Pipe filepath to ImageJ

Assuming the rjp2tif command returns a file-path in standard output, and you want to pass this output as a regular CLI argument to another command, you may be interested in the xargs command. But note that in the general case, you may hit some issue if the file-path contains spaces or so:

Read space, tab, newline and end-of-file delimited arguments from standard input and execute the specified utility with them as arguments.
The arguments are typically a long list of filenames (generated by ls or find, for example) that get passed to xargs via a pipe.

So in this case, assuming each file-path takes only one line (which is obviously the case if there's only one line overall), you can use the following NUL-based tip relying on the tr command.

Here is the command you'd obtain:

rjp2tif /path/to/rjpeg | tr '\n' '\0' | xargs -0 open -a imagej

Note: I have a GNU/Linux OS, so can you please confirm it does work under macOS?

FTR, below is a comprehensive shell code allowing one to test two different modes of xargs: generating one command per line-argument (-n1), or a single command with all line-arguments in one go:

$ printf 'one \ntwo\nthree and four' | tr '\n' '\0' | xargs -0 -n1 \
  bash -c 'printf "Run "; for a; do printf "\"$a\" "; done; echo' bash
Run "one " 
Run "two" 
Run "three and four" 

$ printf 'one \ntwo\nthree and four' | tr '\n' '\0' | xargs -0 \
  bash -c 'printf "Run "; for a; do printf "\"$a\" "; done; echo' bash
Run "one " "two" "three and four" 

######################################
# or alternatively (with no for loop):
######################################

$ printf 'one \ntwo\nthree and four' | tr '\n' '\0' | xargs -0 -n1 \
  bash -c 'printf "Run "; printf "\"%s\" " "$@"; echo' bash
Run "one " 
Run "two" 
Run "three and four" 

$ printf 'one \ntwo\nthree and four' | tr '\n' '\0' | xargs -0 \
  bash -c 'printf "Run "; printf "\"%s\" " "$@"; echo' bash
Run "one " "two" "three and four"