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Exercise 4 - Using pipelines and redirects

You will learn to:

  • Use the output of one program as the input of another program
  • Redirecting output from standard out to a file

📖 The pipeline is an important concept in Bash. Using pipes, we can take the standard output of one program and pass it as standard input to another program.

4.1 - Piping simple commands

Here is a simple example of using a pipe:

ls -l | head -3

This example executes 2 commands. ls -l which lists out the content of our current directory in a readable format. Example from the ./files folder:

$ ls -l
total 48
-rw-r--r--  1 user  staff    26 Aug 18 16:33 config.yaml
-rw-r--r--  1 user  staff    33 Aug 18 16:33 data.json
-rw-r--r--  1 user  staff   445 Aug 18 16:33 file1.txt
-rw-r--r--  1 user  staff   865 Aug 18 16:33 file2.txt
-rw-r--r--  1 user  staff  1001 Aug 22 20:58 got_characters.txt
-rw-r--r--  1 user  staff    43 Aug 18 16:33 startup.yaml

The tail -3 takes some input and returns the 3 last lines on the input. Combining the two commands should give us the following result:

-rw-r--r--  1 user  staff   865 Aug 18 16:33 file2.txt
-rw-r--r--  1 user  staff  1001 Aug 22 20:58 got_characters.txt
-rw-r--r--  1 user  staff    43 Aug 18 16:33 startup.yaml

✏️ Try piping the output into other commands, such as wc -l (counts the number of lines) or grep <searchterm> (text search).

4.2 - More pipes

✏️ Using cat, list out the content of the file /files/got_characters.txt. You should get a list of Game of Thrones characters in your terminal.

✏️ Combine your cat of the file with the sort sort command, in order to output a sorted list of GoT characters.

📖 Oh oh... when we inspect our sorted list of GoT characters, we see that we have some duplicates. E.g. Margaery Tyrell and Tywin Lannister is showing up more than once. Pipe the output of your sorted list of characters into the uniq command to remove any duplicates.

✏️ Using the wc -l command, find our how many unique GoT characters are in the got_characters.txt file.

4.3 - Redirect to file

📖 One of the things we can do in bash, is to redirect the output of a command to a file. We can redirect to a file using the > or >> operator.

  • The > operator will redirect the commands output to the file and remove any content that was already present.
  • The >> operator will redirect the commands output to the file and append it to any existing content.

✏️ Using the > operator, create a new file consisting of unique and sorted Game of Thrones characters.

Go to exercise 5 ➡️