The Universe of Disco

The "z" command: alternative implementations
In yesterday's article I discussed a possibly-useful utility program named z, which has a flaw. To jog your memory, here is a demonstration:

        % z grep immediately *  
        ctime.blog:we want to update.  It is immediately copied into a register, and
        /proc/self/fd/3:All five people who wrote to me about this immediately said "oh, yes,
        /proc/self/fd/5:program continues immediately, possibly posting its message.  (It
        struct-inode.blog:is a symbolic link, its inode is returned immediately; iname() would
        sync.blog:and reports success back to the process immediately, even though the

Fixing this flaw seems difficult-to-impossible. As I said earlier, the trick is to fool the command into reading from a pipe when it thinks it is opening a file, and this is precisely what /proc/self/fd is for. But there is an older, even more widely-implemented Unix feature that does the same thing, namely the FIFO. So an alternative implementation creates one FIFO for each compressed file, with a gzip process writing to the FIFO, and tells the command to read from the FIFO. Since we have some limited control over the name of the FIFO, we can ameliorate the missing-filename problem to some extent. Say, for example, we create the FIFOs in /tmp/PID. Then the broken zgrep example above might look like this instead:

        % z grep immediately *  
        ctime.blog:we want to update.  It is immediately copied into a register, and
        /tmp/7516/env-2.blog.gz:All five people who wrote to me about this immediately said "oh, yes,
        /tmp/7516/qmail-throttle.blog.gz:program continues immediately, possibly posting its message.  (It
        struct-inode.blog:is a symbolic link, its inode is returned immediately; iname() would
        sync.blog:and reports success back to the process immediately, even though the

        #!/bin/sh
        DIR=/tmp/$$
        mkdir $DIR

        COMMAND=$1
        shift
        cp -p "$@" $DIR

        cd $DIR
        gzip -d *
        $COMMAND *

But as I said, I don't know yet.

[ Addendum 20080325: Several people suggested a fix that I had considered so unwise that I didn't even mention it. But after receiving the suggestion repeatedly, I wrote an article about it. ]

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