Lesson 22: Combining commands

One of the most powerful features of Linux is that you can send the output from one command or program to any other command (as long as the other command accepts input of some sort). We do this by using a pipe character, represented as: |. The chain of commands is commonly known as a pipeline.

[learner planets]$ grep surface earth.txt | wc
      1       9      50

The lines within 'earth.txt' that contain the word 'surface' (filtered by grep) are passed into the pipeline to the word count (wc) program to display the line, word and byte counts of the input parameter (retrieved from the pipeline).

Sorting Numerical Lists

Firstly, let's create the directory ~/learning_linux/number and change to the 'number' directory. Now copy the file /opt/guided/numbers.txt file to the current directory.

Try running the sort command on the 'numbers.txt' file - is the output what you expect?

[learner number]$ sort numbers.txt

Without any switches, sort arranges the input by digit, then alphabetically, hence why 112 is listed before 14 (the second digit in 112 is smaller than the second digit in 14). Try running the sort command again with the -n switch. Use the manual page to discover what that switch does. To display unique entries in 'numbers.txt', pipe the output of the previous command into uniq. Please note that for uniq to work, the file contents must first be sorted because adjacent identical lines are collapsed into one. Running uniq before sort -n will yield unwanted results!

Note

Whenever chaining many piped commands together, test each step as you build it!