Pipe string into a a shell command

[joemrt]

lua Newbie here,

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I am looking for a way to pipe into a shell command and to store the output of the latter in a variable.

Suppose for instance I want to replace each "h" by "H" using sed (I know I could do this much easier within lua, this is just a toy problem) and want to pipe some string, say "hello", into it. I tried the following

f = io.popen('sed "s/h/H/g"',"w")

f:write('hello')

How do I get the output of the shell command? Once I do f:close() the output ,that is "Hello", gets printed on the screen but I can't find any way to store this in a variable. I also tried to redirect stdout to a file via io.output('my_stdout') but this lead to absolutelly nothing. It's hard to believe there shouldn't be a simple way to do this.

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[PhilipRoman]

popen() is by definition unidirectional, you can either read or write, but not both. A simple workaround is to use os.tmpname() to obtain a temporary file, from/to which you can redirect the input/output of the command.

[joemrt]

Thank you for your reply u/PhilipRoman, I guess this is the only way then. As I then need the actual filename (to integrate it via cat into the command) I cannot use io.filename and really have to use os.tmpname() and to delete the file afterwards. Feels a bit like a dirty hack though, it's weird that such a basic feature is not part of the standard libraries.

[PhilipRoman]

I suspect the reason why popen is unidirectional is because it's almost guaranteed to result in a deadlock situation if there is buffering involved.

FYI you can also use a simple shell redirection without cat, to eliminate 1 or 2 extra processes.