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. Thegoto
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?.
Take home message from the discussion below.
The `: this is a comment`
is just a command substitution.
The output of : this is a comment
is nothing, and that
gets put in the place of `: this is a comment`
.
A better choice is the following:
echo `# this is a comment` \
foo
Best Answer
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:As for your first proposal, the backslash isn't followed by a newline, you're just quoting the space: it's equivalent to
$(: 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.Another use case is an idiom for setting a variable if it's not already set.
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).