Difference between revisions of "Linux/C3/The-grep-command/English"

grep command. We will do this with the help of an examples.

Ubuntu Linux 12.04 Operating System and

GNU BASH version 4.2.24

Please note, GNU bash version 4 or above is recommended to practice this tutorial.

You should know Basics of linux terminal

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Regular expressions are pattern matching techniques.

When we have to find out whether a pattern exist in a line, paragraph or a file.

For ex. If you want to search a phone number in the telephone directory

Or

To find a keyword in a paragraph or a line, we use grep command Let us move on to grep.

If filename is not mentioned grep search for the patterns in the standard input.

If filename is missing, grep searches for the patterns in the standard input.

“grepdemo.txt” [enter]

Let us see the content of the file

This is a file that has 13 entries.

Each entry has 6 fields roll, name, stream, marks, and stipend amount.

The fields are separated by a bar, which is called a delimiter.

grep “computers” grepdemo.txt [enter]

At the prompt, type:

cat grepdemo.txt

Say we want to use grep command to see who are the students in the computers stream. For this we have to open the terminal. So press CTRL + ALT and T keys simultaneously on your keyboard.

Now type on the terminal:

grep space (within double quotes) computers space grepdemo.txt

This would enlist those entries where stream is computers

Now Compare the result with the original file. Come back to our text editor.

We see that the entry for Zubin is not enlisted.

Why this is so?

This is because grep searched for the pattern “computers” with small c

While for Zubin the stream is “Computers” with a capital C.

“grep -i “computers” grepdemo.txt” [enter]

To make it case insensitive, we need to use the minus i option with grep.

Type:

grep space (minus) i space (within double quotes) “computers” after the double quotes space grepdemo.txt

Press Enter.

This will now enlist all the four entries.

“grep -iv “pass” grepdemo.txt” [enter]

We may do the reverse.

It is possible to make grep enlist only those lines that do not match the pattern.

For that we have the minus v option.

Say we want to enlist those entries of students who have not passed.

“grep -iv “pass” grepdemo.txt > notpass.txt” [enter]

For this Type:

grep space minus iv space in double quotes pass space grepdemo.txt space greater than sign space notpass.txt

Press Enter.

“cat notpass.txt”

cat space notpass.txt

press Enter The output is displayed.

“grep -i “fail” grepdemo.txt” [enter]

At the prompt type:

grep space minus i space within double quotes fail after the double quotes space grepdemo.txt

Press Enter.

This is different.

This will include those students who are failed but their result is incomplete.

“grep -in “fail” grepdemo.txt” [enter]

We have the minus n option

The patterns so far have been of single word.

At the prompt type

"grep space -in space within double quote "fail" after the double quotes space grepdemo.txt Press Enter

The line number is displayed.

“grep -i “ankit saraf” grepdemo.txt” [enter]

The patterns so far have been of single word. We may have multiword patterns as well.

But the entire pattern must be within quotes.

So Type:

grep space minus i space in double quotes ankit space saraf space grepdemo.txt

Press Enter

We see that ankit saraf's record is displayed.

“grep -i “fail” grepdemo.txt notpass.txt” [enter]

Type:

grep space minus i space in double quotes fail space grepdemo.txt space notpass.txt

Press Enter

The output is displayed.

With multiple files grep will write the name of the file in which the entry was found.

grepdemo.txt and notpass.txt

These are the records from notpass.txt file and these are the records from grepdemo.txt file.

“grep -c “Fail” grepdemo.txt” [enter]

For that we have the minus c option

Type:

grep space minus c space in double quotes Fail with a capital F space grepdemo.txt

Press Enter

This will give us the count of number of lines matched.

This brings us to the end of this tutorial.

Let us summarize.

Summary

To see the content of a file

eg. cat filename

To list the entries of a particular stream

eg. grep “computers” grepdemo.txt

To ignore cases

eg. grep -i “computers” grepdemo.txt

Lines that do not match the pattern

eg. grep -iv “pass” grepdemo.txt

To list the line numbers with the entries

eg. grep -in “fail” grepdemo.txt

To store the result in another file

eg. grep -iv “pass” grepdemo.txt > notpass.txt

To know the count

eg. grep -c “Fail” grepdemo.txt

Explore some other commands like -E, + and ?

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