What Is Here Document?

Here Document enables you to "embed" some text chunks inside your scripts, without needing to worry about escaping, line breaks, concatenations etc. It is especially useful when you want to keep some configuration file chunks inside your script file or when you want to write a multiple line text chunk on command line.

Here Doc used on command line:

cat <<END
some text
another new line with variable ${PATH}
the third line 
END

Here Doc used inside script file, write Apache configuration file chunks:

#!/usr/bin/env bash

cat > site.conf << 'END'
<VirtualHost *:80>
    ServerAdmin admin@test.com
    ServerName test.com
    ServerAlias www.test.com
    DocumentRoot /var/www/test.com/public_html
    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined
</VirtualHost>
END

But you must understand how does it work behind the scene, though it looks like a long paragraph of text, Here Doc is passed as a temporary file/stream literal, not a string literal, so when using Here Doc, choose tools and utilities that works with files or streams(like cat, wc, less), those who expecting string(like echo) will not work!

An error seen many times: use echo with Here Doc

# Works, cat works with files
cat <<EOL
cat works with file
so it is find
EOL

# Will not work, echo want a string
echo <<EOL
echo want a string
so it will not work
EOL

By default, Here Doc support variable interpolation, if you don't want this, add single quote to the opening Mark, Here Doc without variable interpolation is also called Now Document.

# Will expand $PATH variable
cat <<EOL
You Path is:
    ${PATH}
EOL

# Will not expand $PATH variable
cat <<'EOL'
You Path is:
    ${PATH}
EOL

What Is Here String?

Here String is something similar(still a stream/file literal, not string literal), but with simpler and different syntax:

read a b c <<< "A B C"
echo $a $b $c

echo "A B C" | read a b c
echo $a $b $c

The first code snippet will print the contents as expected, a very good example of how here string could be useful. The second code snippet will print a blank line, actually the contents are read into $a $b $c, but the read builtin is after a | operator, so it is in a sub-process, when we come back and try to print, those values are already gone with the sub-process.

Compared with here document, the obvious difference is the lackinig of delimiters in here strings.

Here Document In Other Languages

Languages like PHP, Perl support Here Doc and Now Doc that is kinda compatible with Bash.

Languages like C++ has raw string literals which serves similar purpose:

const char *p = R"(
a
b
c
)";

Languages like Javascript also has string template:

var name = 'jack';
console.log(`
    Hi, my name is: ${name}!
    Nice to meet you!
`)

Tags: Bash Command Line