Bash continuation lines

Question

How do you use bash continuation lines?

I realize that you can do this:

echo "continuation \
lines"
>continuation lines

However, if you have indented code, it doesn't work out so well:

    echo "continuation \
    lines"
>continuation     lines

Answer 1

I came across a situation in which I had to send a long message as part of a command argument and had to adhere to the line length limitation. The commands looks something like this:

somecommand --message="I am a long message" args

The way I solved this is to move the message out as a here document (like @tripleee suggested). But a here document becomes a stdin, so it needs to be read back in, I went with the below approach:

message=$(
    tr "\n" " " <<- END
        This is a
        long message
END
)
somecommand --message="$message" args

This has the advantage that $message can be used exactly as the string constant with no extra whitespace or line breaks.

Note that the actual message lines above are prefixed with a tab character each, which is stripped by here document itself (because of the use of <<-). There are still line breaks at the end, which are then replaced by dd with spaces.

Note also that if you don't remove newlines, they will appear as is when "$message" is expanded. In some cases, you may be able to workaround by removing the double-quotes around $message, but the message will no longer be a single argument.

Answer 2

This isn't exactly what the user asked, but another way to create a long string that spans multiple lines is by incrementally building it up, like so:

$ greeting="Hello"
$ greeting="$greeting, World"
$ echo $greeting
Hello, World

Obviously in this case it would have been simpler to build it one go, but this style can be very lightweight and understandable when dealing with longer strings.

Answer 3

This probably doesn't really answer your question but you might find it useful anyway.

The first command creates the script that's displayed by the second command.

The third command makes that script executable.

The fourth command provides a usage example.

john@malkovich:~/tmp/so$ echo $'#!/usr/bin/env python\nimport textwrap, sys\n\ndef bash_dedent(text):\n    """Dedent all but the first line in the passed `text`."""\n    try:\n        first, rest = text.split("\\n", 1)\n        return "\\n".join([first, textwrap.dedent(rest)])\n    except ValueError:\n        return text  # single-line string\n\nprint bash_dedent(sys.argv[1])'  > bash_dedent
john@malkovich:~/tmp/so$ cat bash_dedent 
#!/usr/bin/env python
import textwrap, sys

def bash_dedent(text):
    """Dedent all but the first line in the passed `text`."""
    try:
        first, rest = text.split("\n", 1)
        return "\n".join([first, textwrap.dedent(rest)])
    except ValueError:
        return text  # single-line string

print bash_dedent(sys.argv[1])
john@malkovich:~/tmp/so$ chmod a+x bash_dedent
john@malkovich:~/tmp/so$ echo "$(./bash_dedent "first line
>     second line
>     third line")"
first line
second line
third line

Note that if you really want to use this script, it makes more sense to move the executable script into ~/bin so that it will be in your path.

Check the python reference for details on how textwrap.dedent works.

If the usage of $'...' or "$(...)" is confusing to you, ask another question (one per construct) if there's not already one up. It might be nice to provide a link to the question you find/ask so that other people will have a linked reference.

Answer 4

You could simply separate it with newlines (without using backslash) as required within the indentation as follows and just strip of new lines.

Example:

echo "continuation
of 
lines" | tr '\n' ' '

Or if it is a variable definition newlines gets automatically converted to spaces. So, strip of extra spaces only if applicable.

x="continuation
of multiple
lines"
y="red|blue|
green|yellow"

echo $x # This will do as the converted space actually is meaningful
echo $y | tr -d ' ' # Stripping of space may be preferable in this case

Answer 5

This is what you may want

$       echo "continuation"\
>       "lines"
continuation lines

If this creates two arguments to echo and you only want one, then let's look at string concatenation. In bash, placing two strings next to each other concatenate:

$ echo "continuation""lines"
continuationlines

So a continuation line without an indent is one way to break up a string:

$ echo "continuation"\
> "lines"
continuationlines

But when an indent is used:

$       echo "continuation"\
>       "lines"
continuation lines

You get two arguments because this is no longer a concatenation.

If you would like a single string which crosses lines, while indenting but not getting all those spaces, one approach you can try is to ditch the continuation line and use variables:

$ a="continuation"
$ b="lines"
$ echo $a$b
continuationlines

This will allow you to have cleanly indented code at the expense of additional variables. If you make the variables local it should not be too bad.

Answer 6

Here documents with the <<-HERE terminator work well for indented multi-line text strings. It will remove any leading tabs from the here document. (Line terminators will still remain, though.)

cat <<-____HERE
    continuation
    lines
____HERE

See also http://ss64.com/bash/syntax-here.html

If you need to preserve some, but not all, leading whitespace, you might use something like

sed 's/^  //' <<____HERE
    This has four leading spaces.
    Two of them will be removed by sed.
____HERE

For wrapping long complex strings over many lines, I like printf:

printf '%s' \
    "This will all be printed on a " \
    "single line (because the format string " \
    "doesn't specify any newline)"