Explaining heredocs in Bash — combined with stream redirection.

The aim of this page📝 is to explain initial confusion. Misunderstandings, when learning about heredocs simultaneously with stream redirection. Let’s look at a simple example of using a here document with the cat command to write to a file.

cat > remrem.md << EOT
hello
world
EOT

This command creates a file named remrem.md with the contents hello and world written on separate lines.

• Stream redirection operators in shell scripting, such as <, >, >>, and |, are used to manipulate input and output streams.
• Stream redirection operators are executed in the left-to-right order.
• The << operator, used in the example, is not considered a stream redirection operator; it is used to create a here document.
• More on heredocs: https://medium.com/p/b64337f87bfc
• More on redirection: https://medium.com/p/7e77ccb39a3f
• In the provided example, the << EOT here document marker indicates the start of the here document.
• The lines hello and world following the here document marker are the input for the cat command.
• The output of the cat command, including the input lines, is redirected to the file remrem.md using the > operator.
• The here document redirection (<< EOT) takes precedence over the output redirection (> remrem.md) in this example.
• Placing the here document marker at the rightmost part of the command can be counterintuitive for those (like me) expecting a strict left-to-right structure.
• The here document structure helps provide multiple lines of input to a command in a clear and readable manner.

ANKI

Question:
What does the `<<` operator do in shell scripting?

Answer:
The `<<` operator is used to create a here document in shell scripting.

Question:
What does the provided command `cat > remrem.md << EOT` achieve?

Answer:
This command creates a file named `remrem.md` with the contents `hello` and `world` written on separate lines.

Question:
Are here documents considered stream redirection operators in shell scripting?

Answer:
No, the `<<` operator used for here documents is not considered a stream redirection operator.

Question:
What is the order of processing stream redirection operators in shell scripting?

Answer:
Stream redirection operators are processed from left to right in shell scripting.

Question:
Why might the placement of the here document marker at the rightmost part of the command be counterintuitive?

Answer:
It can be counterintuitive for those expecting a strict left-to-right structure in command execution.

Question:
How can the here document structure help in shell scripting?

Answer:
The here document structure helps provide multiple lines of input to a command in a clear and readable manner.

Question:
What does the `>` operator do in shell scripting?

Answer:
The `>` operator is used for output redirection in shell scripting.

Question:
What does the `cat` command do in the provided example?

Answer:
The `cat` command reads input lines from the here document and writes them to the file `remrem.md`.

Question:
Why is understanding the order of stream redirection operators and here documents important in shell scripting?

Answer:
Understanding the order is essential for effectively manipulating input and output streams in shell scripts.

Question:
How can practicing with examples like the one provided enhance comprehension of shell scripting concepts?

Answer:
Practicing with examples can improve understanding of stream redirection and here documents in shell scripting.