Pass Every Line of Input as stdin for Invocation of Utility

Question

I have a file containing valid xmls (one per line) and I want to execute a utility (xpath) on each line one by one.

I tried xargs but that seems doesn't seem to have an option to pass the line as stdin :-

% cat <xmls-file> | xargs -p -t -L1 xpath -p "//Path/to/node"

Cannot open file '//Path/to/node' at /System/Library/Perl/Extras/5.12/XML/XPath.pm line 53.

I also tried parallel --spreadstdin but that doesn't seem to work either :-

% cat <xmls-file> | parallel --spreadstdin  xpath -p "//Path/to/node"
junk after document element at line 2, column 0, byte 1607

Answer 1

If you want every line of a file to be split off and made stdin for a utility

you could use a for loop in bash shell:

cat xmls-file | while read line
do ( echo $f > /tmp/input$$; 
     xpath -p "//Path/to/node" </tmp/input$$
     rm -f /tmp/input$$
);
done

The $$ appends the process id number, creating a unique name

I assume xmls-file contains, on each line, what you want iterated into $f and that you want this as stdin for a command line, not as a parameter to the command.

On the other hand, your specification may be incorrect and maybe instead you need each line to be part of a command. In that case, delete the echo and rm lines, and change the xpath command to include $f wherever the line from the file is needed.

I've not done much XML so the do command may need to be edited.

Answer 2

You are very close with the GNU Parallel version; only -n1 missing:

cat <xmls-file> | parallel -n1 --spreadstdin  xpath -p "//Path/to/node"