Shell script multi-line comment

Question

I am having a large shell script file. At times while doing modification I want to comment out part of it. But commenting line as shown in the below example is giving me error.

Script:

#!/bin/bash
<<COMMENT1
read build_label
read build_branch_tag
build_date_tag=$(echo $build_label | sed "s/$build_branch_tag//g")
echo $build_path
COMMENT1
echo "HELLO WORLD"

Error Message:

sed: first RE may not be empty

I just want to understand what's wrong with the above script and why comment section is not working properly.

Answer 1

First, using here docs to comment code is really dirty! Use the # instead. If you want to comment multiple lines, use your editor. In vim (commenting lines from 10 to 15 for example):

:10,15s/^/#

However, to solve your current problem you need to enclose the starting here-doc delimiter in single quotes, like this:

<<'COMMENT'
...
COMMENT

Using single quotes you tell bash that it should not attempt to expand variables or expression inside the here doc body.

Answer 2

Traditional UNIX shell doesn't have multiline comment support. What you're doing here is using a so-called "HERE document" without using its value, a common hack to get multiline comment like behaviour.

However, patterns inside the the HERE document are still evaluated, which means that your $(…) is executed. But since build_branch_tag has not been defined before, it will evaluate to an empty string, and the shell will thus execute sed s///g.

You can use a different hack:

: '
Bla bla, no $expansion is taking place here.
'

What this is doing: the : is a no-op command, it simply does nothing. And you're passing it an argument which is a string '…'. Inside the single quotes, no expansion/evaluation is taking place. Beware of ' inside the "commented out" region, though.