piping output from programs that do not output to STDOUT

Say I have a program that outputs to a file which is provided by an (required) argument:

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./foo -o bar.txt

Is there any way to trick it into outputting to a pipe? This just outputs to a file named -.

./foo -o - | less

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You can use /dev/stdout as the file name:

./foo -o /dev/stdout | less

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This will feed the contents of bar.txt to filter as the file grows:

tail -f bar.txt | filter

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other solution, create a named pipe:

mkfifo /tmp/myfifo; ./foo -o /tmp/myfifo
</tmp/myfifo cat -