I am learning Fish and I want to know how to make multiline comments but the docs are hard to understand.
Any Help Apprecited.
You can pass a quoted string that spans multiple lines to a no-op command. This also works in other shells.
: '
This is a multi-line
comment.
'
The : command is an alias to true.
The documentation doesn't seem to include an explicit references for how to add a comment to a fish script. The implication via examples is that the only comment available is the POSIX-shell style rest-of-line comment that starts with an unquoted #.
Related
I was reading a tutorial about bash scripting on https://www.tldp.org/LDP/abs/html/special-chars.html
and noticed that some comments begin with #+ instead of #
echo ls -l | sh
# Passes the output of "echo ls -l" to the shell,
#+ with the same result as a simple "ls -l"
I'm guessing the author decided to use #+ to mention continued comment from the previous line.
Is this the case and is it a common convention in bash?
Comments start with #. There is no convention associated with comments (well, they should be readable and about the code..........). The standard does not mention what syntax should a comment have. The string after # is not parsed, so #+ is equivalent to #!!##$%^&*() and is not anything special when parsed by Bash. The #+ is just the author's convention to continue a comment in the next line if they didn't end the sentence in the previous line.
This is not a convention but here, I assume like you, the author wanted to link both comments.
The real way to do it in bash is to use the multiline comment:
: '
This is a
multiline comment.
'
Please note that the space between the : and ' is important.
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?
Use : ' to open and ' to close.
For example:
: '
This is a
very neat comment
in bash
'
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
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.
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.
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
'
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
}
}
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
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
in plain bash
to comment out
a block of code
i do
:||{
block
of code
}
For example:
code = <<-EOH
bundle install
bundle exec unicorn -c /etc/unicorn.cfg -D
EOH
What does this code do? What is <<- called?
It's called heredoc. An easy way to define multiline strings which may include single or double quotes without needing to escape them.
See more here, for example.
Often you use heredocs to define large chunks of code. Some editors know about this and can highlight syntax for you there (if you specify language). Look:
There is also a newer HEREDOC syntax for Ruby <<~END that more closely resembles what you would typically see in most shells and other languages with the ~ instead of the - to tell Ruby to strip the leading whitespace to match the least indented line in the block.
https://infinum.co/the-capsized-eight/multiline-strings-ruby-2-3-0-the-squiggly-heredoc
Looks to me like heredoc. The - allows the ending delimiter to ignore whitespace before it.
A simple Google Search gave me this.
For example:
code = <<-EOH
bundle install
bundle exec unicorn -c /etc/unicorn.cfg -D
EOH
What does this code do? What is <<- called?
It's called heredoc. An easy way to define multiline strings which may include single or double quotes without needing to escape them.
See more here, for example.
Often you use heredocs to define large chunks of code. Some editors know about this and can highlight syntax for you there (if you specify language). Look:
There is also a newer HEREDOC syntax for Ruby <<~END that more closely resembles what you would typically see in most shells and other languages with the ~ instead of the - to tell Ruby to strip the leading whitespace to match the least indented line in the block.
https://infinum.co/the-capsized-eight/multiline-strings-ruby-2-3-0-the-squiggly-heredoc
Looks to me like heredoc. The - allows the ending delimiter to ignore whitespace before it.
A simple Google Search gave me this.
I am writing a shell (bash) script and I'm trying to figure out an easy way to accomplish a simple task.
I have some string in a variable.
I don't know if this is relevant, but it can contain spaces, newlines, because actually this string is the content of a whole text file.
I want to replace the last occurence of a certain substring with something else.
Perhaps I could use a regexp for that, but there are two moments that confuse me:
I need to match from the end, not from the start
the substring that I want to scan for is fixed, not variable.
for truncating at the start: ${var#pattern}
truncating at the end ${var%pattern}
${var/pattern/repl} for general replacement
the patterns are 'filename' style expansion, and the last one can be prefixed with # or % to match only at the start or end (respectively)
it's all in the (long) bash manpage. check the "Parameter Expansion" chapter.
amn expression like this
s/match string here$/new string/
should do the trick - s is for sustitute, / break up the command, and the $ is the end of line marker. You can try this in vi to see if it does what you need.
I would look up the man pages for awk or sed.
Javier's answer is shell specific and won't work in all shells.
The sed answers that MrTelly and epochwolf alluded to are incomplete and should look something like this:
MyString="stuff ttto be edittted"
NewString=`echo $MyString | sed -e 's/\(.*\)ttt\(.*\)/\1xxx\2/'`
The reason this works without having to use the $ to mark the end is that the first '.*' is greedy and will attempt to gather up as much as possible while allowing the rest of the regular expression to be true.
This sed command should work fine in any shell context used.
Usually when I get stuck with Sed I use this page,
http://sed.sourceforge.net/sed1line.txt