# Heredoc
A Here document (Heredoc) is a type of redirection that allows you to pass multiple lines of input to a command.
The syntax of writing HereDoc takes the following form:
[COMMAND] <<[-] 'DELIMITER'
HERE-DOCUMENT
DELIMITER
- The first line starts with an optional command followed by the special redirection operator
<<and the delimiting identifier. - You can use any string as a delimiting identifier, the most commonly used are EOF or END.
- If the delimiting identifier is unquoted, the shell will substitute all variables, commands and special characters before passing the here-document lines to the command.
- Appending a minus sign to the redirection operator <<-, will cause all leading tab characters to be ignored. This allows you to use indentation when writing here-documents in shell scripts. Leading whitespace characters are not allowed, only tab.
See more here (opens new window)
# Examples
Heredoc is most often used with the cat command.
An example with environment variables:
cat << EOF
The current working directory is: $PWD
You are logged in as: $(whoami)
EOF
Output:
The current working directory is: /home/john
You are logged in as: john
+++
What if we enclose the delimeter with quotes?
cat << "EOF"
The current working directory is: $PWD
You are logged in as: $(whoami)
EOF
Output:
The current working directory is: $PWD
You are logged in as: $(whoami)
Notice that no paramter expansion and command substitution is done by the shell
+++
Instead of displaying the output, we can redirect to file with > or >> operators:
cat << EOF > file.txt
The current working directory is: $PWD
You are logged in as: $(whoami)
EOF
> overwrites the file whilst >> appends to file.
Posted on: Thu, 03 Jun 2021 14:37:56 GMT