Question

Hi : Is there a way to create a file which, when read, is generated dynamically ?

I wanted to create 3 versions of the same file (one with 10 lines, one with 100 lines, one with all of the lines). Thus, I don’t see any need for these to be static, but rather, it would be best if they were proxies from a head/tail/cat command.

The purpose of this is for unit testing – I want a unit test to run on a small portion of the full input file used in production. However, since the code only runs on full files (its actually a hadoop map/reduce application), I want to provide a truncated version of the whole data set without duplicating information.

UPDATE: An Example

more myActualFile.txt 

1
2
3
4
5

more myProxyFile2.txt 

1
2

more myProxyFile4.txt 

1
2
3
4 

etc…. So the proxy files are DIFFERENT named files with content that is dynamically provided by simply getting the first n lines of the main file.

Answer 1

This is hacky, but… One way is to use named pipes, and a looping shell script to generate the content (one per named pipe). This script would look like:

while true; do
    (
        for $(seq linenr); do echo something; done
    ) >thenamedpipe;
done

Your script would then read from that named pipe.

Another solution, if you are ready to dig into low level stuff, is FUSE.