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Info

  • Redirection of output: > vs >>

    • A redirect sends a channel of output to a file.

    • A channel refers to standard input/output as well as standard error (not covered here).

    • You can redirect a file as input to a command using < and << (not covered here).

  • Using pipe “|’

    • A pipe passes standard output as the standard input to another command

  • Examples of the form: 

    • View a text file and pipe to the grep command to filter lines by looking for text.

    • Cat a list and sort by line.

    • Sort and then find unique items.

    • View folder contents and look for a specifically named namefilename.

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Redirection of output: > vs >>

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Info

Over the next set of examples we will pipe (using the “|” character) together a series of commands that will “generate a file that contains a sorted list of unique terms that include the sub string berry”.

Code Block
[intro_to_linux]$ cat fruits.txt
Gooseberry
Apple
Apricot
Avocado
Strawberry
...
[intro_to_linux]$ cat fruits.txt | wc -l
97
Info

We could use the wc command as it is normally intended, and confirm the file contains 97 lines.

Code Block
[intro_to_linux]$ wc -l fruits.txt
97 fruits.txt

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Example continued

Info

First step, find all the terms that contain the sub string berry.

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Code Block
[intro_to_linux]$ cat fruits.txt | grep berry | sort | uniq > berries.txt
[intro_to_linux]$ cat berries.txt
Bilberry
Blackberry
Blueberry
Boysenberry
Cloudberry
Cranberry
Elderberry
Goji berry
Gooseberry
Honeyberry
Huckleberry
Juniper berry
Marionberry
Mulberry
Raspberry
Salal berry
Salmonberry
Strawberry

[intro_to_linux]$ cat berries.txt | wc -l
18
Info

  • Although a simple example, it demonstrates the principle of piping four commands into a single call.

  • Without using the pipe command you would have had to create intermediate files, to store results, after each command.

  • As you get more confident, you can create more elaborate pipelines of commands.

Note

You might also be presented with a long pipeline/list of commands separated by the | character.

Now you know how to separate this pipeline into individual steps, dissecting by the | character, and then running each step individually.

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Example: Pipe from ls command

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Note

Make sure you are use the commands and options as you intend and understand and be are able to describe and explain/justify.

Notice the difference between:

Code Block
[salexan5@mblog1 intro_to_linux]$ cat vegatables.txt | grep beans | sort
...
kidney beans
...
soy beans

and

Code Block
[salexan5@mblog1 intro_to_linux]$ cat vegatables.txt | grep -i beans | sort
...
kidney beans
kidney BEANS
...
soy beans
Soy Beans

  • Should “soy beans” and “Soy Beans” be treated the same or different? You need to decide with respect to the context and use case you are following, and intending.

  • What happens if you use bean instead of beans?

  • What does the uniq -i option do?

Note

AgainNote: Explore and understand how commands work, and the order they are run in. For example, in your own time, understand if there is a difference between sort | uniq versus uniq | sort?

Expand
titleBrief Explanation:
Info

The uniq command isn’t able to detect the duplicate lines unless they are adjacent to each other.

That is why we sort first.

Try it and see…

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