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41 lines
1.8 KiB
Plaintext
41 lines
1.8 KiB
Plaintext
27 years ago
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When MzScheme is installed as part of the standard Unix PLT
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distribution, the executable ${PLTHOME}/bin/mzscheme is actually a
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script that selects an executable based on the current platform. This
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is done to support multiple platforms in a single PLT installation.
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Unfortunately, this causes problems for writing scripts that use
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MzScheme, because the shell's #! convention requires that a binary is
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specified.
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Fortunately, there is a sneaky workaround. To write a MzScheme script,
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use the following at the top of your script:
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#!/bin/sh
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string=? ; exec ${PLTHOME}/bin/mzscheme -r $0 $0 ${1+"$@"}
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... <scheme program> ...
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(It may be better to hardwire the path to mzscheme instead of using
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${PLTHOME}.)
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Within <scheme program>, (vector-ref argv 0) is the name of the
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script; arguments passed to the script start at (vector-ref argv 1).
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How does this work? "#!/bin/sh" causes the shell to run /bin/sh on the
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file. The first command, "string=?" is a harmless assignment, and ";"
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separates this assignment from the "exec" command. The "exec" command
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replaces /bin/sh with mzscheme (so /bin/sh does not see the actual
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Scheme program). The "-r" flag tells mzscheme to load the next
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argument as a script and forces the remaining arguments into
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`argv'. (For information on ${1+"$@"}, consult your `sh' man page.)
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Thus, mzscheme will start up and load this file. As always, mzscheme
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ignores "#!" in the first line of a loaded file as a comment. And
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"string=?" is again a harmless expression (since it's a built-in
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variable). The ";" starts a Scheme comment, so the "exec" expression
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is ignored. Finally, mzscheme continues to load the file and evaluates
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the real Scheme program.
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Thanks to J. P. Lewis <zilla@interval.com> for the basic idea behind
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this trick. Many others have suggested alternatives to `string=?'
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for shells other than /bin/sh.
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