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Consider

echo \ # this is a comment
foo

This gives:

$ sh foo.sh
 # this is a comment
foo.sh: line 2: foo: command not found

After some searching on the web, I found a solution by DigitalRoss on sister site Stack Overflow. So one can do

echo `: this is a comment` \
foo

or alternatively

echo $(: this is a comment) \
foo

However, DigitalRoss didn't explain why these solutions work. I'd appreciate an explanation. He replied with a comment "There used to be a shell goto command which branched to labels specified like : here. The goto is gone but you can still use the : whatever syntax ... : is a sort of parsed comment now.", but I'd like more details and context, including a discussion of portability.

Of course, if anyone has other solutions, that would be good too.

See also the earlier question How to comment multi-line commands in shell scripts?.

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1 Answer

up vote 14 down vote accepted

Comments end at the first newline (see shell token recognition rule 10), without allowing continuation lines, so this code has foo in a separate command line:

echo # this is a comment \
foo

As for your first proposal, the backslash isn't followed by a newline, you're just quoting the space: it's equivalent to

echo ' # this is a comment'
foo

$(: this is a comment) substitutes the output of the command : this is a comment. If the output of that command is empty, this is effectively a highly confusing way to insert a comment in the middle of a line.

There's no magic going on: : is an ordinary command, the colon utility, which does nothing. The colon utility is mostly useful when the shell syntax requires a command but you happen to have nothing to do.

# Sample code to compress files that don't look compressed
case "$1" in
  *.gz|*.tgz|*.bz2|*.zip|*.jar|*.od?) :;; # the file is already compressed
  *) bzip2 -9 "$1";;
esac

Another use case is an idiom for setting a variable if it's not already set.

: "${foo:=default value}"

The remark about goto is a historical one. The colon utility dates back from even before the Bourne shell, all the way to the Thompson shell, which had a goto instruction. The colon then meant a label; a colon is a fairly common syntax for goto labels (it's still present in sed).

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Ok, I see. Are there any one better ways of inserting comments in this context that you are aware of? –  Faheem Mitha Aug 21 '11 at 16:05
3  
@FaheemMitha As recommended in the thread you cite: break up your command into manageable chunks and comment each chunk. If your command is so complicated as to need a comment in the middle, it's time to simplify it! –  Gilles Aug 21 '11 at 16:15
    
Well, the command in question has a lot of arguments... It's converting a bunch of video files into one file. I don't see a direct way to simplify it. Maybe create a list of some sort, and pass it as an argument? I guess that could be another question. –  Faheem Mitha Aug 21 '11 at 16:19
    
@FaheemMitha Example: make_FIND, a quickie script which builds a long list of arguments to find. Here, the motivation for building it chunk by chunk is that each chunk comes from the body of a loop, but the same style allows to comment on each chunk. –  Gilles Aug 21 '11 at 17:46
    
Thanks for the example. My example is basically just a long list of names given as arguments to a command. I want comments attached to each name, because that is the most easy way for me to keep context. I don't see any obvious way to break the command up into pieces, and if I did, it might make it even longer, and it is long enough already. –  Faheem Mitha Aug 21 '11 at 18:42
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