Commenting out a set of lines in a shell script

Question

I was wondering if there is a way to comment out a set of lines in a shell script. How could I do that? We can use /* */ in other programming languages. This is most useful when I am converting/using/modifying another script and I want to keep the original lines instead of deleting.

It seems a cumbersome job to find and prefix # for all the lines which are not used.

Lets say there are 100 lines in the script in consequent lines which are not to used. I want to comment them all out in one go. Is that possible?

Possible duplicate of [Block Comments in a Shell Script](http://stackoverflow.com/questions/947897/block-comments-in-a-shell-script) — azalea, Dec 15 '16 at 14:21

score 49 · Answer 1 · answered Oct 04 '13 at 13:23

The most versatile and safe method is putting the comment into a void quoted here-document, like this:

<<"COMMENT"
    This long comment text includes ${parameter:=expansion}
    `command substitution` and $((arithmetic++ + --expansion)).
COMMENT

Quoting the COMMENT delimiter above is necessary to prevent parameter expansion, command substitution and arithmetic expansion, which would happen otherwise, as Bash manual states and POSIX shell standard specifies.

In the case above, not quoting COMMENT would result in variable parameter being assigned text expansion, if it was empty or unset, executing command command substitution, incrementing variable arithmetic and decrementing variable expansion.

Comparing other solutions to this:

Using if false; then comment text fi requires the comment text to be syntactically correct Bash code whereas natural comments are often not, if only for possible unbalanced apostrophes. The same goes for : || { comment text } construct.

Putting comments into a single-quoted void command argument, as in :'comment text', has the drawback of inability to include apostrophes. Double-quoted arguments, as in :"comment text", are still subject to parameter expansion, command substitution and arithmetic expansion, the same as unquoted here-document contents and can lead to the side-effects described above.

Using scripts and editor facilities to automatically prefix each line in a block with '#' has some merit, but doesn't exactly answer the question.

score 47 · Accepted Answer · answered Sep 18 '09 at 12:55

47

if false
then

...code...

fi

false always returns false so this will always skip the code.

answered Sep 18 '09 at 12:55

Artelius

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bash is not a pre-processed language... this trick is useful when used in C as a preprocessor command. If you do it as you suggested you can't avoid possible syntax errors. – drAlberT Sep 18 '09 at 12:57
4

I actually don't mind being informed of syntax errors in code that is "disabled". – Artelius Sep 18 '09 at 13:04
6

:/ This is certainly not the best answer, IMO. – Artelius Oct 09 '09 at 22:55
2

The best answer is below, by @asev69 – th3an0maly May 23 '14 at 03:44
1

@Artelius Sometimes, you want to comment *because* there are unsolved syntax errors that you're not bothered about at the moment – th3an0maly May 23 '14 at 03:45

score 27 · Answer 3 · edited Jun 20 '20 at 09:12

27

You can also put multi-line comments using:

: '
comment1comment1
comment2comment2
comment3comment3
comment4comment4
'

As per the Bash Reference for Bourne Shell builtins

: (a colon)

: [arguments]

Do nothing beyond expanding arguments and performing redirections. The return status is zero.

Thanks to Ikram for pointing this out in the post Shell script put multiple line comment

edited Jun 20 '20 at 09:12

Community

1
1

answered Jul 10 '13 at 15:33

asev69

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It is also important to note that there is a space between the colon and the single apostrophe. Even though the author has made it clear, some like yours truly might miss it and it will not run without the space. – ConfusedMan Dec 18 '15 at 17:47
I think this is the best answer. It is very clear and concise. – consuela May 31 '16 at 10:53
1

You can open and close your comment with `: ' :`. :) – Константин Ван Aug 11 '17 at 19:10
This won't work and would be dangerous if the commented code contains single quotes. e.g. `: 'echo foo'<(echo bar >&2)'baz'`. – spbnick Apr 11 '23 at 11:21

score 21 · Answer 4 · answered Sep 18 '09 at 12:56

21

You can use a 'here' document with no command to send it to.

#!/bin/bash
echo "Say Something"
<<COMMENT1
    your comment 1
    comment 2
    blah
COMMENT1
echo "Do something else"

Wikipedia Reference

answered Sep 18 '09 at 12:56

Buggabill

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The contents of an unquoted here-document is still subject to parameter expansion, command substitution and arithmetic expansion and thus to possible side-effects, like executing text in backticks as commands. – spbnick Oct 04 '13 at 13:26

score 10 · Answer 5 · answered Sep 18 '09 at 12:55

10

Text editors have an amazing feature called search and replace. You don't say what editor you use, but since shell scripts tend to be *nix, and I use VI, here's the command to comment lines 20 to 50 of some shell script:

:20,50s/^/#/

answered Sep 18 '09 at 12:55

kdgregory

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That's how I would do it. To uncomment, try this: :20,50s/^#// – Hai Vu Sep 20 '09 at 17:56

score 5 · Answer 6 · answered Dec 18 '09 at 11:08

5

: || {
your code here
your code here
your code here
your code here
}

answered Dec 18 '09 at 11:08

jj_5698

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This is no better than `if false; then … fi`, because it does not hide the [possibly ill-formed] code from syntax analysis. – Anton Samsonov May 06 '16 at 12:41

score 2 · Answer 7 · answered Nov 08 '16 at 12:50

2

What if you just wrap your code into function?

So this:

cd ~/documents
mkdir test
echo "useless script" > about.txt

Becomes this:

CommentedOutBlock() {
  cd ~/documents
  mkdir test
  echo "useless script" > about.txt
}

answered Nov 08 '16 at 12:50

Buksy

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score 1 · Answer 8 · answered Dec 26 '16 at 04:02

1

As per this site:

#!/bin/bash
foo=bar
: '
This is a test comment
Author foo bar
Released under GNU 
'

echo "Init..."
# rest of script

answered Dec 26 '16 at 04:02

gjois

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score 0 · Answer 9 · answered Sep 18 '09 at 12:55

0

Depending of the editor that you're using there are some shortcuts to comment a block of lines.

Another workaround would be to put your code in an "if (0)" conditional block ;)

answered Sep 18 '09 at 12:55

Carlos Tasada

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0 is truthy in bash, though. if I'm not mistaken, `if (0); then` will always execute – sig_seg_v Jan 16 '17 at 14:45

score -1 · Answer 10 · answered Sep 18 '09 at 13:56

This Perl one-liner comments out lines 1 to 3 of the file orig.sh inclusive (where the first line is numbered 0), and writes the commented version to cmt.sh.

perl -n -e '$s=1;$e=3; $_="#$_" if $i>=$s&&$i<=$e;print;$i++' orig.sh > cmt.sh

Obviously you can change the boundary numbers as required.

If you want to edit the file in place, it's even shorter:

perl -in -e '$s=1;$e=3; $_="#$_" if $i>=$s&&$i<=$e;print;$i++' orig.sh

Demo

$ cat orig.sh 
a
b
c
d
e
f

$ perl -n -e '$s=1;$e=3; $_="#$_" if $i>=$s&&$i<=$e;print;$i++' orig.sh > cmt.sh

$ cat cmt.sh 
a
#b
#c
#d
e
f

sed is simpler: `echo -e "1\n2\n3\n4\n5\n6" | sed -e "2,4 s/^/#/"` — vp_arth, May 04 '14 at 13:58

Commenting out a set of lines in a shell script

10 Answers10

Demo