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Home » Bash stdin, stdout, and stderr: A Deep Dive

Bash stdin, stdout, and stderr: A Deep Dive

Standard Input (stdin)

Standard input is a stream where data is sent to a command or program to be processed. By default, stdin is associated with the keyboard, allowing you to enter data manually. However, you can also redirect stdin from a file or another command using the < operator.

For example, consider the wc command, which counts the number of lines, words, and characters in a file or stdin. If you want to count the number of lines in the file file.txt, you can use the following command:

wc -l < file.txt

This would output the number of lines in the file file.txt.

Standard Output (stdout)

Standard output is the stream where a command or program writes its results. By default, stdout is associated with the terminal, allowing you to see the output of a command. Similar to stdin, you can redirect stdout to a file or another command using the > operator.

For example, if you want to save the output of the ls command to a file, you can use the following command:

ls > list.txt

This would create a new file list.txt with the list of files and directories in the current directory.

Standard Error (stderr)

Standard error is a stream where a command or program writes error messages. By default, stderr is also associated with the terminal, allowing you to see the error messages. You can redirect stderr to a file or another command using the 2> operator.

For example, if you want to save the error messages of the cat command to a file, you can use the following command:

cat file.txt 2> error.txt

This would create a new file error.txt with any error messages generated by the cat command.

Surprising Fact

Did you know that you can combine stdout and stderr into a single stream using the &> operator? This can be useful when you want to redirect both streams to a file or another command.

For example, the following command redirects both stdout and stderr to a file named output.txt:

itvraag.nl &> output.txt

Key Takeaways

  • Standard input (stdin) is a stream where data is sent to a command or program for processing.
  • Standard output (stdout) is a stream where a command or program writes its results.
  • Standard error (stderr) is a stream where a command or program writes error messages.
  • You can redirect stdin, stdout, and stderr to a file or another command using the <, >, and 2> operators respectively.
  • You can combine stdout and stderr into a single stream using the &> operator.

Examples and Tips for Improved Productivity

  1. Use the tee command to save stdout to a file while still displaying it in the terminal. For example:
itvraag.nl | tee output.txt
  1. Use the | operator (also known as a “pipe”) to redirect stdout to another command. For example:
itvraag.nl | grep keyword

This would search for the keyword “keyword” in the output of the itvraag.nl command.

  1. Use the xargs command to execute a command for each line of stdin. For example:
echo "file1.txt file2.txt file3.txt" | xargs rm

This would delete the files file1.txt, file2.txt, and file3.txt.

  1. Use the 2>&1 operator to redirect stderr to stdout. For example:
itvraag.nl 2>&1 | grep error

This would search for the keyword “error” in both the stdout and stderr streams of the itvraag.nl command.

  1. Use the > /dev/null 2>&1 operator to suppress stdout and stderr. For example:
itvraag.nl > /dev/null 2>&1

This would execute the itvraag.nl command without displaying any output or error messages.

Challenge

Try using the find command to list all the files in the current directory with the .txt extension, and redirect the output to a file named txt-files.txt. Hint: you can use the -name option to search for a specific pattern.

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