How do I break up an extremely long string literal in bash?

I would like to embed a long command like this in a bash script:

mycommand \
    --server myserver \
    --filename extremely/long/file/name/that/i/would/like/to/be/able/to/break/up/if/possible \
    --otherflag \
    --anotherflag

with the long filename broken up.

I could do this:

# Insufficiently pretty
mycommand \
    --server myserver \
    --filename extremely/long/file/name/\
that/i/would/like/to/be/able/to/break/\
up/if/possible \
    --otherflag \
    --anotherflag \

but it breaks the flow. I would like to be able to write this:

# Doesn't work
mycommand \
    --server myserver \
    --filename extremely/long/file/name/\
         that/i/would/like/to/be/able/to/break/\
         up/if/possible \
    --otherflag \
    --anotherflag

but that doesn't work because it breaks up the string literal.

Is there a way to tell bash to break a string literal but ignore any leading spaces?

It's a bit of a hack, but this works:

mycommand \
    --server myserver \
    --filename "extremely/long/file/name/"`
               `"that/i/would/like/to/be/able/to/break/"`
               `"up/if/possible" \
    --otherflag \
    --anotherflag

Bash concatenates string literals that are adjacent, so we take advantage of that. For example, echo "hi" "there" prints hi there whereas echo "hi""there" prints hithere.

It also takes advantage of the backtick operator, and the fact that a bunch of spaces evaluates to nothing.

You can use a variable :

file=extremely/long/file/name
file+=/that/i/would/like/to/be/able/to/break
file+=/up/if/possible

mycommand\
    --server myserver\
    --filename $file\
    --flag flag

I define a short strcat function at the top of my bash script and use an inline invocation to split things up. I sometimes prefer it to using a separate variable because I can define the long literal in-line with the command invocation.

function strcat() {
  local IFS=""
  echo -n "$*"
}

mycommand \
  --server myserver \
  --filename "$(strcat \
      extremely/long/file/name/ \
      that/i/would/like/to/be/able/to/break/ \
      up/if/possible)" \
  --otherflag \
  --anotherflag \

I also like this approach for when I have to enter a long CSV of values as a flag parameter because I can use it to avoid typing the comma between values:

function strjoin() {
  local IFS="$1"
  shift
  echo -n "$*"
}

csv_args=(
  foo=hello
  bar=world
  "this=arg  has  spaces  in  it"
)
mycommand \
  --server myserver \
  --csv_args "$(strjoin , "${csv_args[@]}")" \
  --otherflag \
  --anotherflag \

Which is equivalent to

mycommand \
  --server myserver \
  --csv_args "foo=hello,bar=world,this=arg  has  spaces  in  it" \
  --otherflag \
  --anotherflag \

One can also use an array variable

file=(extremely/long/file/name
	/that/i/would/like/to/be/able/to/break
	/up/if/possible)
IFS=''

echo mycommand\
    --server myserver\
    --filename "${file[*]}"\
    --flag flag

Basically, there is nothing built into bash to do this.
A wrapper is typically more trouble than it's worth, but that said, you could try an alias or a funciton, eg. j

j(){sed -e ':a;$!N;s/ *\n *//g;ta' <<<"$1"}

echo "$(j "3   spaces  
           /hello
           /world
           /this
           /is
           /a
           /long
           /path
          ")"

# 3   spaces/hello/world/this/is/a/long/path

Another way of writing a long string to a variable while keeping the maximum line length at bay:

printf -v fname '%s' \
    'extremely/long/file/name/that/i/' \
    'would/like/to/be/able/to/break/up/' \
    'if/possible'

Because there are more arguments than formatting directives, %s is just repeated and we get

$ declare -p fname
declare -- fname="extremely/long/file/name/that/i/would/like/to/be/able/to/break/up/if/possible"

which an be used like

mycommand \
    --server myserver \
    --filename "$fname" \
    --otherflag \
    --anotherflag

This is extra handy when setting long variables with inherently separated contents such as CDPATH (or PATH, of course):

printf -v CDPATH '%s' \
	':/Users/benjamin/here/is/a/long/path' \
	':/Users/benjamin/and/here/is/another/one' \
    ':/Users/benjamin/and/a/third/line'
export CDPATH

as opposed to

export CDPATH=':/Users/benjamin/here/is/a/long/path:/Users/benjamin/and/here/is/another/one:/Users/benjamin/and/a/third/line'

or the clunky

export CDPATH=':/Users/benjamin/here/is/a/long/path'
CDPATH+=':/Users/benjamin/and/here/is/another/one'
CDPATH+=':/Users/benjamin/and/a/third/line'