CODEWITHSUNDEEP
shellcommentsmultiline
Enes Malik Turhan
Enes Malik Turhan

Reputation: 4882

Way to create multiline comments in Bash?

I have recently started studying shell script and I'd like to be able to comment out a set of lines in a shell script. I mean like it is in case of C/Java :

/* comment1
   comment2 
   comment3
*/

How could I do that?

Upvotes: 478

Views: 443759

Answers (13)

asmeurer
asmeurer

Reputation: 91460

This is perhaps a bit of a non-answer, but the idiomatic way to handle multiline comments in Bash is to comment each line individually with the comment character #. For example:

# This is a multiline comment
# 
# It spans multiple lines, but each line 
# has its own comment character.

Any modern text editor will support selecting multiple lines at once and commenting them all out with a keyboard shortcut, and a good text editor will support reflowing the text automatically (for example, in emacs, this can be done with M-q).

It's best to use the idiomatic method with normal comment characters. It is what people will expect to see, it will avoid bugs, and text editors will actually syntax highlight the text as a comment (notice how the above example is colored gray by StackOverflow's Markdown, because it's actually colored as a comment).

Upvotes: 2

Just Saying
Just Saying

Reputation: 99

In plain bash, to comment out a block of code, I do:

:||{
block
of code
}

Upvotes: 9

xgqfrms
xgqfrms

Reputation: 12136

Here Documents

<<EOF & EOF solution

#!/usr/bin/env bash

echo "just use the \`EOF\` for multi-line comments in your shell script. 🚀"

<<EOF
  comment1
  comment2 
  comment3
  # ...
EOF

By the way: you can also rename EOF to other unique name, such SHELL_MULTI_LINE_COMMENTS.

#!/usr/bin/env bash

echo "just use the \`SHELL_MULTI_LINE_COMMENTS\` for multi-line comments in your shell script. 🚀"

<<SHELL_MULTI_LINE_COMMENTS
  comment1
  comment2 
  comment3
  # ...
SHELL_MULTI_LINE_COMMENTS

test ✅


#!/usr/bin/env bash

echo "just use the \`EOF\` for multi-line comments in your shell script. 🚀"

<<EOF
  comment1
  comment2
  comment3
  # ...
EOF

echo "just use the \`SHELL_MULTI_LINE_COMMENTS\` for multi-line comments in your shell script. 🚀"

<<SHELL_MULTI_LINE_COMMENTS
  comment1
  comment2
  comment3
  # ...
SHELL_MULTI_LINE_COMMENTS


$ chmod +x ./multi-line-comments.sh

$ ./multi-line-comments.sh
just use the `EOF` for multi-line comments in your shell script. 🚀
just use the `SHELL_MULTI_LINE_COMMENTS` for multi-line comments in your shell script. 🚀

screenshots

enter image description here

fefs

https://en.wikipedia.org/wiki/Here_document

https://tldp.org/LDP/abs/html/here-docs.html

Upvotes: 7

Daniel YC Lin
Daniel YC Lin

Reputation: 15992

After read all solutions prior to 2023.03.25, I use this solution. The benefit of it:

  1. Support nested comment.
  2. Support comment out some multi-line comment by prefix with #
  3. Readable.
  4. Allow single quote in comment.
  5. Pass shellcheck

The cons:

  1. The last command exit code will be reset as zero before comment.
  2. vim color syntax still can not detect it correctly.
  3. longer(but readable).

Here is the example of "disable COMMENT1" and "enable COMMENT2".

#!/bin/bash

echo "line 1"
#: <<'#END_COMMENT1'
echo "line 2"
echo "line 3"
false
: <<'#END_COMMENT2'
echo "line 4"
echo "line 5"
#END_COMMENT2
echo "status code=$?"
#END_COMMENT1
echo "line 6"

output

line 1
line 2
line 3
status code=0
line 6

If you care about the exit code side effect, use the single line comment like

echo "line 1"
echo "line 2"
echo "line 3"
false
#echo "line 4"
#echo "line 5"
echo "status code=$?"
echo "line 6"

output

line 1
line 2
line 3
status code=1
line 6

Upvotes: 3

clarke
clarke

Reputation: 31

Not even if you use the inner apostrophe, this method does not allow the following content:

: '
' => &#39;
(') => &#39;
'

The best is the shortest:

<</
' => &#39;
(') => &#39;
\/ or #/
/

Upvotes: 0

Oliver
Oliver

Reputation: 29463

Note: I updated this answer based on comments and other answers, so comments prior to May 22nd 2020 may no longer apply. Also I noticed today that some IDE's like VS Code and PyCharm do not recognize a HEREDOC marker that contains spaces, whereas bash has no problem with it, so I'm updating this answer again.

Bash does not provide a builtin syntax for multi-line comment but there are hacks using existing bash syntax that "happen to work now".

