Day 4: grep, less and wc

04_librecatprojectYesterday we gave you a little introduction into UNIX programming. Today we will show you how many words there are in Tolstoy’s War and Peace. First, as always, you need to startup your Virtual Catmandu (hint: see our day 1 tutorial) and open the UNIX prompt (hint: see our day 2 tutorial). As a result you should see this on your screen:



First we are going to learn you a new UNIX command, cat. In our Catmandu project the cat command is our favorite. With this command you can read War and Peace in 2 seconds! Lets try it out, type ‘cat Documents/war_and_peace.txt‘ on the UNIX prompt and press ‘enter’.

$ cat Documents/war_and_peace.txt

“What was that?!” you might wonder? Well, that was the complete War and Peace running across your screen. We provided cat command with one argument ‘Documents/war_and_peace.txt’ which is the filename that contains the complete text of War and Peace (this text I downloaded from the Gutenberg Project for you).

In UNIX it is possible to glue the output from one command to the input of another command this is called a pipe. To use a pipe in a command you have to find a funny little key on your computer which has this sign ‘|’. On my computer it looks like this:



With this pipe symbol ‘|’ you can glue the output of one command to another command. We will use a pipe to count the number of lines, words and characters in a file with the UNIX wc command like this:

$ cat Documents/war_and_peace.txt | wc
64620 563290 3272023

The output contains three numbers: 64620 , 563290 and 327203. The first number counts the number of lines in the file Documents/war_and_peace.txt. The second number counts the number of words in Documents/war_and_peace.txt. And the third number counts the number of characters in Documents/war_and_peace.txt.

Five hundred sixty-three thousand two hundred and ninety words counted with one simple command! This is the power of command line processing.

The file Documents/war_and_peace.txt contains the English translation of War and Peace. We can count the number of times the word ‘war’ is mentioned in this novel. You need to use a new UNIX command, grep,  to do this trick. Type the following commands at the UNIX prompt and I will explain in a moment what happens.

$ cat  Documents/war_and_peace.txt | grep -ow war | wc
274     274    1096

We count 274 occurrences of the word ‘war’ in War and Peace. With the cat command we read the document Documents/war_and_peace.txt. With the pipe ‘|’ symbol we send all the text of this document to the grep command, where we use the -ow option to search for the word ‘war’. With the pipe ‘|’ symbol we send all the ‘war’ words to the wc command which will count the number of ‘war’-s.

You can experiment with these commands to better understand what happens. If you type:

$ cat  Documents/war_and_peace.txt

, then you will see the complete War and Peace in the output. When you type:

$ cat  Documents/war_and_peace.txt | grep -ow war

, then you will see many lines containing ‘war’, ‘war’, ‘war’ (one line for every occurence of the word ‘war’ in War and Peace). When you type:

$ cat  Documents/war_and_peace.txt | grep -ow war | wc

, then you will count all these ‘war’ lines. Pretty neat, eh?

What about ‘war’ at the beginning of the sentence like ‘War’ or someone shouting ‘WAR!’? To have a correct count we need to be case-insensitive. You can go this by adding the -i option to the grep command like this:

$ cat  Documents/war_and_peace.txt | grep -i -ow war | wc
297     297    1188

We get 23 more occurences.

Now we can try to find out if War and Peace is more about ‘war’ than ‘peace’ by counting the number of times ‘peace’ is mentioned:

$ cat  Documents/war_and_peace.txt | grep -i -ow peace | wc
110     110     660

This proves ‘peace’ is mentioned only 110 times and ‘war’ 297 times!

To finish this tutorial I will learn you one more UNIX command: less. When experimenting with commands like cat and grep you might want to inspect intermediary results. With the less command you page through output. Lets try this out. Type:

$ cat  Documents/war_and_peace.txt | less

Your screen will now show the first page of War and Peace. When you press the spacebar the next page will be displayed. Pressing spacebar again you see again a next page, etc etc. This way you can slowly page through long result lists. When you press the ‘b’ key you will go one page back. To exit this less command press the ‘q’ key.

Some more examples of what we have learned.

Show all the lines which contain the word Bolkonski

$ cat  Documents/war_and_peace.txt | grep Bolkonski

Or, use ‘less’ to page through the results:

$ cat  Documents/war_and_peace.txt | grep Bolkonski | less

Count all the lines which contain the word Bolkonski

$ cat  Documents/war_and_peace.txt | grep Bolkonski | wc
178 1848 11705

The answer is 178 lines.

Count all the number of times Napeleon is mentioned

$ cat  Documents/war_and_peace.txt | grep -ow Napoleon | wc
580 580 5220

This answer is 580 times.

Continue to Day 5: Editing text with nano >>


One comment

  1. Pingback: Day 3: Bash basics | LibreCat

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