTAX = grep <SWITCH> pattern_list <FILE>

Below  are the switches which we uses regularly to perform day to day activities.

i => Case Insensitive Search
w => Search for particular Word
r => Search in all the files under the current directory and its sub directory
v => Invert the Search.
c => Count, how many lines matches the given pattern/string
n => Shows line number while displaying the output.

Input File: F_Data_File.txt

EMPID|EMPNAME|EMPDEPT|EMPSAL|LOCATION
10001|A1|HR|10000|USA
10002|A2|FIN|20000|USA
10003|A3|NSS|30000|IND
10004|A4|SEC|40000|USA
10005|A5|TECH|50000|IND
10006|A6|TECH|60000|IND
10007|A7|TECH|70000|IND


Examples:

1.  Search word TECH in file F_Input_Data.txt and list all the lines.

      $ grep "tech" F_Input_Data.txt

      Above will not return anything as grep is case sensitive.

      $ grep "TECH" F_Input_Data.txt

      Will return the below output:

      10005|A5|TECH|50000|IND
   10006|A6|TECH|60000|IND
   10007|A7|TECH|70000|IND

if you want to achieve same output using first syntax, you need to add switch -i as given below:

      $ grep -i "tech" F_Input_Data.txt

Switch i will force grep to do the search by ignoring the case. It will return the same output as above.

      10005|A5|TECH|50000|IND
   10006|A6|TECH|60000|IND
   10007|A7|TECH|70000|IND

2.  Search TEC in file F_Input_Data.txt and list all the lines.
               
      $ grep -i "tec" F_Input_Data.txt
               
      Above will return below output as all theses lines are having TEC
               
      10005|A5|TECH|50000|IND
   10006|A6|TECH|60000|IND
   10007|A7|TECH|70000|IND
               
If you want to check whether word TEC is available in the file or not then you need to use -w switch.
               
      $ grep -w "TEC" F_Input_Data.txt
               
It will not return anything as word TEC is not available in the file. hence if you are strictly looking for a word use -w switch.

3.  print all the lines other than lines which are having TECH in it.

     $ grep -v "TEC" F_Input_Data.txt
               
     Will return the below output:
               
      10001|A1|HR|10000|USA
   10002|A2|FIN|20000|USA
   10003|A3|NSS|30000|IND
   10004|A4|SEC|40000|USA
   
Switch -v negate the search pattern, means it will return everything other than the pattern.

4. Search a pattern in all the files under the current directory and its sub directory
    There is a directory "test" in current working directory. this directory also have the     file F_Input_Data.txt.
   
    $ grep -r "TECH"

    Output: -r switch has searched the word TECH in all the files in current directory as           well as in all the files in subdirectory.

  F_Data_File.txt:10005|A5|TECH|50000|IND
  F_Data_File.txt:10006|A6|TECH|60000|IND
  F_Data_File.txt:10007|A7|TECH|70000|IND
  test/F_Data_File.txt:10005|A5|TECH|50000|IND
  test/F_Data_File.txt:10006|A6|TECH|60000|IND
  test/F_Data_File.txt:10007|A7|TECH|70000|IND

5. Count the number of line contains the word TECH in it.

    $ grep -c "TECH" F_Input_Data.txt

    Output: 3

6. Print the line number in front of the line returned by grep as a result of search.
   
   $ grep -n "TECH" F_Input_Data.txt
   
   Output:
    6:10005|A5|TECH|50000|IND
  7:10006|A6|TECH|60000|IND
  8:10007|A7|TECH|70000|IND