Personally I think the simplest (ie least noisy, least weird, easiest to type, most explicit) is to use a quoted HEREDOC, but make it obvious what you are doing, and use the same HEREDOC marker everywhere:

<<'###BLOCK-COMMENT'
line 1
line 2

line 3
line 4
###BLOCK-COMMENT

Single-quoting the HEREDOC marker avoids some shell parsing side-effects, such as weird subsitutions that would cause crash or output, and even parsing of the marker itself. So the single-quotes give you more freedom on the open-close comment marker.

For example the following uses a triple hash which kind of suggests multi-line comment in bash. This would crash the script if the single quotes were absent. Even if you remove ###, the FOO{} would crash the script (or cause bad substitution to be printed if no set -e) if it weren't for the single quotes:

set -e

<<'###BLOCK-COMMENT'
something something ${FOO{}} something
more comment
###BLOCK-COMMENT

ls

You could of course just use

set -e

<<'###'
something something ${FOO{}} something
more comment
###

ls

but the intent of this is definitely less clear to a reader unfamiliar with this trickery.

Note my original answer used '### BLOCK COMMENT', which is fine if you use vanilla vi/vim but today I noticed that PyCharm and VS Code don't recognize the closing marker if it has spaces.

Nowadays any good editor allows you to press ctrl-/ or similar, to un/comment the selection. Everyone definitely understands this:

# something something ${FOO{}} something
# more comment
# yet another line of comment

although admittedly, this is not nearly as convenient as the block comment above if you want to re-fill your paragraphs.

There are surely other techniques, but there doesn't seem to be a "conventional" way to do it. It would be nice if ###> and ###< could be added to bash to indicate start and end of comment block, seems like it could be pretty straightforward.

Upvotes: 86

Nikhil VJ
Nikhil VJ

Reputation: 6112

I tried the chosen answer, but found when I ran a shell script having it, the whole thing was getting printed to screen (similar to how jupyter notebooks print out everything in '''xx''' quotes) and there was an error message at end. It wasn't doing anything, but: scary. Then I realised while editing it that single-quotes can span multiple lines. So.. lets just assign the block to a variable.

x='
echo "these lines will all become comments."
echo "just make sure you don_t use single-quotes!"

ls -l
date

'

Upvotes: 8

xerostomus
xerostomus

Reputation: 557

Simple solution, not much smart:

Temporarily block a part of a script:

if false; then
    while you respect syntax a bit, please
    do write here (almost) whatever you want.
    but when you are
    done # write
fi

A bit sophisticated version:

time_of_debug=false # Let's set this variable at the beginning of a script

if $time_of_debug; then # in a middle of the script  
    echo I keep this code aside until there is the time of debug!
fi

Upvotes: 6

Vegas
Vegas

Reputation: 8907

Use : ' to open and ' to close.

For example:

: '
This is a
very neat comment
in bash
'

Upvotes: 848

bugi
bugi

Reputation: 153

Here's how I do multiline comments in bash.

This mechanism has two advantages that I appreciate. One is that comments can be nested. The other is that blocks can be enabled by simply commenting out the initiating line.

#!/bin/bash
# : <<'####.block.A'
echo "foo {" 1>&2
fn data1
echo "foo }" 1>&2
: <<'####.block.B'
fn data2 || exit
exit 1
####.block.B
echo "can't happen" 1>&2
####.block.A

In the example above the "B" block is commented out, but the parts of the "A" block that are not the "B" block are not commented out.

Running that example will produce this output:

foo {
./example: line 5: fn: command not found
foo }
can't happen

Upvotes: 5

Imre
Imre

Reputation: 332

what's your opinion on this one?

function giveitauniquename()
{
  so this is a comment
  echo "there's no need to further escape apostrophes/etc if you are commenting your code this way"
  the drawback is it will be stored in memory as a function as long as your script runs unless you explicitly unset it
  only valid-ish bash allowed inside for instance these would not work without the "pound" signs:
  1, for #((
  2, this #wouldn't work either
  function giveitadifferentuniquename()
  {
    echo nestable
  }
}

Upvotes: 7

noamtm
noamtm

Reputation: 12953

After reading the other answers here I came up with the below, which IMHO makes it really clear it's a comment. Especially suitable for in-script usage info:

<< ////

Usage:
This script launches a spaceship to the moon. It's doing so by 
leveraging the power of the Fifth Element, AKA Leeloo.
Will only work if you're Bruce Willis or a relative of Milla Jovovich.

////

As a programmer, the sequence of slashes immediately registers in my brain as a comment (even though slashes are normally used for line comments).

Of course, "////" is just a string; the number of slashes in the prefix and the suffix must be equal.

Upvotes: 51

David Okwii
David Okwii

Reputation: 7830

Multiline comment in bash

: <<'END_COMMENT'
This is a heredoc (<<) redirected to a NOP command (:).
The single quotes around END_COMMENT are important,
because it disables variable resolving and command resolving
within these lines.  Without the single-quotes around END_COMMENT,
the following two $() `` commands would get executed:
$(gibberish command)
`rm -fr mydir`
comment1
comment2 
comment3
END_COMMENT

Upvotes: 230

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