Redirect input and output using `>`, `>>`, and `|`

[jbeadling]

Summary

Learn how to redirect input and output in the shell using the >, >>, and | operators. Mastering these techniques allows you to manipulate how data flows between commands, files, and other I/O streams.

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Description

• Objective: Get a firm grasp on the input/output redirection concept in the command line and become proficient in using >,``>>, and | for various tasks.

• Scope:

  • Understanding I/O redirection
  • How to use > for overwriting
  • How to use >> for appending
  • How to use | for piping

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Learning Tasks

1. Understanding I/O Redirection:

   • Learn what I/O redirection is and why it is essential for efficient shell scripting and command-line use.

2. How to Use > for Overwriting:

   • Understand how to use the > operator to redirect output from a command to a file, overwriting the file if it already exists.

3. How to Use >> for Appending:

   • Learn how to use the >> operator to append output from a command to an existing file.

4. How to Use | for Piping:

   • Learn how to use the pipe (|) operator to send the output of one command as the input to another.

5. Hands-on Practice:

   • Exercise 1: Use the > operator to redirect the output of ls to a file, thereby saving the listing of a directory to that file.
   • Exercise 2: Utilize the >> operator to append the output of another ls command to the same file.
   • Exercise 3: Employ the | operator to pipe the output of ls into grep to filter the results.
   • Exercise 4: Combine multiple redirection operators in a single command line to perform complex tasks.

6. Troubleshooting:

   • Go through common pitfalls and problems that might occur while redirecting I/O and how to solve them.

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Learning Goals

• Acquire a strong understanding of I/O redirection and its practical use cases.
• Learn how to effectively use >, >>, and | to manipulate I/O streams.
• Gain hands-on experience using I/O redirection for file creation, modification, and data filtering tasks.

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Priority

• Medium

[jbeadling]

There are 3 redirects in unix systems:

• Standard input (stdin) and it has file descriptor number as 0
• Standard output(stdout) and it has file descriptor number as 1
• Standard error(stderr) and it has file descriptor number as 2

By default when running a command its output goes to the terminal. The output of a command can be routed to a file using the > symbol.

In the below screenshot we redirect the output of ls to a file name directory.txt using stdin and > as shown below:

You can use >> to direct output and append any file you direct it too. Using > will replace the text with whatever is redirected but using >> will just append it. As shown below:

Below we employ the | to redirect the output of ls into a grep command

And finally we take a txt file grep the results to show only files that end in .txt and then append that do a different file: